90
Chapter 3
urea.
Thus, urea, which is relatively nontoxic, is the major
nitrogenous waste product of protein catabolism. It enters the
blood from the liver and is excreted by the kidneys into the
urine.
Thus far, we have discussed mainly amino acid catabo-
lism; now we turn to amino acid synthesis. The keto acids
pyruvic acid and
α
-ketoglutaric acid can be derived from the
breakdown of glucose; they can then be transaminated, as
described previously, to form the amino acids glutamate and
alanine. Thus glucose can be used to produce certain amino
acids, provided other amino acids are available in the diet to
supply amino groups for transamination. However, only 11 of
the 20 amino acids can be formed by this process because 9 of
the specifi c keto acids cannot be synthesized from other inter-
mediates. We have to obtain the 9 amino acids corresponding
to these keto acids from the food we eat, and they are thus
known as
essential amino acids.
Figure 3–52
provides a summary of the multiple routes
by which the body handles amino acids. The amino acid
pools, which consist of the body’s total free amino acids, are
derived from (1) ingested protein, which is degraded to amino
acids during digestion in the intestinal tract, (2) the synthesis
of nonessential amino acids from the keto acids derived from
carbohydrates and fat, and (3) the continuous breakdown
of body proteins. These pools are the source of amino acids
for the resynthesis of body protein and a host of specialized
amino acid derivatives, as well as for conversion to carbohy-
drate and fat. The body loses a very small quantity of amino
acids and protein via the urine, skin, hair, fi ngernails, and, in
women, the menstrual fl uid. The major route for the loss of
amino acids is not their excretion but rather their deamina-
tion, with the eventual excretion of the nitrogen atoms as urea
in the urine. The terms
negative nitrogen balance
and
posi-
tive nitrogen balance
refer to whether there is a net loss or
gain, respectively, of amino acids in the body over any period
of time.
If any of the essential amino acids are missing from
the diet, a negative nitrogen balance—that is, loss greater
than gain—always results. The proteins that require a miss-
ing essential amino acid cannot be synthesized, and the other
amino acids that would have been incorporated into these pro-
teins are metabolized. This explains why a dietary requirement
for protein cannot be specifi ed without regard to the amino
acid composition of that protein. Protein is graded in terms
of how closely its relative proportions of essential amino acids
approximate those in the average body protein. The highest
quality proteins are found in animal products, whereas the
quality of most plant proteins is lower. Nevertheless, it is quite
possible to obtain adequate quantities of all essential amino
acids from a mixture of plant proteins alone.
CH
2
COOH
CH
Coenzyme
2H
Oxidative
deamination
Transamination
COOH
O
C
NH
2
Coenzyme
H
2
O
NH
3
CH
3
COOH
O
C
CH
2
COOH
CH
2
COOH
CH
2
CH
NH
2
CH
3
COOH
Glutamic acid
α
-Ketoglutaric acid
Pyruvic acid
Alanine
Figure 3–51
Oxidative deamination and transamination of the amino acids
glutamic acid and alanine produce keto acids that can enter the
carbohydrate pathways.
Excretion as
sloughed hair,
skin, etc.
(very small)
Dietary
proteins and
amino acids
Body
proteins
Amino acid
pools
Urinary
excretion
(very small)
Nitrogen-containing
derivatives of
amino acids
Nucleotides, hormones,
creatine, etc.
Carbohydrate
and fat
NH
3
NH
3
Urea
Urinary
excretion
Figure 3–52
Pathways of amino acid metabolism.
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