In humans (and other organisms) one may find as many as a few
hundred nonstandard amino acids. These are synthesized through
specialized enzymatic reactions from various metabolic precursors. For
example, some standard amino acids (or their derivative residues in proteins)
are found to be phosphorylated, acetylated, hydroxylated, alkylated, or
carboxylated, for example. Specific enzymes catalyze each of these
reactions. Many of the nonstandard amino acids are found as intermediates
in various metabolic pathways.
Perhaps the most interesting class of nonstandard
amino acids are the stereoisomers of the standard amino acids. Of
the 20 standard amino acids, 17 have 1 mirror-image stereoisomer,
and 2 have 1 mirror-image stereoisomer as well as 2 additional diastereoisomers.
Even though each stereoisomer, or diastereoisomer, is chemically equivalent to
its standard amino acid counterpart -- except for its inherent asymmetry --
evolution has selected for the genetic code only the
standard amino acids (for unknown reasons). Thus, the genetically-encoded amino
acids include only L-amino acid stereoisomers even
though
their D-amino acid stereoisomer mirror images are chemically
equivalent, except for their relative asymmetry. Because most biological
reactions inherently depend on the symmetry as well as the chemical composition
of the reactants (and products), evolution has preserved the imbalance of L-
vs. D-amino acid stereoisomers by the conservation of the genetic
code from the lowest to the highest forms of life.
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