II. LITERATURE REVIEW
2.1 Protein and Amino Acid
Protein is a very important constituent of diet, both qualitatively and quantitatively, as it is the building material for the growing animal organism and
is also important for the production of enzymes, among other things. Amino acids are the building blocks which formed proteins. There are 20 amino acids used as
building blocks for the proteins in all living organisms. Amino acids composition in a feedstuff protein is of overriding importance for definition of its quality, and
determines its value as a feed constituent Steffens 1989. Amino acids may be classified as either essential or non-essential.
Ten amino acids are essential for fish, meaning
that they cannot be synthesized in sufficient quantity for body needs; thus, they must be supplied in the diet. A non-
essential amino acid is one that can be synthesized by the animal in sufficient quantity for maximal growth.
Both essential and non-essential amino acids are required by body cells whenever proteins are being synthesized.
2.1.1 Fate of Protein
Utilization of dietary proteins in metabolism requires that the protein molecules were degraded to their constitutive amino acids. The reaction is
catalyzed by a group of enzymes referred collectively as the proteolytic enzymes. The action of these enzymes is rather selective. The different enzymes
preferentially split peptide bonds adjacent to particular amino acids. This selectivity depends upon the nature of the side chains of the amino acids.
Protein digestion takes place in the stomach and upper small intestine. When protein is digested, amino acids are released and absorbed into the body
either as individual amino acids or as shorter chains of amino acids, di- and tri- peptides. It is rapid, and the products the amino acid are rapidly absorbed from
the digestive tract. Dietary protein not only stimulates the secretion of proteolytic enzymes but it protects the enzymes against proteolysis, apparently because
dietary protein denatured in the stomach is the preferred substrate for proteolytic enzymes.
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Dietary protein
Undigested Digested and absorbed
Feces Digestive enzymes
CO
2
H
2
O NH
3
Deaminated and used for energy
Used to meet protein needs
Urine
Dietary protein enters the portal bloodstream as free amino acid. These amino acids mix with those of endogenous origin to produce the amino acid
patterns that are found in physiological fluids after ingestion of a protein- containing meal. Free amino acids within the body may follow three different
metabolic pathways: 1 they can be used for protein synthesis; 2 they can serve as precursors in the synthesis of nitrogen-containing compounds such as nucleic
acids, creatine, choline, or thyroxine; or 3 they can be degraded, the nitrogen being excreted mainly as ammonia and carbon skeleton entering energy
metabolism.
Figure 1. The fate of dietary protein. Source: Lloyd et al. 1978
Although proteins may serve as a source of energy, the primary reason for including protein in diet is to provide amino acids for the synthesis of essential
tissues and fluid components, and to meet other specific needs for nitrogen.
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2.1.2 Amino Acid Requirement
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