Why only L amino acids?

Wed Jul 6 18:18:46 EST 1994

> 6566friedman at vmsa.csd.mu.edu on 6-JUL-1994 15:40 :
> 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: 6566FRIEDMAN at VMS.CSD.MU.EDU
> Alan Friedman  Dept Biology  Marquette University  Milwaukee, WI  
Since no experiments can really be done to test the origins of only L-alpha-
amino acids in proteins, I would suggest a simple answer, "We don't know."

On the other hand, studies on L- and D- amino acids in peptides/proteins exist 
plentifully.  They show, for example, that a helix of L-amino acids folds
to yield a right-handed structure, whereas the identical sequence in D-amino
acids folds to yield the opposite hand (left handed helix).

Finally, you might also point out that D-amino acids do exist 'naturally'.
Glycine has no stereochemistry at the alpha carbon, but is found to assume
phi/psi angles permitted for both D- and L- amino acids.  Also, take a look
at the cyclic peptide Gramicidin-S which contains D-Phe.  D-amino acids may
also form at certain residues in senescent proteins.

Enjoy,  Shaun   (shaun at jason.uthct.edu)

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