Combinations of turns in proteins

We observed that beta- and gamma-turns in protein structure may be associated as peptides representing combinations of turns that span between nine and 26 amino acid residues along the polypeptide backbone chain and often correspond to loops in the protein structure. Around 475 peptides resulted from the analysis of a non-redundant data set corresponding to 248 protein crystal structures selected from the Protein Data Bank. Nearly 40% protein chains are associated with two or more peptides and the peptides with nine and 10 amino acid residues are more frequent. A maximum of four distinct peptides varying in number of amino acid residues were observed in at least 10 proteins along the same protein chain. Nearly 80% peptides comprise type IV beta-turns that are associated with irregular dihedral angle values suggesting this may be important for the conformational diversity associated with the loops in proteins. In general, predominant interactions that possibly stabilize these peptides involve main-chain and side-chain interactions with solvent, in addition to hydrogen bond, salt-bridge and non-bonded interactions. Majority of the peptides were observed in hydrolase, oxidoreductase, transferase, serine proteinase/inhibitor complex, electron transport/electron transfer and lyase proteins.