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236
Concerted hydrogen-atom abstraction in photosynthetic
water oxidation
Kristi L Westphal*, Cecilia Tommos†, Robert I Cukier* and
Gerald T Babcock*
Photosystem II evolves oxygen by using water in the unlikely role
of a reductant. The absorption of sunlight by chlorophyll
produces highly oxidizing equivalents that are filled with
electrons stripped from water. This proton-coupled redox
chemistry occurs at the oxygen-evolving complex, which contains
a tetramanganese cluster, a redox-active tyrosine amino acid
hydrogen-bonded to a histidine amino acid, a calcium ion and
chloride. Hydrogen-atom abstraction by the tyrosyl radical from
water bound to the manganese cluster is now widely held to
occur in this process, at least for some of the steps in the
catalytic cycle. We discuss kinetic and energetic constraints on
the hydrogen-atom abstraction process.
Addresses
*Department of Chemistry, Michigan State University, East Lansing,
Michigan 48824, USA
† Department of Biochemistry, Arrhenius Laboratories for Natural
Sciences, Stockholm University, S106 91 Stockholm, Sweden
† Current address: Department of Biochemistry, Stockholm University,
S-106 91 Stockholm, Sweden
Current Opinion in Plant Biology 2000, 3:236–242
1369-5266/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Abbreviations
ε
dielectric constant
∆G
free energy change
OEC
oxygen-evolving complex
PSII
photosystem II
QA
initial plastoquinone electron acceptor in photosystem II
Sn
State at which n oxidizing equivalents are accumulated
(where n is 0—4)
Yz
redox-active tyrosine at position 161 of the D1 protein
Introduction
Photosynthetic organisms use photosystem II (PSII) to catalyze the light-driven oxidation of water according to the
following half-cell reaction:
2 H2O → O2 + 4 H+ + 4 e–
The protons and electrons released in this process are ultimately used in starch production; to the plant, O2 is simply
a waste product. This remarkable redox chemistry is initiated by photon absorption by the PSII reaction-center
chlorophyll, P680, which produces the charge-separated
state, P680+QA–. Reduction of P680+ by water is mediated
by the oxygen-evolving complex (OEC), which is largely,
if not exclusively, associated with D1, one of the principal
reaction-center proteins in PSII. The OEC consists of four
manganese ions, a redox-active tyrosine called Yz (tyrosine
161), histidine 190, and Ca2+ and Cl– ions. The cluster containing the four Mn ions [i.e. (Mn)4] stores oxidizing
equivalents upon photon absorption; this process is annotated by the Sn nomenclature, where n is the number of
oxidizing equivalents accumulated. Four successive lightinduced charge separations lead to the formation of S4 from
which O2 is released, thereby resetting the system to S0
(for recent reviews see [1••,2,3]).
It has been postulated that Yz• accepts both electrons and
protons from water during catalysis [4,5], and a mechanism
for the water-splitting chemistry of PSII based on H-atom
abstraction has been developed [1••,6]. Several functional
models have been proposed recently within this theme.
These models differ primarily in the extent to which Yz
acts as a H-atom abstractor during the catalytic cycle and in
the O=O bond-forming step [7•–9•,10••]. In this review, we
discuss the mechanistic details of coupled electron–proton
transfer. First, we describe the original H-atom abstraction
model, then we discuss Yz oxidation briefly, and finally we
analyze Yz• reduction.
The hydrogen-atom abstraction model
Essential features of this mechanism are summarized in
Figure 1. Upon photon absorption and generation of
P680+, Yz reduces P680+ and simultaneously deprotonates
through the H190–H-bonded network, which is postulated to extend to bulk, to produce Yz• (reaction 1 in
Figure 1). In the second step, Yz• abstracts a hydrogen
atom from the substrate water/hydroxide that is ligated to
the (Mn)4 cluster to advance the S-state (Figure 1, reaction 2). Four consecutive photon absorption/H-atom
abstraction events strip four hydrogen atoms from two
substrate water molecules; the two oxygen atoms collapse
to form O2, which is released as the system resets to the
S0 state by loading the next two water molecules. Within
this mechanism, each cofactor in the OEC plays specific
roles that are described below.
Yz carries out at least three functions that are critical to the
oxidation of water. First, Yz reduces P680+ on the ns time
scale, easily outcompeting the P680+QA– chargerecombination reaction, which prevents energy loss as heat.
Because of this rapid reduction of P680+, PSII operates with
a quantum efficiency that is close to unity. Second, the
oxidation of water requires strongly oxidizing conditions.
These are initially created at P680+ and preserved by the
high reduction potential of the Yz•/Yz redox couple. Third,
Yz interfaces the electron-tunneling reactions that are
carried out by the P680/QA pair at the reaction center to the
proton-coupled redox chemistry in the OEC. Thus, the
proton currents associated with water oxidation are
switched on at Yz•.
Concerted hydrogen-atom abstraction in photosynthetic water oxidation Westphal et al.
237
Figure 1
Reaction 1
P680+
e−
O
Mn
O
O
O
H
Mn
O
H
H
N
H190
N
H-bonded chain
to bulk
O
H
O
O
H
H
N
H190
Mn
O
S0
H
S0YzP680+
Mn
Yz•
Mn
O
H
H
O
O
H+
O
Mn
O
S0
Yz
Mn
H
H
Mn
P680
N
H
S0Yz
H-bonded chain
to bulk
•P
680
Reaction 2
P680
P680
O
Mn
O
H
S0
O
H
O
O
Mn
H
Mn
O
H
H
H
N
Mn
H190
N
S1
Yz
Mn
O
O
H
S0Yz•P680
O
H•
O
H
Mn
Yz•
Mn
O
O
O
H
O
Mn
H
H
N
H190
N
H
H-bonded chain
to bulk
S1YzP680
H-bonded chain
to bulk
Current Opinion in Plant Biology
Essential features of the H-atom abstraction model for photosynthetic
water oxidation. The postulated relative arrangement of the catalytic
center of PSII is shown. Substrate water binds terminally to the Mn
ions within hydrogen bonding (……) distance of the Yz phenol group.
The arrows show the flow of electrons (open arrow) and protons
(solid arrow) on the proton-coupled oxidation of Yz (Reaction 1) and
on the subsequent H-atom transfer (broken arrow) to Yz• from water
bound to the (Mn)4 cluster (Reaction 2). The figure shows the S0 to
S1 transition. Similar reactions are postulated to occur in the higher
S-states [1••].
