Human Physiology, 14th edition (2016)

122 Chapter 5

Amino Acid Metabolism

Nitrogen is ingested primarily as proteins, enters the body as
amino acids, and is excreted mainly as urea in the urine. In
childhood, the amount of nitrogen excreted may be less than
the amount ingested because amino acids are incorporated into
proteins during growth. Growing children are thus said to be in
a state of positive nitrogen balance. People who are starving or
suffering from prolonged wasting diseases, by contrast, are in a
state of negative nitrogen balance; they excrete more nitrogen
than they ingest because they are breaking down their tissue
proteins.
Healthy adults maintain a state of nitrogen balance in which
the amount of nitrogen excreted is equal to the amount ingested.
This does not imply that the amino acids ingested are unneces-
sary; on the contrary, they are needed to replace the protein that
is “turned over” each day. When more amino acids are ingested
than are needed to replace proteins, the excess amino acids
are not stored as additional protein (one cannot build muscles
simply by eating large amounts of protein). Rather, the amine
groups can be removed, and the “carbon skeletons” of the
organic acids that are left can be used for energy or converted to
carbohydrate and fat.

Transamination

An adequate amount of all 20 amino acids is required to build
proteins for growth and to replace the proteins that are turned
over. However, only 8 of these (9 in children) cannot be pro-
duced by the body and must be obtained in the diet. These are
the essential amino acids ( table 5.3 ). The remaining amino
acids are “nonessential” only in the sense that the body can
produce them if provided with a sufficient amount of carbohy-
drates and the essential amino acids.
Pyruvic acid and the citric acid cycle acids are collectively
termed keto acids because they have a ketone group; these
should not be confused with the ketone bodies (derived from
acetyl CoA) discussed in the previous section. Keto acids can
be converted to amino acids by the addition of an amine (NH 2 )

Figure 5.15 Two important transamination

reactions. The areas shaded in blue indicate the parts of the

molecules that are changed. (AST 5 aspartate transaminase;

ALT 5 alanine transaminase. The amino acids are identified in

boldface.)

OH O

C

Glutamic acid Oxaloacetic acid α-Ketoglutaric acid Aspartic acid

O

O

HH

HO

+

HO

C

C

C

C

O

O

HH

H

OH

H

HO O

C

C

C

C

C

AST

O

+

OH O

H

N

H

HO

H

C

H

H

O

C

C

C

H

OH O

C

OC

OH

C

H CH

H

H

H

O

Glutamic acid Pyruvic acid α-Ketoglutaric acid Alanine

+ ALT +

N

H

H

H

H

H

O

H

H

H

C

C

C N

CH

C

C

H

HO

OH O

C

H

H

H

O

N

H

H

C

CH

C

C

H

HO

O

O

HH

H

OH

H

HO O

C

C

C

C

C

CLINICAL APPLICATION

Ketone bodies are small, water-soluble molecules produced

in the liver and derived from fatty acids during the break-

down of stored triglycerides (fat). Ketone body production is

thereby increased during fasting and stringent diets, result-

ing in ketosis (increased ketone bodies in the blood). This

may be detected by the fruity odor of acetone in the breath,

and by ketone bodies in the urine ( ketonuria ). The brain dur-

ing fasting can use ketone bodies for some of its energy.

Type 1 diabetes mellitus may cause an excessive amount

of ketone body production, resulting in ketoacidosis. For

osmotic reasons, ketonuria is accompanied by excessive

urinary water loss and dehydration.

Essential Amino Acids Nonessential Amino Acids

Lysine Aspartic acid

Tryptophan Glutamic acid

Phenylalanine Proline

Threonine Glycine

Valine Serine

Methionine Alanine

Leucine Cysteine

Isoleucine Arginine

Histidine (in children) Asparagine

Glutamine

Tyrosine

Table 5.3 | The Essential and Nonessential

Amino Acids

group. This amine group is usually obtained by “cannibal-

izing” another amino acid; in this process, a new amino acid

is formed as the one that was cannibalized is converted to a

new keto acid. This type of reaction, in which the amine group

is transferred from one amino acid to form another, is called

transamination ( fig. 5.15 ).