Intramolecular assistance of cis/trans isomerization of the histidine-proline moiety

Peptidyl-prolyl cis/trans isomerization is a slow conformational interconversion in the polypeptide backbone that is frequently rate-limiting in refolding of proteins and is thought to play a role in cellular restructuring of proteins. In order to probe the influence of positively charged amino acids located in sequence segments adjacent to proline, the rotational barriers of Arg-Pro- and His-Pro-containing peptides were determined by isomer-specific proteolysis and dynamic NMR spectroscopy for Suc-Ala-His-Pro-Phe-NH-Np, Ac-Ala-Arg-Pro-Ala-Lys-NH2, Ac-Ala-His-Pro-Ala-Lys-NH2, angiotensin III, thyrotropin-releasing hormone (TRH), and [His(3-Me)2]TRH in aqueous solution. In contrast to the guanidinium group of arginine, the protonated side chain of histidine preceding proline led to an acceleration of the prolyl isomerization up to 10-fold relative to the unprotonated state. Both arginine and histidine residues succeeding proline in an amino acid sequence proved to be ineffective. Under basic and acidic conditions the kinetic solvent deuterium isotope effects Kc-->tH20/Kc-->tD20 for angiotensin III were 1.0 +/- 0.1 and 2.0 +/- 0.1, respectively. The results are interpreted in terms of intramolecular general acid catalysis of prolyl bond rotation by the imidazolium group that is without precedent in intermolecular catalysis.