Isomerizations at Nonprolyl Peptide Bonds

Cis/trans isomerism is not confined to prolyl bonds. Cis peptide bonds to residues other than proline (cis nonprolyl bonds) are, however, extremely rare in folded proteins because the trans form is strongly favored over cis. In short unstructured peptides 99.5—99.9% of nonprolyl peptide bonds are in the trans state (Scherer et al., 1998). Proteins that contain nonprolyl cis peptide bonds in their native states must therefore undergo trans —cis isomerizations of these bonds in virtually all refolding molecules. 

Together, the results for RNase T1 and -lactamase show that the trans — cis isomerizations of nonprolyl peptide bonds control the folding of proteins with such cis bonds. The trans — cis isomerizations of prolyl and nonprolyl bonds show similar rates, the reverse cis — trans isomerizations are 50-100-fold faster, however, for the nonprolyl peptide bonds. 

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