928
UNIT 4
Maintenance of the Body
24
use them to
replace
tissue proteins at the rate of about 100 grams
each day.
Although popular opinion has it that excess protein can be
stored by the body, nothing is farther from the truth. When
more protein is available than is needed for anabolic purposes,
amino acids are oxidized for energy or converted to fat for fu-
ture energy needs.
Oxidation of Amino Acids
Before amino acids can be oxidized for energy, they must be
deaminated
, that is, their amine group (NH
2
) must be removed.
(In sulfur-containing amino acids, such as methionine and
cysteine, sulfur is released prior to deamination.) Te resulting
molecule is then converted to pyruvic acid or to one of the keto
acid intermediates in the Krebs cycle [acetyl CoA, alpha (
) ke-
toglutaric acid, succinyl CoA, fumaric acid, or oxaloacetic acid].
Te key molecule in these conversions is the nonessential
amino acid
glutamic acid
(gloo-tam
9
ik). As
Figure 24.16
shows, the following events occur:
Check Your Understanding
16.
Which part of triglyceride molecules enters the glycolytic
pathway?
17.
What is the central molecule in fat metabolism?
18.
What are the products of beta oxidation?
For answers, see Appendix H.
Protein Metabolism
Describe how amino acids are metabolized for energy.
Describe the need for protein synthesis in body cells.
Like all other biological molecules, proteins have a limited life
span and must be broken down and replaced before they de-
teriorate. As proteins are broken down, their amino acids are
recycled and used to build new proteins or modified to form a
different N-containing compound. Cells take up newly ingested
amino acids from the blood by active transport processes and
Krebs
cycle
Oxidative
deamination
Transamination
Amino acid + Keto acid
(
α
-keto-
glutaric acid)
Keto acid + Amino acid
(glutamic acid)
Keto acid
modification
Modified
keto acid
Enter Krebs
cycle in body cells
Liver
NH
3
(ammonia)
Kidney
Blood
Excreted in urine
Urea
Urea
1
Transamination:
An amine
group is switched from an
amino acid to a keto acid.
2
Oxidative
deamination:
The amine
group of glutamic acid is
removed as ammonia
and combined with CO
2
to form urea.
3
Keto acid modification:
The keto acids formed
during transamination are
altered so they can easily
enter the Krebs cycle
pathways.
CO
2
Figure 24.16
Transamination, oxidative deamination, and keto acid modification:
processes that occur when amino acids are utilized for energy.
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