Side-chain effects on peptidyl-prolyl cis/trans isomerisation.

Peptidyl-prolyl cis/trans isomerisation has been frequently found as a rate limiting step in the folding of proteins. In order to determine whether the nature of the amino acid preceding proline controls the probability of cis prolyl bonds in native proteins, systematic studies on the thermodynamics and kinetics of the prolyl isomerisation in the pentapeptide series Ac-Ala-Xaa-Pro-Ala-Lys-NH2 were performed. All proteinogenic amino acids were substituted in the position preceding proline. When measured by 1H-NMR and CD spectroscopy both isomers proved to be devoid of ordered structure in the whole series of the oligopeptides in aqueous solution. Thus, isomerization rates and cis/trans ratios calculated from solvent jump and 1H-NMR magnetisation transfer experiments exclusively reflect the side-chain effects of the Xaa position in the peptide series. There is a rough correlation between the cis content in the oligopeptides and the propensity of Xaa-Pro cis prolyl bonds in proteins. This correlation suggests that the prolyl bond conformation is mainly determined by local effects in proteins. The rate constants kc-->t of pentapeptides containing unionised amino acids preceding proline range from 3.2 x 10(-3) s-1 (Xaa = Ala) to 0.5 x 10(-3) s-1 (Xaa = Trp) at 4 degrees C. Proline clustering led to an isomerisation cycle indicating considerable influence on the isomerisation rates of the peptide bond conformations flanking the rotating bond. Both tyrosine and histidine specifically reduce isomerisation rates severalfold by deprotonation of their respective side-chains.