~ 11 2) ij1 " 3 44 2: Bioorganic Chemistry of Amino Acids and Polypeptides electrons into the carbonyl function than will the sulfur atom. Such delocalization will reduce the electrophilicity of the carbonyl function. Furthermore, the thiol is a much better leaving group than the corresponding alcohol. Remember, the pKa of a mercaptan is approximately 10, while that of an alcohol is approximately 15. In the next step, proline attached to the heavy enzyme by a thiol ester linkage attacks the light enzyme, thus transferring the dipeptide to the heavy enzyme.
Bioorganic Chemistry: A Chemical Approach to Enzyme Action by Hermann Dugas