atom (H), and a fourth group, referred to as the
R-group, that determines the structural identity and
chemical properties of the amino acid. The first three
groups are common to all amino acids. The basic
amino acid structure is R-CH(NH 2 )-COOH.
There are twenty different forms of amino acids
that the human body utilizes. These forms are distin-
guished by the fourth variable substituent, the R-
group, which can be a chain of different lengths or a
carbon-ring structure. For example, if hydrogen rep-
resents the R-group, the amino acid is known asgly-
cine, a polar but uncharged amino acid, while methyl
(CH 3 ) group is known asalanine,a nonpolar amino
acid. Thus, the chemical components of the R-group
essentially determine the identity, structure, and func-
tion of the amino acid.
The structural and chemical relatedness of the
R-groups allows classification of the twenty amino
acids into chemical groups. Amino acids can be clas-
sified according to optical activity (the ability to polar-
ize light), acidity and basicity, polarity and nonpolarity,
or hydrophilicity (water-loving) and hydrophobicity
(water-fearing). These categories offer clues to the func-
tion and reactivity of the amino acids in proteins. The
biochemical properties of amino acids determine the
role and function of protein in the human body.
Of the twenty amino acids, eleven are considered
nonessential(ordispensable), meaning that the body is
able to adequately synthesize them, and nine areessen-
tial(orindispensable), meaning that the body is unable
to adequately synthesize them to meet the needs of the
cell. They must therefore be supplied through the diet.
Foods that have protein contain both nonessential
and essential amino acids, the latter of which the
body can use to synthesize some of the nonessential
amino acids. A healthful diet, therefore, should consist
of a sufficient and balanced supply of both essential
and nonessential amino acids in order to ensure high
levels of protein production.Protein Quality: Nutritive Value
The quality of protein depends on the level at
which it provides the nutritional amounts of essential
amino acids needed for overall body health, mainte-
nance, and growth. Animal proteins, such as eggs,
cheese, milk, meat, and fish, are consideredhigh-
quality,orcomplete, proteinsbecause they provide
sufficient amounts of the essential amino acids. Plant
proteins, such as grain, corn, nuts, vegetables and fruits,
arelower-quality,orincomplete, proteinsbecause many
plant proteins lack one or more of the essential amino
acids, or because they lack a proper balance of amino
acids. Incomplete proteins can, however, be combined
to provide all the essential amino acids, though com-
binations of incomplete proteins must be consumed at
the same time, or within a short period of time (within
four hours), to obtain the maximum nutritive value
from the amino acids. Such combination diets gener-
ally yield a high-quality protein meal, providing suffi-
cient amounts and proper balance of the essential
amino acids needed by the body to function.Protein Processing: Digestion, Absorption,
and Metabolism
Protein digestion begins when the food reaches
the stomach and stimulates the release of hydrochloric
acid (HCl) by the parietal cells located in the gastric
mucosa of the GI (gastrointestinal) tract. Hydro-
chloric acid provides for a very acidic environment,
which helps the protein digestion process in two ways:
(1) through an acid-catalyzedhydrolysisreaction of
breaking peptide bonds (the chemical process of
breaking peptide bonds is referred to as a hydrolysis
reaction becausewateris used to break the bonds);
and (2) through conversion of the gastric enzyme pep-
sinogen (an inactive precursor) to pepsin (the active
form). Pepsinogen is stored and secreted by the ‘‘chiefKEY TERMS
adipose tissue—Tissue containing fat deposits.
amino acid—Building block of proteins, necessary
dietary nutrient.
anabolic—Promoting building up.
catabolism—Breakdown of complex molecules.
epithelial cell—Sheet of cells lining organs
throughout the body.
glycolysis—Cellular reaction that begins the break-
down of sugars.
hydrolyze—To break apart through reaction with
water.
kwashiorkor—Severe malnutrition characterized
by swollen belly, hair loss, and loss of skin pigment.
nonpolar—Without a separation if charge within
the molecule; likely to be hydrophobic.
polar—Containing regions of positive and negative
charge; likely to be soluble in water.Protein