STRUCTURAL CHARACTERISTICS OF PROLYL RESIDUES—A REVIEW.

Abstract

The structural and conformational characteristics of prolylresidues are discussed on the basis of the available crystal structure data on peptides and proteins. The conformational flexibility of the pyrrolidine ring often leads to the apparent shortening of the bond lengths involving CP and CI. The value of some angles in the residue vary substantially as a function of the nature and the geometrical disposition of the preceding residue. Almost all the X-Pro bonds in small linear peptides exist in the trans configuration. The pyrrolidine ring exists in C, or C2 conformation with d and/or 0 deviating from the plane defined by the rest of the atoms, in all linear peptides of naturally occurring amino acids. Other ring  conformations occur only in conditions of strain. The observed conformations in peptides suggest that the proly) residue has a high propensity to assume the polyproline 11 structure. The residue has a high propensity to be at the second position in beta-turns also. It occurs at the third position only under unusual circumstances and on rare occasions. A bend involving a cis X-Pro bond turns out to be an ideal structural feature for bringing about chain reversal in proteins. Prolyl residues occur extensively in beta-turns in peptides containing D and unusual residues as well.