Amino Acids in Overall Metabolism:
Once the keto acids have been formed from the appropriate
amino acids by transamination, they may be used for several purposes.
The most obvious is the complete metabolism into carbon dioxide
and water by the citric acid cycle.
However, if there are excess proteins in the diet those amino
acids converted into pyruvic acid and acetyl CoA can be converted
into lipids by the lipogenesis process. If carbohydrates are
lacking in the diet or if glucose cannot get into the cells (as
in diabetes), then those amino acids converted into pyruvic acid
and oxaloacetic acids can be converted into glucose or glycogen.
The hormones cortisone and cortisol from the adrenal cortex
stimulate the synthesis of glucose from amino acids in the liver
and also function as antagonists to insulin.
Synthesis of New Amino Acids:
In addition to the catabolic function of transamination reactions,
these reactions can also be used to synthesize amino acids needed
or not present in the diet. An amino acid may be synthesized
if there is an available "root" ketoacid with a synthetic
connection to the final amino acid. Since an appropriate "root"
keto acid does not exist for eight amino acids, (lys, leu, ile,
met, thr, try, val, phe), they are essential and must be included
in the diet because they cannot be synthesized.
Glutamic acid usually serves as the source of the amine group
in the transamination synthesis of new amino acids. The reverse
of the reactions mentioned earlier are the most obvious methods
for producing the amino acids alanine and aspartic acid.
Several nonessential amino acids are made by processes other
than transamination. Cysteine is made from methionine, and serine
and glycine are synthesized from phosphoglyceric acid - an intermediate