Ganong's Review of Medical Physiology, 23rd Edition

16 SECTION ICellular & Molecular Basis of Medical Physiology

THE AMINO ACID POOL

Although small amounts of proteins are absorbed from the
gastrointestinal tract and some peptides are also absorbed,
most ingested proteins are digested and their constituent ami-
no acids absorbed. The body’s own proteins are being contin-
uously hydrolyzed to amino acids and resynthesized. The
turnover rate of endogenous proteins averages 80–100 g/d, be-
ing highest in the intestinal mucosa and practically nil in the
extracellular structural protein, collagen. The amino acids
formed by endogenous protein breakdown are identical to
those derived from ingested protein. Together they form a
common amino acid pool that supplies the needs of the body
(Figure 1–16).

PROTEINS

Proteins are made up of large numbers of amino acids linked
into chains by peptide bonds joining the amino group of one
amino acid to the carboxyl group of the next (Figure 1–17). In
addition, some proteins contain carbohydrates (glycopro-
teins) and lipids (lipoproteins). Smaller chains of amino acids
are called peptides or polypeptides. The boundaries between
peptides, polypeptides, and proteins are not well defined. For

this text, amino acid chains containing 2–10 amino acid resi-

dues are called peptides, chains containing more than 10 but

fewer than 100 amino acid residues are called polypeptides,

and chains containing 100 or more amino acid residues are

called proteins.

TABLE 1–3 Amino acids found in proteins.*

Amino acids with aliphatic side chains Amino acids with acidic side chains, or their amides

Alanine (Ala, A) Aspartic acid (Asp, D)

Valine (Val, V) Asparagine (Asn, N)

Leucine (Leu, L) Glutamine (Gln, Q)

Isoleucine (IIe, I) Glutamic acid (Glu, E)

Hydroxyl-substituted amino acids γ-Carboxyglutamic acidb (Gla)

Serine (Ser, S) Amino acids with side chains containing basic groups

Threonine (Thr, T) Argininec (Arg, R)

Sulfur-containing amino acids Lysine (Lys, K)

Cysteine (Cys, C) Hydroxylysineb (Hyl)

Methionine (Met, M) Histidinec (His, H)

Selenocysteinea Imino acids (contain imino group but no amino group)

Amino acids with aromatic ring side chains Proline (Pro, P)

Phenylalanine (Phe, F) 4-Hydroxyprolineb (Hyp)

Tyrosine (Tyr, Y) 3-Hydroxyprolineb

Tryptophan (Trp, W)

*Those in bold type are the nutritionally essential amino acids. The generally accepted three-letter and one-letter abbreviations for the amino acids are shown in parentheses.
aSelenocysteine is a rare amino acid in which the sulfur of cysteine is replaced by selenium. The codon UGA is usually a stop codon, but in certain situations it codes for selenocysteine.
bThere are no tRNAs for these four amino acids; they are formed by post-translational modification of the corresponding unmodified amino acid in peptide linkage. There are
tRNAs for selenocysteine and the remaining 20 amino acids, and they are incorporated into peptides and proteins under direct genetic control.
cArginine and histidine are sometimes called “conditionally essential”—they are not necessary for maintenance of nitrogen balance, but are needed for normal growth.

FIGURE 1–16 Amino acids in the body. There is an extensive

network of amino acid turnover in the body. Boxes represent large

pools of amino acids and some of the common interchanges are rep-

resented by arrows. Note that most amino acids come from the diet

and end up in protein, however, a large portion of amino acids are in-

terconverted and can feed into and out of a common metabolic pool

through amination reactions.

Inert protein

(hair, etc)

Amino acid

pool

Body

Diet protein

Urea

NH 4 +

Common

metabolic

pool

Transamination

Amination

Deamination

Purines,

pyrimidines

Hormones,

neurotransmitters

Creatine

Urinary

excretion