Strong hydrophobic nature of cysteine residues in proteins

The differences between disulfide-bonding cystine (Cys_SS) and free cystein e (Cys_SH) residues were examined by analyzing the statistical distribution of both types of residue in proteins of known structure. Surprisingly, Cys _SH residues display stronger hydrophobicity than Cys_SS residues, A detail ed survey of atoms which come into contact with. the sulfhydryl group (sulf ur atom) of Cys_SH revealed those atoms are essentially the same in number and variety as those of the methyl group of isoleucine, but are quite diffe rent to those of the hydroxyl group of serine, Moreover, the relationships among amino acids were also determined using the 3D-profile table of known protein structures. Cys_SH was located in the hydrophobic cluster, along wi th residues such as Met, Trp and Tyr, and was clearly separated from Ser an d Thr in the polar cluster, These results imply that free cysteines behave as strongly hydrophobic, and not hydrophilic, residues in proteins, (C) 199 9 Federation of European Biochemical Societies.