ABSTRACT
We report here two sets of results on proline-containing linear peptides, one of which brings out the role of the β-turn conformation in the structure of nascent collagen while the other points to the functional importance of the β-turn in calcium-binding proteins. Based on the data on peptides containing the -Pro-Gly- sequence, we had proposed and experimentally verified that the β-turn conformation in these peptides is a structural requirement for the enzymic hydroxylation of the proline residues in the nascent (unhydroxylated) procollagen molecule. Our recent data, presented here, on the conformation of peptides containing both the -Pro-Gly- and -Gly-Pro- sequences reveal that while the β-turn in the substrate molecule is required at the catalytic site of prolyl hydroxylase, the polyproline-II structure is necessary for effective binding at the active site of the enzyme. Thus, peptides containing either the β-turn or the polyproline-II structure alone are found to act only as inhibitors while those with the polyproline-II followed by β-turn serve as substrates of the enzyme. In another study, we have synthesized the two linear peptides: Boc-Pro-D-Ala-Ala- NHCH3 and Boc-Pro-Gly-Ala-NHCH3 each of which adopts, in solution, a structure with two consecutive β-turns, as judged from circular dichroism, infrared and nuclear magnetic resonance data. Drastic spectral changes are seen in these peptides on binding to Ca2+. Both the peptides show a distinct specificity to Ca2+ over Mg2+, Na+ and Li+. A conformational change in the peptides occurs on Ca2+ binding which brings together the carbonyl groups to coordinate with the metal ion. These results imply a functional role for the β-turn in Ca2+ - binding proteins.