Site-directed mutagenesis ([11••] and references therein)
and kinetic data [12•,13•] have shown that Yz is hydrogen
bonded to H190. If this histidine is mutated, Yz oxidation
slows and takes ms rather than ns. Furthermore, these
results suggest that when hydrogen-bond formation is curtailed, Yz cannot deprotonate efficiently and is not able to
reduce P680+. Thus, the oxidation of Yz is a proton-coupled
event, and H190 serves as the initial proton acceptor. The
electron made available in this process reduces P680+, and
the proton migrates through the H190/H-bonded network
to bulk ([14•,15••], but see [10••]). Together, these
processes form the neutral radical, Yz•, which is now able to
mediate the H-atom abstractions.
the function of the (Mn)4 cluster is at least three-fold.
First, it serves to delocalize the oxidizing equivalents generated as the S-state advances. The general consensus of
opinion is that the (Mn)4 cluster is oxidized on each S-state
transition, with the possible exception of the S2→S3 transition [17,18]. Second, it reduces the bond-dissociation
energy of bound substrate [1••,19,20•]. Third, the (Mn4)
cluster acts as a template for the formation of the O=O
bond [6]. The C-shaped structure promoted by Klein,
Sauer, Yachandra and their co-workers [21] is ideal in this
regard, as it holds the two oxygen atoms in a geometry that
allows facile O=O bond formation. The structure of the
(Mn)4 cluster is controversial but a templating function of
the metal cluster is likely.
Yz• and the (Mn)4 cluster, which have a magnetic center-tocenter separation of about 8 Å ([16•] and references
within), form a single catalytic site. Within this complex,
Ca2+ and Cl– are closely associated with the catalytic site
[21]. We have proposed that Ca2+ serves as a Cl– docking
238
Physiology and metabolism
Figure 2
(a) Holo-PSII
(ns kinetics)
(b) Apo-PSII
(µs kinetics)
CH2
CH2
e−
slow
fast
+
Disordered
H
H
N
N
H190
P680+
H w
sl o
O
+
O
H t
s
fa
Wellordered
e−
P680+
Rate-limiting factors for Yz oxidation. (a) The
OEC is intact, proton transfer to H190 occurs
on the ns time scale or faster, and the
reduction of P680+ is limited by the rate of
electron transfer. (b) PSII is depleted of Mn or
Ca and the oxidation of Yz takes µs rather than
ns. The movement of the phenol proton now
becomes rate-limiting for the overall protoncoupled reduction of P680+.
N
CH2
H
H190
N
CH2
H
Current Opinion in Plant Biology
site, and that Cl– neutralizes charge upon the S2→S3 transition as the result of Jahn-Teller effects [1••]. The role of
these two essential ions is elusive, however, and several
other functions have been suggested (e.g. [7•–9•]).
There is considerable experimental support for H-atom
abstraction in the catalytic cycle of PSII. In the sections
that follow, we provide further insight into the mechanisms
of H-atom transfer from and to Yz during water oxidation.
The kinetics of Yz oxidation
The oxidation of Yz has S-state dependent, multiphasic
electron-transfer kinetics that occur in the ns to µs
timescale. Moreover, its oxidation rate is highly sensitive
to the integrity of the OEC (Figure 2). The different
kinetics for Yz oxidation in holo- versus apo-PSII have
recently been considered in detail experimentally
[11••,12•,13•,22,23] and in a review article [15••]. In the
intact system (i.e. holo-PSII), it appears that proton transfer is facilitated by a well-ordered H-bonding structure
(Figure 2a). Under these conditions, proton transfer is not
rate limiting and the kinetics of Yz oxidation can be accurately predicted [15••] using an expression developed by
Dutton, Moser, and coworkers for estimating the rates of
intraprotein, non-adiabatic electron tunneling [24••].
When PSII is depleted of Mn or Ca (i.e. apo-PSII), hydrogen bonding at the Yz site becomes disordered (Figure 2b).
Under these conditions, the deprotonation of Yz during
electron transfer is no longer facile and the rate of the proton transfer event limits that of Yz oxidation. The overall
reaction slows to the µs timescale and becomes pH dependent with a substantial kinetic isotope effect. These
advances in our understanding of the experimental details
of Yz oxidation have been accompanied by a satisfying theoretical description of the process as an example of
dissociative proton-coupled electron transfer [25•].
The kinetics of Yz reduction
The reduction of Yz• is S-state dependent and occurs
over the µs to ms time scale (Table 1). Assuming reasonable values for driving force, reorganization energy,
protein packing and distance, we find that the expression developed by Page et al. [24 ••] predicts
electron-tunneling rates that are far more rapid than the
observed rates of Yz• reduction [15••]. Yz deprotonates to
H190 upon oxidation and reprotonates upon reduction.
It follows that an accurate analysis of Yz• reduction
should take into account the movement of a proton as
well as an electron. The Yz•/Yz and Sn+1/Sn reduction
potentials, and the activation energies and pre-exponential factors for the S-state transitions, are required for this
analysis and are given in Table 2.
Three mechanisms for the transfer of an electron and a
proton are illustrated in Figure 3, where the Sn→Sn+1
advance is represented by Mn(H2O)→Mn+(OH-). Two
are sequential and involve either electron transfer followed by proton transfer [path (i)], or proton transfer
followed by electron transfer [path (ii)]. The third mechanism invokes concerted electron/proton transfer
[path (iii)].
Table 1
Half-times for S-state advance..
S-state
Thylakoids
PSII
membranes
Core
particles
Yz•S0→YzS1
Yz•S1→YzS2
Yz•S2→YzS3
Yz•S3→YzS0
40–60 µs
65–86 µs
140–245 µs
750 µs;1.3 ms
30–70 µs
30–70 µs
55–110 µs
1.2–1.4 ms
60 µs
75–95 µs
225–380 µs
4.1–4.6 ms
Compiled from [14•,38,39].
Concerted hydrogen-atom abstraction in photosynthetic water oxidation Westphal et al.
For path (i), the initial transfer of an electron results in the
reduction of Yz• to a tyrosinate [intermediate (b)], which
subsequently protonates in the second step of the process.
