Single Amino Acid Preferences for Specific Locations at Type-I α-Turns in Globular Proteins

Using both dihedral angles and hydrogen-bond definitions, 225 type-I α-turns were extracted from 125 analyzed proteins. The extracted α-turns were built up from 5% (1125) of the total amino acid residues (23286) found in the proteins. The type-I α-turn is an independent kind of secondary structural unit, although it has been frequently classified as (i, i+1) double β-turns. Both the nucleation of a helix with the type-I β-turn and its propagation with the repetitive addition of the type-I β-turn suggest that the origin of the helix in a globular protein might be the type-I β-turn. Single amino acid preferences for specific locations at type-I α-turns were compared to those at type-I β-turns and the ends of helices. The result indicates that the type-I α-turn, the type-I β-turn, and the helix are of similar kin. Similarities in the structure and amino acid preferences for specific locations among them suggest that the amino acid residues on defined positions along both turns could be properly assigned to t...