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RE: [ccp4bb]: Chirality Issue

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> Never paid attention to it before. Isn't there a chirality issue with the
> side chain of Threonine at the Cbeta position???
> Is it always R (rectus) or can it be S also????

The amino acid threonine contains two chiral carbons and thus can 
exist in four possible conformations.  However, only a single 
conformation (2S, 3R) of threonine is incorporated in proteins 
by the cellular translation material.  The conformation at the 
alpha carbon (carbon 2) is the same as that of the other amino 
acids in biological proteins, and there is no explanation for 
why this enantiomer was selected.  Further, there is no explanation 
for the choice of conformation at the 3 carbon in threonine.