The Yz•/Yz potential (shown in Table 2), corresponds to the
overall H+/e– transfer, but to judge the viability of the
sequential process we must consider the energetics of
forming the transient intermediate (b). The potential of
Y•/Y– is 0.68 V in water [15••] and is expected to decrease
in the low protein dielectric. This effect can be estimated
by using the Born model [26]:
∆G = 14.397/εr
Figure 3
(b) Yz−/Mn+ (H2O)
e−
(i)
(1)
where ∆G (in units of volts) is the electrostatic penalty for
introducing a charged species with radius (r; in Å) in a
homogenous medium of dielectric constant ε. Assuming a
radius of 3 Å and an ε of 10, we calculate a Yz•/Yz– potential
of 0.26 V. Thus, in a protein milieu, Yz• is a relatively weak
oxidant if Yz– is the product. For the Sn+1/Sn potential, the
opposite trend occurs, that is, to oxidize [Mn–H2O] or
[Mn–OH] to charged products requires a stronger oxidant
than when the process is proton coupled. With ε and r as
used above for Yz, Equation 1 predicts that the S1/S0
potential increases from 0.7 V to 1.12 V, if a positive charge
is formed. For the higher S-state transitions, correspondingly higher reduction potentials are predicted. As a result,
the driving force for forming (b) during S0→S1 is very
endergonic, (+0.26)–(+1.12) = –0.86 V. The electrostatic
interaction between [Mn-OH2]+ and Yz– in (b) lowers this
value by –0.18 V. Thus, the resulting difference in reduction potential is –0.68 V, which equates to a free energy
difference (∆G) of 15.7 kcal/mol. For the higher S-states,
the endergonicity will be even greater. These free energy
values obtained for formation of (b) lead to activation energies that are substantially higher than the activation
energies for the S-state transitions presented in Table 2,
and suggest that path (i) in Figure 3 is improbable.
The second possibility is also sequential [path (ii) in Figure 3]. The pKas of the proton donor and acceptor can be
used to determine the energetics of the initial proton transfer. For the individual S-states, the pKa is expected to be
greater than or equal to seven [1••,20•], whereas the pKa of
Y•+ in water is minus two [15••]. Therefore, the ∆pKa for
the proton transfer is at least nine, resulting in a ∆G of at
239
(a) Yz•/Mn (H2O)
H+ (i)
(iii)
e− and H+
H+ (ii)
(d) Yz/Mn+ (OH−)
e−
(ii)
(c) Yz•+/Mn (OH–)
Current Opinion in Plant Biology
Possible reaction paths for electron and proton transfer. Four species
are shown as follows: (a) The reactant state Yz•/Mn(H2O), (d) the
product state Yz/Mn+(OH-), and two intermediate states, (b) Yz/Mn+(H2O) and (c) Yz•+/Mn(OH-). Along the edges of the square, two
sequential pathways are shown. In path (i), electron transfer is
followed by proton transfer (et/pt), while in path (ii), the reverse
sequence of proton and electron steps occurs (pt/et). In path (iii) the
electron and proton move in a concerted process.
least 12 kcal/mol. The ∆G is as high as 23 kcal/mol when
Equation 1 is used to estimate the pKa in a protein environment. Consequently, the ∆G calculated for the transfer
of a proton prior to electron transfer gives activation energies that are considerably higher than those that have been
observed for any of the S-state transitions (Table 2). We
conclude that reduction of Yz• is unlikely to take place by
this mechanism.
As the sequential transfer of an electron and a proton
involves the formation of high energy intermediates, concerted proton/electron transfer in PSII becomes a viable
mechanism. These processes have been studied theoretically by Cukier [25•,27••], and by Hammes-Schiffer [28],
and experimentally by researchers including Mahoney and
DaRooge [29], Ingold and colleagues [30,31], Nocera and
Table 2
Kinetic and thermodynamic parameters for the S-state transitions..
Reaction
Yz•S0→YzS1
Yz•S1→YzS2
Yz•S2→YzS3
Yz•S3→YzS0
Ea (kcal/mol)
A (s–1)
KIE
Yz•/Yz (V)
Sn+1/Sn
1.2
3.0
9.0
5.0†
9.2‡
4.0 × 106
5.4 × 109
8.9 × 105†
2.9 × 1014‡
1.3; 2.9
1.3
1.4–1.6
0.97
0.97
0.97
0.97
0.70
0.93
0.93
0.93
A, arrhenius pre-exponential factor; Ea, activation energy; KIE, kinetic isotope effect; Sn+1/Sn, reduction potential of Sn+1/Sn;Yz•/Yz, reduction
potential of Yz•/Yz. †T > 280 K. ‡T < 280 K. Compiled from [14•,15••,38].
240
Physiology and metabolism
Energy
Figure 4
extracted. The activation energies of these processes are
generally very low. For example, activation energies in the
1.7–4.9 kcal/mol range were reported in studies on the
reactivity of phenoxy radicals in H-atom abstraction reactions [29,30] and values of 5 kcal/mol were measured for
H-atom transfer between two ferric complexes [34]. The
activation energies measured for reduction of Yz• are
1–9 kcal/mol (Table 2) and fit well within the range of values for H-atom abstraction reactions.
Sequential
Yz•+/Mn (OH−)
or
Yz−/Mn+ (H2O)
Concerted
Yz•/Mn (H2O)
Yz/Mn+ (OH−)
Reaction Path
Current Opinion in Plant Biology
Potential energy surfaces showing the relation between the activation
energies (Eas) for the sequential processes and for the concerted
H-atom transfer. The Eas for the sequential processes are considerably
higher than those for the concerted transfer (as detailed in the text).
However, for the concerted process the pre-exponential (A factor) is
significantly smaller than that for the sequential transfer. In PSII,
therefore, the concerted process describes the S-state advance more
accurately than either of the possible sequential routes.
colleagues [27••,32], Mayer and colleagues [33••,34], and
Sjödin et al. [35]. Concerted processes are expected to be
unlikely, a priori, as they require simultaneous tunneling of
the electron and proton. The proton contributes to the proton-coupled electron transfer rate through Franck-Condon
factors whose magnitudes can be quite small in view of the
large displacement of the proton upon transfer. Cukier
refers to this as ‘Franck-Condon drag’. Thus, coupled electron-transfer/proton-transfer processes are inherently low
probability events, and are associated with substantially
negative activation entropies, and hence smaller Arrhenius
pre-exponential factors (A), that are a hallmark of atom
transfers. These A values, when converted to s–1 units, as
appropriate to unimolecular reactions, are around 108.5 s–1
[29,36]. These values are significantly lower than the 1013
s–1 that is taken for van der Waal’s electron transfer [24••].
In essence, then, there is a competition between concerted
and sequential routes (Figure 3). A sequential process may
be slow because of a rate-limiting endergonic step, whereas the concerted process may be slow because of
Franck-Condon limitations. If the sequential process proceeds through an intermediate [in the specific case of PSII,
intermediates (b) or (c) in Figure 3] that is accessible thermally, then the sequential route will dominate [24••].
Conversely, if the sequential intermediates lie too high in
energy, as they appear to in PSII, then the concerted pathway dominates (Figure 4).
