In article <94168.181148MISENERS at QUCDN.QueensU.CA>,
Stephen Misener <MISENERS at QUCDN.QueensU.CA> wrote:
>In article <2ts7va$fdo at babyblue.cs.yale.edu>, johnk at loglady.ninds.nih.gov (John
>Kuszewski) says:
>>|>>>>>Both your proposals are complete crap. "Essential" amino acids are those
>>that people can't synthesize for themselves, but plants can. We don't
>>bother having synthetic mechanisms for all the AAs for the same reason
>>that we don't bother having a synthetic mechanism for glucose (from CO2,
>>that is)--it's perfectly easy just to eat plants, letting them do all
>>the work.
>>>Well...I am aware of the fact that we are unable to synthesize essential amino
>acids...hence the name. But I think you missed the point. I am trying to
>come up with an explanation for why we can synthesize *any* amino acids given
>the fact that they occur in our diet at adequate levels (for the most part).
>I argue that we have maintained these pathways because these amino acids play
>some important regulatory role *besides* protein synthesis. For example
>arginine biosynthesis in the urea cycle. I don't think we can lose this
>biosynthetic pathway. What I am *most* interested in is people's opinion
>on whether/not all essential amino acids must play some important role, and
>that all non-essential ones have no role besides protein synthesis. Even
>I think this may be too simplistic a view...but I can think of no examples
>of essential amino acids having some important biological role.
>
If I understand you correctly you are postulating that the NON-ESSENTIAL
amino acids are required for metabolic roles other than protein synthesis
and the ESSENTIAL amino acids are only used in protein synthesis. Thus, the
reason why mammals have lost the ability to synthesize the ESSENTIAL amino
acids is that they are only needed for translation. Is this correct?
Methionine is an essential amino acid in mammals. In addition to its role
in protein synthesis it is a precursor in the biosynthesis of the methyl
group donor S-adenosylmethionine. SAM is required for many reactions.
Histidine is an essential amino acid. It is the precusor of histamine which
controls constriction of blood vessels and acid secretion in the stomach.
The essential amino acid tryptophan is used in the synthesis of serotonin,
a hormone and a neurotransmitter.
Several of the essential amino acids are required for the synthesis of some
of the other amino acids. For example, methionine is a presursor
of cysteine, phenylalanine is converted to tyrosine, and threonine is
a presursor in the pathway leading to isoleucine. Cysteine and tyrosine
are non-essential and isoleucine is essential.
There doesn't seem to be any logical reason why mammals have lost the ability
to synthesize some amino acids. Perhaps it is simply one more example of
genetic drift in the absence of natural selection. In general, the non-
essential amino acids are made in short pathways that begin with one of
the intermediates of glycolysis of the citric acid cycle. The essential
amino acids are usually the end products of longer pathways involving
several reactions. There is a greater chance of a mutation knocking out
one of the enzymes required for the longer pathways.
Laurence A. Moran (Larry)
