essential amino acids (methionine) biosynthesis ?

[George M. Carter]

Peter.Kulmburg at AUG.UKL.UNI-FREIBURG.DE
(kulmburg at aug.ukl.uni-freiburg.de) wrote:


>8 of 20 amino acids are known to be essential for human beings, any book of
>biology tells us. But none told me yet, which (key) enzymes lack in humans
>or mice for the production of the respective amino acid.

There are around 80 amino acids.  20 are used by humans.  Of these,
the following (11) are not synthesized by the body (i.e., essential):

histidine, isoleucine, leucine, lysine, methionine, cysteine,
phenylalanine, tyrosine, threonine, tryptophan and valine.
 (Source: Taber's Cyclopedic Medical Dictionary)

For infants, arginine is additionally classifed as essential.
Tyrosine is considered "conditionally essential," and can be
metabolized from phenylalanine in the liver.  Others suggest that
cystine (a homodimer of cysteine) is synthesized from methionine and
thus cysteine is not "essential."

Obviously, there is more to amino acid production than just enzymes
(since various amino acids can be converted to others).  Such a
differential analysis of pathways of amino acid production would be
interesting to see but beyond the scope of a quick email. Does anyone
here have good references for this kind of thing?  And what is the
rationale for wanting to know "what's missing" (so to speak) from
humans that cannot synthesize essential aa's?

			George M. Carter