From model compound studies, general characteristics of
concerted electron and proton transfer reactions can be
Similarly, moderate kinetic isotope effects, typically less
than 2.5, and reaction rates in the range 103–106 s–1, despite
short distances, are typical for atom abstraction processes
[25•,27••,29–31,33••,34]. These results are well in line with
those that have been obtained for PSII (Table 2). Thus,
there is good agreement between the kinetic characteristics
of S-state advance and observations of H-atom abstraction
in model systems. Moreover, the above analysis indicates
that sequential processes proceed through intermediates
that are inaccessible energetically. Accordingly, it appears
that the reduction of Yz• involves the concerted movement
of a proton and an electron from the Mn–water (hydroxide)
complex on each S-state transition.
Conclusions
The original model for H• atom abstraction as the underlying mechanism for water oxidation in PSII stressed the
necessity of managing both proton and electron currents
closely [4,37]. The mutagenesis and kinetic results for Yz
oxidation by P680+ obtained in the past two years reinforce
this point: PSII in the absence of either H190 or the (Mn)4
cluster is not an efficient oxidant of Yz. The dramatic kinetic slow-downs of Yz oxidation rate when either H190 or the
(Mn)4 cluster is missing are related to the extent that the
local H-bond network around Yz is disrupted by their
absence. The analysis presented above shows that the same
principles apply for the reduction of Yz•. The available data
indicate that protons and electrons move together in a concerted process as the S-state advance occurs.
Acknowledgements
We would like to thank The Swedish Foundation for International
Cooperation in Research and Higher Education, the US Department of
Agriculture Competitive Grants Office, and NIH Grants GM-47274 and
GM-37300 for financial support. We would also like to thank Chris Moser
and Les Dutton for useful discussions along with Jim Mayer and Curt
Hoganson for discussion and for careful reading of the manuscript.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
••
Tommos C, Babcock GT: Oxygen production in nature: a light
driven metalloradical enzyme process. Accounts Chem Res 1998,
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The authors summarize our knowledge of H-atom abstraction in the wateroxidation process. This information is complemented with a discussion of the
pKas, bond dissociation energies and local thermodynamic properties of
manganese-bound water.
Concerted hydrogen-atom abstraction in photosynthetic water oxidation Westphal et al.
2.
Yocum CF, Pecoraro VL: Recent advances in the understanding of
the biological chemistry of manganese. Curr Opin Chem Biol
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Hoganson CW, Lydakis-Simantiris N, Tang X-S, Tommos C, Warncke K,
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manganese cluster of photosystem-II to the redox-active tyrosine
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•
Pecoraro VL, Baldwin MJ, Caudle MT, Hsieh W-Y, Law NA: A
proposal for water oxidation in photosystem II. Pure Appl Chem
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The role of H-atom abstraction, manganese-cluster redox potentials, and the
possible involvement of Ca2+ in presenting a hydroxide nucleophile close
enough to a putative Mnv=O species to form the critical 0=0 bond in the S4
state is presented.
8.
•
Limburg J, Vrettos JS, Liable-Sands LM, Rheingold AL, Crabtree RH,
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A binuclear manganese center is described that promotes formation of the
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9.
•
Siegbahn PEM, Crabtree RH: Manganese oxyl radical
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from hydroxide that is ligated to a six-coordinate manganese ion. A mechanism to overcome this problem, which involves oxyl radical formation and
Ca2+ involvement, is presented.
10. Haumann M, Junge W: Photosynthetic water oxidation: a simplex
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1411:86-91.
An interpretation of proton release and chromophore bandshifts that are
attributed to the formation of charge is presented that differs from the one
described in this review. A complete model for water oxidation is presented
that incorporates both charge-accumulating and charge-neutral transitions at
the Mn complex.
11. Hays A-MA, Vassiliev IR, Golbeck JH, Debus RJ: Role of D1-His190 in
•• the proton-coupled oxidation of tyrosine Yz in manganesedepleted photosystem II. Biochemistry 1999, 38:11851-11865.
Mutations at H190, with and without chemical rescue by exogenous bases,
are used to establish this central role of the histidine in promoting deprotonation of Yz as it is oxidized by P680+.
12. Schilstra MJ, Rappaport F, Nugent JHA, Barnett CJ, Klug DR:
•
Proton/hydrogen transfer affects the S-state-dependent
microsecond phases of P680+ reduction during water splitting.
Biochemistry 1998, 37:3974-3981.
The role of proton transfer in facilitating the slower phases of P680+ reduction that involve proton/hydrogen motion in water-splitting PSII preparations
is described.
13. Christen G, Seeliger A, Renger G: P680+* reduction kinetics and
•
redox transition probability of the water oxidizing complex as a
function of pH and H/D isotope exchange in spinach thylakoids.
Biochemistry 1999, 38:6082-6092.
The authors show that both proton transfer events and deuterium kinetic isotope effects are apparent in the slower phases of P680+ reduction in watersplitting PSII preparations. This behavior contrasts with that exhibited during
the ns time-scale phases of P680+ reduction in the same preparations and
leads to the development of the model described in [15••].
14. Tommos C, Hoganson CW, Di Valentin M, Lydakis-Simantiris N,
•
Dorlet P, Westphal K, Chu H-A, McCracken J, Babcock GT:
Manganese and tyrosyl radical function in photosynthetic oxygen
evolution. Curr Opin Chem Biol 1998, 2:244-252.
The H-atom transfer model is discussed and interpretations of electrochromism that do not involve charge acccumulations are considered.
15. Tommos C, Babcock GT: Proton and hydrogen currents in
•• photosynthetic water oxidation. Biochim Biophys Acta 2000, in press.
This review considers the oxidation of tyrosine in aqueous solution and of Yz by
P680+. It summarizes results that support the model in Figure 2 for the oxidation
241
process and shows that the reduction of Yz• during the S-state advances cannot be explained by simple electron transfer.
16. Lakshmi KV, Eaton SS, Eaton GR, Brudvig GW: Orientation of the
•
tetranuclear manganese cluster and tyrosine Z in the O2-evolving
complex of photosystem II: an EPR study of the S2Yz* state in
oriented acetate-inhibited photosystem II membranes.
Biochemistry 1999, 38:12758-12767.
The proximity of Yz and the (Mn)4 cluster is demonstrated by using electron
paramagnetic resonance (EPR) spectroscopy on oriented samples; earlier
work using EPR that reached the same conclusion is summarized.
17.
Roelofs TA, Liang WC, Latimer MJ, Cinco RM, Rompel A,
Andrews JC, Sauer K, Yachandra VK, Klein MP: Oxidation states of
the manganese cluster during the flash-induced S-state cycle of
the photosynthetic oxygen-evolving complex. Proc Natl Acad Sci
USA 1996, 93:3335-3340.
