HELIX GEOMETRY IN PROTEINS

In this report we describe a general survey of all helices found in 57 of the known protein crystal structures, together with a detailed analysis of 48 .alpha.-helices found in 16 of the structures that are determined to high resolution. The sruvey of all helices reveals a total of 291 .alpha.-helices, 71 310-helices and no examples of .pi.-helices. The conformations of the observed helices are significantly different from the "ideal" linear structures. The mean .vphi., .psi. angles for the .alpha.- and 310-helices found in proteins are, respectively, (-62.degree., -41.degree.) and (-71.degree., -18.degree.). A computer program, HBEND, is used to characterize and to quantify the different types of helix distortion. .alpha.-Helices are classified as regular or irregular, linear, curved or kinked. Of the 48 .alpha.-helices analysed, only 15% are considered to be linear; 17% are kinked, and 58% are curved. The curvature of helices is caused by differences in the peptide hydrogen bonding on opposite faces of the helix, reflecting carbonyl-solvent/side-chain interactions for the exposed residues, and packing constraints for residues involved in the hydrophobic core. Kinked helices arise either as a result of included proline residues, or because of conflicting requirements for the optimal packing of the helix side-chains. In .alpha.-helices where there are kinks caused by proline residues, we show that the angle of kink is relatively constant (.apprx.26.degree.), and that there is minimal disruption of the helix hydrogen bonding. The proline residues responsible for the kinks are highly conserved, suggesting that these distortions may be structurally/functionally important.