chapter  5
Amino Acids
Pages 42

Proteins have many functions in the organism and

constitute key compounds for survival of animals and

humans. Proteins are naturally constituted by 20

amino acids, which act as basic components of the

polymeric structure. Once proteins are ingested, amino

acids are released by enzymatic digestion and absorbed

into the body. So, protein quality strongly depends on

its amino acid content and digestibility (1). Amino

acids participate in many biochemical pathways for

growth, maintenance, and metabolic activity of cells

and organs and their requirements vary, depending on

the stage of life (2). However, the quality of proteins

may be affected by processing and storage (3,4). Thus,

knowledge of the amino acid profile and the limiting

amino acids in a protein is very important not only to

check the quality of that protein, but also to improve

the nutritional quality by supplementation with the

required amino acids. Essential amino acids (listed in Table 1) cannot be

synthesized in adult humans andmust be supplied in the

diet. Histidine is also essential for infants. The rest of

natural amino acids are considered as nonessential (see

Table 2) because they are efficiently synthesized in the

body. There is a third group of amino acids, usually not

present in proteins but with significant roles in foods

(see Table 3). The molecular mass and structure of all

these amino acids are compiled in Tables 1, 2, and 3. All

amino acids, except proline, contain one primary amino

group (-NH2) in the position in relation to the carboxy

group (-COOH). Each amino acid has a characteristic

side chain (R group), which has a strong influence on its

physicochemical properties. So, amino acids are usually

grouped according to the polarity of this group

(nonpolar or hydrophobic side chains, polar or hydro-

philic side chains, positively or negatively charged

groups). The stereochemical L isomer is the most usual

form in nature and, in practice, all amino acids found in

hydrolyzates from animal and vegetable proteins are

isomers of the L form. The D isomers may be found in

some specific cases like in the cell walls of certain

microorganisms and polypeptides with antibiotic