Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
929
24
1
Transamination
(trans
0
am-ĭ-na
9
shun). A number of amino
acids can transfer their amine group to
-ketoglutaric
acid (a Krebs cycle keto acid), thereby transforming
-ketoglutaric acid to glutamic acid. In the process, the
original amino acid becomes a keto acid (that is, it has an
oxygen atom where the amine group formerly was). Tis
reaction is fully reversible.
2
Oxidative deamination.
In the liver, the amine group
of glutamic acid is removed as
ammonia (NH
3
)
, and
-ketoglutaric acid is regenerated. Te liberated NH
3
molecules are combined with CO
2
, yielding
urea
and wa-
ter. Te urea is released to the blood and excreted from
the body in urine. Because ammonia is toxic to body cells,
the ease with which glutamic acid funnels amine groups
into the
urea cycle
is extremely important. Tis cycle
rids the body not only of NH
3
produced during oxidative
deamination, but also of bloodborne NH
3
produced by
intestinal bacteria.
3
Keto acid modification.
Te goal of amino acid degra-
dation is to produce molecules that can be oxidized in
the Krebs cycle or converted to glucose. Keto acids re-
sulting from transamination are altered as necessary to
produce metabolites that can enter the Krebs cycle. Te
most important of these metabolites are pyruvic acid,
acetyl CoA,
-ketoglutaric acid, and oxaloacetic acid (see
Figure 24.7). Because the reactions of glycolysis are re-
versible, deaminated amino acids that are converted to
pyruvic acid can be reconverted to glucose and contribute
to gluconeogenesis.
Protein Synthesis
Amino acids are the most important anabolic nutrients. Not
only do they form all protein structures, but they form the bulk
of the body’s functional molecules as well. As we described in
Chapter 3, protein synthesis occurs on ribosomes, where ri-
bosomal enzymes oversee the formation of peptide bonds
linking the amino acids together into protein polymers. Hor-
mones (growth hormone, thyroxine, sex hormones, and others)
precisely control the amount and type of protein synthesized,
and protein anabolism reflects hormonal balance at each stage
of life.
During your lifetime, your cells will synthesize 225–450 kg
(about 500–1000 lb) of proteins, depending on your size. How-
ever, you do not need to consume anywhere near that amount of
protein because the body easily forms nonessential amino acids
by siphoning keto acids from the Krebs cycle and transferring
amine groups to them. Most of these transformations occur in
the liver, which provides nearly all the nonessential amino acids
needed to produce the relatively small amount of protein that
the body synthesizes each day.
However, as noted earlier, a complete set of amino acids must
be present for protein synthesis to take place, so the diet must
provide all essential amino acids. If some are lacking, the rest
are oxidized for energy even though they may be needed for
anabolism. In such cases, body protein is broken down to sup-
ply the essential amino acids needed, and negative nitrogen bal-
ance results.
±o review the various metabolic reactions described so far,
consult
Table 24.4
.
Table 24.4
Thumbnail Summary of Metabolic Reactions
Carbohydrates
Cellular respiration
Reactions that together complete the oxidation of glucose, yielding CO
2
, H
2
O, and ATP
Glycolysis
Converts glucose to pyruvic acid
Glycogenesis
Polymerizes glucose to form glycogen
Glycogenolysis
Hydrolyzes glycogen to glucose monomers
Gluconeogenesis
Forms glucose from noncarbohydrate precursors
Krebs cycle
Completely breaks down pyruvic acid to CO
2
, yielding small amounts of ATP and reduced coenzymes
Electron transport chain
Splits H removed during oxidations to H
1
and e
2
and creates a proton gradient used to bond ADP to P
i
(forming ATP)
Lipids
Beta oxidation
Converts fatty acids to acetyl CoA
Lipolysis
Breaks down lipids to fatty acids and glycerol
Lipogenesis
Forms lipids from acetyl CoA and glyceraldehyde phosphate
Proteins
Transamination
Transfers an amine group from an amino acid to
-ketoglutaric acid, thereby transforming
-ketoglutaric acid
to glutamic acid
Oxidative deamination
Removes an amine group from glutamic acid as ammonia and regenerates
-ketoglutaric acid (the liver
converts NH
3
to urea)
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