Conformational similarity indices between different residues in proteins and alpha-helix propensities

Various amino acid similarity matrices have been derived using data on physicochemical properties and molecular evolution. Conformational similarity indices, CS(XX'), between different residues are computed here using the distribution of the main-chain and side-chain torsion angles and the values have been used to cluster amino acids in proteins. A subset of these parameters, CS(AX') indicates the extent of similarity in the main-chain and side-chain conformations (phi,psi and chi1) of different residues (X) with Ala (A) and is found to have strong correlation with alpha-helix propensities. However, no subset of CS(XX') provides any linear relationship with beta-sheet propensities, suggesting that the conformational feature favouring the location of a residue in an alpha-helix is different from the one favouring the beta-sheet. Conformationally similar residues (close CS(AX) values) have similar steric framework of the side-chain (linear/branched, aliphatic/aromatic), irrespective of the polarity or hydrophobicity. Cooperative nucleation of helix may be facile for a contiguous stretch of residues with high overall CS(AX) values.