Invariant features of the structure of pancreatic ribonuclease. A test of different predictive models

(1)The primary structures of 24 homologous ribonucleases which differ in up to 34% of their amino acids are compared to determine which features are essential for the three-dimensional structure.(2)The distribution pattern of helix-forming residues, as defined by Chou & Fasman (1974a), was rather constant. However, predictions of the helices, based on such a distribution, were not satisfactory, although the method of Chou & Fasman (1974b) gave more uniform results and a better agreement with the X-ray structure of bovine RNA ase than the method of Burgess et al. (1974). The prediction of the β-sheet and the β-bend using these statistical methods was less accurate than the helix prediction.(3)The prediction of the secondary structure by the method of Lim (1974b) which is based on the relative positions of the hydrophobic residues in the α-helix and β-sheet, gave good results in the case of ribonuclease. It turned out that all residues which, according to Lim's theory, are essential for the formation of secondary structure are invariant in all ribonucleases tested.(4)The invariability of the distribution of the hydrophobic residues suggests strongly that this is a very important feature of the primary structure. The role of these residues in forming secondary and tertiary structure is discussed.