ABSTRACT
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
action.