Geometric characteristics of hydrogen bonds involving sulfur atoms in proteins

Sulfur atoms have been known to participate in hydrogen bonds (H-bonds) and these sulfur-containing H-bonds (SCHBs) are suggested to play important roles in certain biological processes. This study aims to comprehensively characterize all the SCHBs in 500 high-resolution protein structures (<= 1.8 angstrom). We categorized SCHBs into six types according to donor/acceptor behaviors and used explicit hydrogen approach to distinguish SCHBs from those of nonhydrogen bonding interactions. It is revealed that sulfur atom is a very poor H-bond acceptor, but a moderately good H-bond donor. In alpha-helix, considerable SCHBs were found between the sulphydryl group of cysteine residue i and the carbonyl oxygen of residue i-4, and these SCHBs exert effects in stabilizing helices. Although for other SCHBs, they possess no specific secondary structural preference, their geometric characteristics in proteins and in free small compounds are significantly distinct, indicating the protein SCHBs are geometrically distorted. Interestingly, sulfur atom in the disulfide bond tends to form bifurcated H-bond whereas in cysteine-cysteine pairs prefer to form dual H-bond. These special H-bonds remarkably boost the interaction between H-bond donor and acceptor. By oxidation/reduction manner, the mutual transformation between the dual H-bonds and disulfide bonds for cysteine-cysteine pairs can accurately adjust the structural stability and biological function of proteins in different environments. Furthermore, few loose H-bonds were observed to form between the sulphydryl groups and aromatic rings, and in these cases the donor H is almost over against the rim rather than the center of the aromatic ring.