For proline, unlike the other amino acids, the cis form is not so energetically

Question

For proline, unlike the other amino acids, the cis form is not so energetically unfavorable that it rarely occurs. In fact, some 10-40% of proline residues share a cis peptide bond with the prior amino acid residue, whereas among all other amino acid residues only 0.01% of peptide bonds are cis. Given this information, roughly how many times greater is the energetic penalty associated with assuming cis form for typical peptide bonds compared to those involving a proline residue at body temperature (37 C)? Approximate that 1/4 prolines are cis, and that the given percentages represent the thermodynamic equilibrium.

Explanation / Answer

The cis bonds of proline amino acids favor the formation of beta-stands of protein secondary structures. Because, it is a small amino acid and has imino-nitrogen group and the conversion of Trans form to Cis form is a thermodynamically favorable reaction. It is conformed, because about 6% of the peptide bonds involving the imino nitrogen of proline, however, are in the cis configuration, and many of these occur at turns.         

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