18. Iuzzolino L, Dittmer J, Dorner W, Meyer-Klaucke W, Dau H: X-ray
absorption spectroscopy on layered photosystem II membrane
particles suggests manganese-centered oxidation of the oxygenevolving complex for the S0–S1, S1–S2, and S2–S3 transitions of
the water oxidation cycle. Biochemistry 1998, 37:17112-17119.
19. Blomberg MRA, Seigbahn PEM, Styring S, Babcock GT, Åkermark B,
Korall P: A quantum chemical study of hydrogen abstraction from
manganese-coordinated water by a tyrosyl radical: a model for
water oxidation in photosystem II. J Am Chem Soc 1997,
119:8285-8292.
20. Law NA, Caudle MT, Pecoraro VL: Manganese redox enzymes and
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model systems: properties, structures, and reactivity. Adv Inorg
Chem 1999, 46:305-440.
This general review of manganese function in biology discusses photosynthetic water-oxidation in depth.
21. Cinco RM, Robblee JH, Rompel A, Fernandez C, Yachandra VK,
Sauer K, Klein MP: Strontium EXAFS reveals the proximity of
calcium to the manganese cluster of oxygen-evolving
photosystem II. J Phys Chem 1998, 102:8248-8256.
22. Diner BA, Force DA, Randall DW, Britt RD: Hydrogen bonding, solvent
exchange, and coupled proton and electron transfer in the oxidation
and reduction of redox-active tyrosine Yz in Mn-depleted core
complexes of photosystem II. Biochemistry 1998, 37:17931-17943.
23. Tommos C, McCracken J, Styring S, Babcock GT: Stepwise
disintegration of the photosynthetic oxygen-evolving complex.
J Am Chem Soc 1998, 120:10441-10452.
24. Page CC, Moser CC, Chen X, Dutton PL: Natural engineering
•• principles of electron tunneling in biological oxidation-reduction.
Nature 1999, 402:47-52.
This paper provides a seminal discussion of electron transfer in biological
systems. The role of endergonic steps in an overall exergonic electron transfer process is considered.
25. Cukier RI: A theory for the rate constant of a dissociative proton
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coupled electron-transfer reaction. J Phys Chem A 1999,
103:5989-5995.
Dissociative proton-coupled electron transfer, which is typified by the oxidation of Yz by P680+ in PSII, is analysed in considerable theoretical depth.
26. Rich PR, Meunier B, Mitchell R, Moody AJ: Coupling of charge and
proton movement in cytochrome c oxidase. Biochim Biophys Acta
1996, 1275:91-95.
27. Cukier RI, Nocera DG: Proton-coupled electron transfer. Annu Rev
•• Phys Chem 1998, 49:337-369.
The theory of coupled proton/electron transfer in both chemistry and biology is summarized and selected applications are presented.
28. Soudackov A, Hammes-Schiffer S: Multistate continuum theory for
multiple charge transfer reactions in solution. J Chem Phys 1999,
111:4672-4687.
29. Mahoney LR, DaRooge MA: The kinetic behavior and
thermochemical properties of phenoxy radicals. J Am Chem Soc
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30. Foti M, Ingold KU, Lusztyk J: The surprisingly high reactivity of
phenoxy radicals. J Am Chem Soc 1994, 116:9440-9447.
31. MacFaul PA, Ingold KU, Lusztyk J: Kinetic solvent effects on
hydrogen atom abstraction from phenol, aniline, and
diphenylamine. The importance of hydrogen bonding on their
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32. Kirby JP, van Dantzig NA, Chang CK, Nocera DG: Formation of
porphryn donor-acceptor complexes via an amidiniumcarboxylate salt bridge. Tetrahedron Lett 1995, 36:3477-3480.
36. Benson SW: Thermochemical Kinetics: Methods for The Estimation
of Thermochemical Data And Rate Parameters, edn 2. New York:
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33. Mayer JM: Hydrogen atom abstraction by metal-oxo complexes:
•• understanding the analogy with organic radical reactions.
Accounts Chem Res 1998, 31:441-450.
The basic thermodynamic and kinetic principles, such as low activation energies and low rates of H-atom abstraction by metal-oxo complexes, are presented. These are compared to analogous processes in which the H-atom
acceptor is an organic moiety.
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34. Roth JP, Lovell S, Mayer JM: Intrinsic barriers for electron and
hydrogen atom transfer reactions of biomimetic iron complexes.
J Am Chem Soc 2000, 122:in press.
35. Sjödin M, Styring S, Åkermark B, Sun L, Hammarström L: Protoncoupled electron transfer from tyrosine in a tyrosine-rutheniumtris-bipyridine complex; comparison with tyrosinez oxidation in
photosystem II. J Am Chem Soc 2000, 122:in press.
Tommos C, Tang X-S, Warncke K, Hoganson CW, Styring S,
McCracken J, Diner BA, Babcock GT: Spin-density distribution,
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Yz in photosystem-II from multiple electron magnetic resonance
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evolution. J Am Chem Soc 1995, 117:10325-10335.
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Biophys Acta 1997, 1322:141-150.
Concerted hydrogen-atom abstraction in photosynthetic
water oxidation
Kristi L Westphal*, Cecilia Tommos†, Robert I Cukier* and
Gerald T Babcock*
Photosystem II evolves oxygen by using water in the unlikely role
of a reductant. The absorption of sunlight by chlorophyll
produces highly oxidizing equivalents that are filled with
electrons stripped from water. This proton-coupled redox
chemistry occurs at the oxygen-evolving complex, which contains
a tetramanganese cluster, a redox-active tyrosine amino acid
hydrogen-bonded to a histidine amino acid, a calcium ion and
chloride. Hydrogen-atom abstraction by the tyrosyl radical from
water bound to the manganese cluster is now widely held to
occur in this process, at least for some of the steps in the
catalytic cycle. We discuss kinetic and energetic constraints on
the hydrogen-atom abstraction process.
