dreyer at bio5.chemie.uni-freiburg.de (Matthias Dreyer) writes:
>|> Does anyone have a plausible hypothesis to explain why only L amino
>|> acids are used in proteins? I am teaching an introductory course in
>|> biochemistry this summer and this question was raised by a student.
>|> Please send your answers to:
>|>>
>Schulz & Schirmer (Prinicples of Protein Structure,1985) dropped a few
>lines about this problem:
>"... Although it had been shown that the intrinsic asymmetric beta-decay
>expresses itself as molecular asymmetry by preferentially destroying
>D-amino acids, the observed effect of a few percent is too small to explain
>the selection.
Dawkins in 'The extended Phenotype' refers to a model that shows that a
selction pressure of 1% is enough to wipe out a gene in 1000
generations. I don't have the book here but I"m pretty sure he gives a
reference.
For my two cents worth. It appears from a brief glance at my old
textbook (Biochemistry, Stryer, 3rd edition) that the assymetry is
induced in all the amino acids at teh point of transamination of the
alpha keto acid to the alpha amino acid. The transmainase enzymes are
all (presumably) stereospecific, so at least there is a common point,in
the 'proto-transmainase'
where the selection pressure could have acted to affect all aa. Could be
by chance I suppose, but I'd tend to favour something that gave a good
reason why one was chosen over the other.
