Since proteins are composed of amino
acids, of which 20 kinds commonly
occur, the central topic of our
consideration is amino acid metabolism.
Amino acid metabolism is complex.
For one thing, each of the 20
amino acids requires a separate pathway
for biosynthesis and degradation.
For another, amino acids are precursors
to tissue proteins, enzymes, nucleic
acids, and other nitrogenous constituents
that form the fabric of cells.
The central purpose of carbohydrate
and fat oxidation is to provide energy
needed to construct and maintain
these vital macromolecules.
|Figure 4-18 Fate of dietary protein.
Let us begin with the amino acid
in blood and extracellular fluid
from which the tissues draw their
requirements. When animals eat proteins,
most are digested in the gut,
releasing the constituent amino acids,
which are then absorbed (Figure 4-18).
Tissue proteins also are hydrolyzed
during normal growth, repair, and tissue
restructuring; their amino acids
join those derived from protein foodstuffs
to enter the amino acid pool. A
portion of the amino acid pool is used
to rebuild tissue proteins, but most animals
ingest a surplus of protein. Since
amino acids are not excreted as such in
any significant amounts, they must be
disposed of in some other way. In fact,
amino acids can be and are metabolized
through oxidative pathways to
yield high-energy phosphate. In short,
excess proteins serve as fuel as do carbohydrates
and fats. Their importance
as fuel obviously depends on the
nature of the diet. In carnivores that
ingest a diet of almost pure protein
and fat, nearly half of their high-energy
phosphate is derived from amino acid
Before an amino acid molecule
may enter the fuel depot, nitrogen
must be removed by deamination (the
amino group splits to form ammonia
and a keto acid) or by transamination
(the amino group is transferred to a
keto acid to yield a new amino acid).
Thus amino acid degradation yields
two main products, carbon skeletons
and ammonia, which are handled in
different ways. Once nitrogen atoms
are removed, the carbon skeletons of
amino acids can be completely oxidized,
usually by way of pyruvic acid
or acetic acid. These residues then
enter regular routes used by carbohydrate
and fat metabolism.
The other product of amino acid
degradation is ammonia. Ammonia is
highly toxic because it reacts with -
ketoglutaric acid to form glutamic
acid (an amino acid). Any accumulation
of ammonia effectively removes -
ketoglutarate from the Krebs cycle (see
Figure 4-13) and inhibits respiration.
Disposal of ammonia offers little problem
to aquatic animals because it is
soluble and readily diffuses into the
surrounding medium through respiratory
surfaces. Terrestrial forms cannot
get rid of ammonia so conveniently and
must detoxify it by converting it to a relatively
nontoxic compound. The two
principal compounds formed are urea
acid, although a variety of
other detoxified forms of ammonia are
excreted by different invertebrate and
vertebrate groups. Among vertebrates,
amphibians and especially mammals
produce urea. Reptiles and birds, as well
as many terrestrial invertebrates, produce
The key feature that seems to
determine choice of nitrogenous waste
is availability of water in the environment.
When water is abundant, the
chief nitrogenous waste is ammonia.
When water is restricted, it is urea. And
for animals living in truly arid habitats,
it is uric acid. Uric acid is highly insoluble
and easily precipitates from solution,
allowing its removal in solid form.
The embryos of birds and reptiles benefit
greatly from excretion of nitrogenous
waste as uric acid, because the
waste cannot be eliminated through
their shells. During embryonic development,
harmless, solid uric acid is
retained in one of the extraembryonic
membranes. When a hatchling
emerges into its new world, accumulated
uric acid, along with the shell and
membranes that supported development,