Addresses
*Department of Chemistry, Michigan State University, East Lansing,
Michigan 48824, USA
† Department of Biochemistry, Arrhenius Laboratories for Natural
Sciences, Stockholm University, S106 91 Stockholm, Sweden
† Current address: Department of Biochemistry, Stockholm University,
S-106 91 Stockholm, Sweden
Current Opinion in Plant Biology 2000, 3:236–242
1369-5266/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Abbreviations
ε
dielectric constant
∆G
free energy change
OEC
oxygen-evolving complex
PSII
photosystem II
QA
initial plastoquinone electron acceptor in photosystem II
Sn
State at which n oxidizing equivalents are accumulated
(where n is 0—4)
Yz
redox-active tyrosine at position 161 of the D1 protein
Introduction
Photosynthetic organisms use photosystem II (PSII) to catalyze the light-driven oxidation of water according to the
following half-cell reaction:
2 H2O → O2 + 4 H+ + 4 e–
The protons and electrons released in this process are ultimately used in starch production; to the plant, O2 is simply
a waste product. This remarkable redox chemistry is initiated by photon absorption by the PSII reaction-center
chlorophyll, P680, which produces the charge-separated
state, P680+QA–. Reduction of P680+ by water is mediated
by the oxygen-evolving complex (OEC), which is largely,
if not exclusively, associated with D1, one of the principal
reaction-center proteins in PSII. The OEC consists of four
manganese ions, a redox-active tyrosine called Yz (tyrosine
161), histidine 190, and Ca2+ and Cl– ions. The cluster containing the four Mn ions [i.e. (Mn)4] stores oxidizing
equivalents upon photon absorption; this process is annotated by the Sn nomenclature, where n is the number of
oxidizing equivalents accumulated. Four successive lightinduced charge separations lead to the formation of S4 from
which O2 is released, thereby resetting the system to S0
(for recent reviews see [1••,2,3]).
It has been postulated that Yz• accepts both electrons and
protons from water during catalysis [4,5], and a mechanism
for the water-splitting chemistry of PSII based on H-atom
abstraction has been developed [1••,6]. Several functional
models have been proposed recently within this theme.
These models differ primarily in the extent to which Yz
acts as a H-atom abstractor during the catalytic cycle and in
the O=O bond-forming step [7•–9•,10••]. In this review, we
discuss the mechanistic details of coupled electron–proton
transfer. First, we describe the original H-atom abstraction
model, then we discuss Yz oxidation briefly, and finally we
analyze Yz• reduction.
The hydrogen-atom abstraction model
Essential features of this mechanism are summarized in
Figure 1. Upon photon absorption and generation of
P680+, Yz reduces P680+ and simultaneously deprotonates
through the H190–H-bonded network, which is postulated to extend to bulk, to produce Yz• (reaction 1 in
Figure 1). In the second step, Yz• abstracts a hydrogen
atom from the substrate water/hydroxide that is ligated to
the (Mn)4 cluster to advance the S-state (Figure 1, reaction 2). Four consecutive photon absorption/H-atom
abstraction events strip four hydrogen atoms from two
substrate water molecules; the two oxygen atoms collapse
to form O2, which is released as the system resets to the
S0 state by loading the next two water molecules. Within
this mechanism, each cofactor in the OEC plays specific
roles that are described below.
Yz carries out at least three functions that are critical to the
oxidation of water. First, Yz reduces P680+ on the ns time
scale, easily outcompeting the P680+QA– chargerecombination reaction, which prevents energy loss as heat.
Because of this rapid reduction of P680+, PSII operates with
a quantum efficiency that is close to unity. Second, the
oxidation of water requires strongly oxidizing conditions.
These are initially created at P680+ and preserved by the
high reduction potential of the Yz•/Yz redox couple. Third,
Yz interfaces the electron-tunneling reactions that are
carried out by the P680/QA pair at the reaction center to the
proton-coupled redox chemistry in the OEC. Thus, the
proton currents associated with water oxidation are
switched on at Yz•.
Concerted hydrogen-atom abstraction in photosynthetic water oxidation Westphal et al.
237
Figure 1
Reaction 1
P680+
e−
O
Mn
O
O
O
H
Mn
O
H
H
N
H190
N
H-bonded chain
to bulk
O
H
O
O
H
H
N
H190
Mn
O
S0
H
S0YzP680+
Mn
Yz•
Mn
O
H
H
O
O
H+
O
Mn
O
S0
Yz
Mn
H
H
Mn
P680
N
H
S0Yz
H-bonded chain
to bulk
•P
680
Reaction 2
P680
P680
O
Mn
O
H
S0
O
H
O
O
Mn
H
Mn
O
H
H
H
N
Mn
H190
N
S1
Yz
Mn
O
O
H
S0Yz•P680
O
H•
O
H
Mn
Yz•
Mn
O
O
O
H
O
Mn
H
H
N
H190
N
H
H-bonded chain
to bulk
S1YzP680
H-bonded chain
to bulk
Current Opinion in Plant Biology
Essential features of the H-atom abstraction model for photosynthetic
water oxidation. The postulated relative arrangement of the catalytic
center of PSII is shown. Substrate water binds terminally to the Mn
ions within hydrogen bonding (……) distance of the Yz phenol group.
The arrows show the flow of electrons (open arrow) and protons
(solid arrow) on the proton-coupled oxidation of Yz (Reaction 1) and
on the subsequent H-atom transfer (broken arrow) to Yz• from water
bound to the (Mn)4 cluster (Reaction 2). The figure shows the S0 to
S1 transition. Similar reactions are postulated to occur in the higher
S-states [1••].
Site-directed mutagenesis ([11••] and references therein)
and kinetic data [12•,13•] have shown that Yz is hydrogen
bonded to H190. If this histidine is mutated, Yz oxidation
slows and takes ms rather than ns. Furthermore, these
results suggest that when hydrogen-bond formation is curtailed, Yz cannot deprotonate efficiently and is not able to
reduce P680+. Thus, the oxidation of Yz is a proton-coupled
event, and H190 serves as the initial proton acceptor. The
electron made available in this process reduces P680+, and
the proton migrates through the H190/H-bonded network
to bulk ([14•,15••], but see [10••]). Together, these
processes form the neutral radical, Yz•, which is now able to
mediate the H-atom abstractions.
the function of the (Mn)4 cluster is at least three-fold.
First, it serves to delocalize the oxidizing equivalents generated as the S-state advances. The general consensus of
opinion is that the (Mn)4 cluster is oxidized on each S-state
transition, with the possible exception of the S2→S3 transition [17,18]. Second, it reduces the bond-dissociation
energy of bound substrate [1••,19,20•]. Third, the (Mn4)
cluster acts as a template for the formation of the O=O
bond [6]. The C-shaped structure promoted by Klein,
Sauer, Yachandra and their co-workers [21] is ideal in this
regard, as it holds the two oxygen atoms in a geometry that
allows facile O=O bond formation. The structure of the
(Mn)4 cluster is controversial but a templating function of
the metal cluster is likely.
