Figure 39-1. Schematic diagram showing the source and utilization of a-
amino acids in metabolic processes.
than ingested. This occurs during prolonged periods of calorie restriction
(e.g., starvation, anorexia nervosa), when protein is degraded to release amino
acids as a utilizable energy source.
A general tactic employed during catabolism of amino acids is the removal
of the a-amino group, followed by conversion of the remaining carbon skele-
ton into a major metabolic intermediate. The main way in which a-amino
groups are removed is through transamination, the transfer of an amino
group from an a-amino acid to an a-keto acid. This transamination reaction is
catalyzed by a class of enzymes called transaminases (or aminotransferases).
These enzymes use pyridoxal phosphate, a vitamin B
derivative, during
the catalytic process. Pyridoxal phosphate acts as an initial acceptor of the
a-amino group, forming a Schiff base (-CH = N-) intermediate. The pyridox-
amine phosphate intermediate subsequently donates the amino group to an
acceptor a-keto acid, forming a new a-amino acid. The acceptor a-keto acid
is most often a-ketoglutarate, therefore resulting in glutamate generation. Not
all amino acids are substrates for transaminases. Certain amino acids are ini-
tially converted into an intermediate that is subsequently transaminated (e.g.,
asparagine is hydrolyzed to asparate, the latter of which is transaminated
by aspartate aminotransferase, forming oxaloacetate). Given that the majority
of amino acids use transaminases in their pathways of degradation (although a
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