Relationship between helix-coil transition parameters for synthetic polypeptides and helix conformation parameters for globular proteins. A simple model

Abstract

o (1) The Zimm-Bragg formulation for helix-coil transitions in simple synthetic polypeptides is generalized to handle the formation of α-helical regions during the folding of globular proteins. (2) The relation between the helix-coil transition parameters for this process and other conformational parameters (derived by statistical analysis of the sequences and conformations of globular proteins_ is determined. (3) It is shown that the generalized Zimm-Bragg model can be used graphically to predict the helix-coil transition parameters from the statistical parameters, and vice versa . (4) It is also shown that the Zimm-Bragg co-operativity parameter σ implied by the lengths of helices observed in globular proteins is of the order of 10 −1 , and therefore much larger than the values obtained from simple synthetic polypeptides. (5) It is postulated that a similarly large value of σ persists throughout the folding process from the moment the helical regions are “trapped” by interactions between residues far apart in the amino acid sequence, and the nature of these interactions is discussed.