Yz• and the (Mn)4 cluster, which have a magnetic center-tocenter separation of about 8 Å ([16•] and references
within), form a single catalytic site. Within this complex,
Ca2+ and Cl– are closely associated with the catalytic site
[21]. We have proposed that Ca2+ serves as a Cl– docking
238
Physiology and metabolism
Figure 2
(a) Holo-PSII
(ns kinetics)
(b) Apo-PSII
(µs kinetics)
CH2
CH2
e−
slow
fast
+
Disordered
H
H
N
N
H190
P680+
H w
sl o
O
+
O
H t
s
fa
Wellordered
e−
P680+
Rate-limiting factors for Yz oxidation. (a) The
OEC is intact, proton transfer to H190 occurs
on the ns time scale or faster, and the
reduction of P680+ is limited by the rate of
electron transfer. (b) PSII is depleted of Mn or
Ca and the oxidation of Yz takes µs rather than
ns. The movement of the phenol proton now
becomes rate-limiting for the overall protoncoupled reduction of P680+.
N
CH2
H
H190
N
CH2
H
Current Opinion in Plant Biology
site, and that Cl– neutralizes charge upon the S2→S3 transition as the result of Jahn-Teller effects [1••]. The role of
these two essential ions is elusive, however, and several
other functions have been suggested (e.g. [7•–9•]).
There is considerable experimental support for H-atom
abstraction in the catalytic cycle of PSII. In the sections
that follow, we provide further insight into the mechanisms
of H-atom transfer from and to Yz during water oxidation.
The kinetics of Yz oxidation
The oxidation of Yz has S-state dependent, multiphasic
electron-transfer kinetics that occur in the ns to µs
timescale. Moreover, its oxidation rate is highly sensitive
to the integrity of the OEC (Figure 2). The different
kinetics for Yz oxidation in holo- versus apo-PSII have
recently been considered in detail experimentally
[11••,12•,13•,22,23] and in a review article [15••]. In the
intact system (i.e. holo-PSII), it appears that proton transfer is facilitated by a well-ordered H-bonding structure
(Figure 2a). Under these conditions, proton transfer is not
rate limiting and the kinetics of Yz oxidation can be accurately predicted [15••] using an expression developed by
Dutton, Moser, and coworkers for estimating the rates of
intraprotein, non-adiabatic electron tunneling [24••].
When PSII is depleted of Mn or Ca (i.e. apo-PSII), hydrogen bonding at the Yz site becomes disordered (Figure 2b).
Under these conditions, the deprotonation of Yz during
electron transfer is no longer facile and the rate of the proton transfer event limits that of Yz oxidation. The overall
reaction slows to the µs timescale and becomes pH dependent with a substantial kinetic isotope effect. These
advances in our understanding of the experimental details
of Yz oxidation have been accompanied by a satisfying theoretical description of the process as an example of
dissociative proton-coupled electron transfer [25•].
The kinetics of Yz reduction
The reduction of Yz• is S-state dependent and occurs
over the µs to ms time scale (Table 1). Assuming reasonable values for driving force, reorganization energy,
protein packing and distance, we find that the expression developed by Page et al. [24 ••] predicts
electron-tunneling rates that are far more rapid than the
observed rates of Yz• reduction [15••]. Yz deprotonates to
H190 upon oxidation and reprotonates upon reduction.
It follows that an accurate analysis of Yz• reduction
should take into account the movement of a proton as
well as an electron. The Yz•/Yz and Sn+1/Sn reduction
potentials, and the activation energies and pre-exponential factors for the S-state transitions, are required for this
analysis and are given in Table 2.
Three mechanisms for the transfer of an electron and a
proton are illustrated in Figure 3, where the Sn→Sn+1
advance is represented by Mn(H2O)→Mn+(OH-). Two
are sequential and involve either electron transfer followed by proton transfer [path (i)], or proton transfer
followed by electron transfer [path (ii)]. The third mechanism invokes concerted electron/proton transfer
[path (iii)].
Table 1
Half-times for S-state advance..
S-state
Thylakoids
PSII
membranes
Core
particles
Yz•S0→YzS1
Yz•S1→YzS2
Yz•S2→YzS3
Yz•S3→YzS0
40–60 µs
65–86 µs
140–245 µs
750 µs;1.3 ms
30–70 µs
30–70 µs
55–110 µs
1.2–1.4 ms
60 µs
75–95 µs
225–380 µs
4.1–4.6 ms
Compiled from [14•,38,39].
Concerted hydrogen-atom abstraction in photosynthetic water oxidation Westphal et al.
For path (i), the initial transfer of an electron results in the
reduction of Yz• to a tyrosinate [intermediate (b)], which
subsequently protonates in the second step of the process.
The Yz•/Yz potential (shown in Table 2), corresponds to the
overall H+/e– transfer, but to judge the viability of the
sequential process we must consider the energetics of
forming the transient intermediate (b). The potential of
Y•/Y– is 0.68 V in water [15••] and is expected to decrease
in the low protein dielectric. This effect can be estimated
by using the Born model [26]:
∆G = 14.397/εr
Figure 3
(b) Yz−/Mn+ (H2O)
e−
(i)
(1)
where ∆G (in units of volts) is the electrostatic penalty for
introducing a charged species with radius (r; in Å) in a
homogenous medium of dielectric constant ε. Assuming a
radius of 3 Å and an ε of 10, we calculate a Yz•/Yz– potential
of 0.26 V. Thus, in a protein milieu, Yz• is a relatively weak
oxidant if Yz– is the product. For the Sn+1/Sn potential, the
opposite trend occurs, that is, to oxidize [Mn–H2O] or
[Mn–OH] to charged products requires a stronger oxidant
than when the process is proton coupled. With ε and r as
used above for Yz, Equation 1 predicts that the S1/S0
potential increases from 0.7 V to 1.12 V, if a positive charge
is formed. For the higher S-state transitions, correspondingly higher reduction potentials are predicted. As a result,
the driving force for forming (b) during S0→S1 is very
endergonic, (+0.26)–(+1.12) = –0.86 V. The electrostatic
interaction between [Mn-OH2]+ and Yz– in (b) lowers this
value by –0.18 V. Thus, the resulting difference in reduction potential is –0.68 V, which equates to a free energy
difference (∆G) of 15.7 kcal/mol. For the higher S-states,
the endergonicity will be even greater. These free energy
values obtained for formation of (b) lead to activation energies that are substantially higher than the activation
energies for the S-state transitions presented in Table 2,
and suggest that path (i) in Figure 3 is improbable.
