ALPHA-TURNS IN PROTEIN STRUCTURES

The occurrence of 5 --> 1 type of hydrogen bonds (alpha-turn) in prote ins has been studied using a data set comprising of 107 proteins with resolution less than or equal to 2.0 Angstrom. A very large majority o f such alpha-turn segments (96%) form part of regular alpha-helices. T he examples (84) which do not form part of an alpha-helix are termed ' isolated alpha-turns' and are grouped into two major families and seve n minor groups along with two isolated examples based on the similarit y of conformational angles, The family with large number of examples ( 50) have (phi, psi) angles close to an alpha-helix and hence belong to the class of the shortest alpha-helices, The 'end to end' distances o f these alpha-turns vary between 4.7 and 6.7 Angstrom, the range being nearly the same as that of alpha-helices. The propensity calculations show that some amino acids such as Glu, Ser and Thr have statisticall y significant higher preferences to occur in alpha-turns than in alpha -helices, In addition to the 5 --> 1 type, the residues in the alpha-t urn are involved in hydrogen bonds with other parts of the chain, The residues are in general more hydrophilic compared to those in alpha-he lices, In many cases (70%) the alpha-turn occurs at the ends of extend ed strands, and whenever it occurs at the loop regions connecting two extended strands, it brings about a hairpin bend.