The second possibility is also sequential [path (ii) in Figure 3]. The pKas of the proton donor and acceptor can be
used to determine the energetics of the initial proton transfer. For the individual S-states, the pKa is expected to be
greater than or equal to seven [1••,20•], whereas the pKa of
Y•+ in water is minus two [15••]. Therefore, the ∆pKa for
the proton transfer is at least nine, resulting in a ∆G of at
239
(a) Yz•/Mn (H2O)
H+ (i)
(iii)
e− and H+
H+ (ii)
(d) Yz/Mn+ (OH−)
e−
(ii)
(c) Yz•+/Mn (OH–)
Current Opinion in Plant Biology
Possible reaction paths for electron and proton transfer. Four species
are shown as follows: (a) The reactant state Yz•/Mn(H2O), (d) the
product state Yz/Mn+(OH-), and two intermediate states, (b) Yz/Mn+(H2O) and (c) Yz•+/Mn(OH-). Along the edges of the square, two
sequential pathways are shown. In path (i), electron transfer is
followed by proton transfer (et/pt), while in path (ii), the reverse
sequence of proton and electron steps occurs (pt/et). In path (iii) the
electron and proton move in a concerted process.
least 12 kcal/mol. The ∆G is as high as 23 kcal/mol when
Equation 1 is used to estimate the pKa in a protein environment. Consequently, the ∆G calculated for the transfer
of a proton prior to electron transfer gives activation energies that are considerably higher than those that have been
observed for any of the S-state transitions (Table 2). We
conclude that reduction of Yz• is unlikely to take place by
this mechanism.
As the sequential transfer of an electron and a proton
involves the formation of high energy intermediates, concerted proton/electron transfer in PSII becomes a viable
mechanism. These processes have been studied theoretically by Cukier [25•,27••], and by Hammes-Schiffer [28],
and experimentally by researchers including Mahoney and
DaRooge [29], Ingold and colleagues [30,31], Nocera and
Table 2
Kinetic and thermodynamic parameters for the S-state transitions..
Reaction
Yz•S0→YzS1
Yz•S1→YzS2
Yz•S2→YzS3
Yz•S3→YzS0
Ea (kcal/mol)
A (s–1)
KIE
Yz•/Yz (V)
Sn+1/Sn
1.2
3.0
9.0
5.0†
9.2‡
4.0 × 106
5.4 × 109
8.9 × 105†
2.9 × 1014‡
1.3; 2.9
1.3
1.4–1.6
0.97
0.97
0.97
0.97
0.70
0.93
0.93
0.93
A, arrhenius pre-exponential factor; Ea, activation energy; KIE, kinetic isotope effect; Sn+1/Sn, reduction potential of Sn+1/Sn;Yz•/Yz, reduction
potential of Yz•/Yz. †T > 280 K. ‡T < 280 K. Compiled from [14•,15••,38].
240
Physiology and metabolism
Energy
Figure 4
extracted. The activation energies of these processes are
generally very low. For example, activation energies in the
1.7–4.9 kcal/mol range were reported in studies on the
reactivity of phenoxy radicals in H-atom abstraction reactions [29,30] and values of 5 kcal/mol were measured for
H-atom transfer between two ferric complexes [34]. The
activation energies measured for reduction of Yz• are
1–9 kcal/mol (Table 2) and fit well within the range of values for H-atom abstraction reactions.
Sequential
Yz•+/Mn (OH−)
or
Yz−/Mn+ (H2O)
Concerted
Yz•/Mn (H2O)
Yz/Mn+ (OH−)
Reaction Path
Current Opinion in Plant Biology
Potential energy surfaces showing the relation between the activation
energies (Eas) for the sequential processes and for the concerted
H-atom transfer. The Eas for the sequential processes are considerably
higher than those for the concerted transfer (as detailed in the text).
However, for the concerted process the pre-exponential (A factor) is
significantly smaller than that for the sequential transfer. In PSII,
therefore, the concerted process describes the S-state advance more
accurately than either of the possible sequential routes.
colleagues [27••,32], Mayer and colleagues [33••,34], and
Sjödin et al. [35]. Concerted processes are expected to be
unlikely, a priori, as they require simultaneous tunneling of
the electron and proton. The proton contributes to the proton-coupled electron transfer rate through Franck-Condon
factors whose magnitudes can be quite small in view of the
large displacement of the proton upon transfer. Cukier
refers to this as ‘Franck-Condon drag’. Thus, coupled electron-transfer/proton-transfer processes are inherently low
probability events, and are associated with substantially
negative activation entropies, and hence smaller Arrhenius
pre-exponential factors (A), that are a hallmark of atom
transfers. These A values, when converted to s–1 units, as
appropriate to unimolecular reactions, are around 108.5 s–1
[29,36]. These values are significantly lower than the 1013
s–1 that is taken for van der Waal’s electron transfer [24••].
In essence, then, there is a competition between concerted
and sequential routes (Figure 3). A sequential process may
be slow because of a rate-limiting endergonic step, whereas the concerted process may be slow because of
Franck-Condon limitations. If the sequential process proceeds through an intermediate [in the specific case of PSII,
intermediates (b) or (c) in Figure 3] that is accessible thermally, then the sequential route will dominate [24••].
Conversely, if the sequential intermediates lie too high in
energy, as they appear to in PSII, then the concerted pathway dominates (Figure 4).
From model compound studies, general characteristics of
concerted electron and proton transfer reactions can be
Similarly, moderate kinetic isotope effects, typically less
than 2.5, and reaction rates in the range 103–106 s–1, despite
short distances, are typical for atom abstraction processes
[25•,27••,29–31,33••,34]. These results are well in line with
those that have been obtained for PSII (Table 2). Thus,
there is good agreement between the kinetic characteristics
of S-state advance and observations of H-atom abstraction
in model systems. Moreover, the above analysis indicates
that sequential processes proceed through intermediates
that are inaccessible energetically. Accordingly, it appears
that the reduction of Yz• involves the concerted movement
of a proton and an electron from the Mn–water (hydroxide)
complex on each S-state transition.
Conclusions
The original model for H• atom abstraction as the underlying mechanism for water oxidation in PSII stressed the
necessity of managing both proton and electron currents
closely [4,37]. The mutagenesis and kinetic results for Yz
oxidation by P680+ obtained in the past two years reinforce
this point: PSII in the absence of either H190 or the (Mn)4
cluster is not an efficient oxidant of Yz. The dramatic kinetic slow-downs of Yz oxidation rate when either H190 or the
(Mn)4 cluster is missing are related to the extent that the
local H-bond network around Yz is disrupted by their
absence. The analysis presented above shows that the same
principles apply for the reduction of Yz•. The available data
indicate that protons and electrons move together in a concerted process as the S-state advance occurs.
Acknowledgements
We would like to thank The Swedish Foundation for International
Cooperation in Research and Higher Education, the US Department of
Agriculture Competitive Grants Office, and NIH Grants GM-47274 and
GM-37300 for financial support. We would also like to thank Chris Moser
and Les Dutton for useful discussions along with Jim Mayer and Curt
Hoganson for discussion and for careful reading of the manuscript.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
••
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•
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•
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•
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241
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