Cysteine residues pair to form unique cross-links in proteins. These bridges have an important role in stability and are common in extracellular proteins. The prevailing view is that disulphide bonds are present in order to enhance the thermal stability of proteins. However, not all the engineered disulphide bonds enhance protein stability. In some cases, no increase or even decrease in protein stability was observed [1-3]. It seems likely that the conformation and stability of disulphide bonds will be determined both by its geometry and by its interactions with the rest of the protein [4].

The contacts that cysteines make with residues in their 3D environment and a comprehensive analysis of conformational features have been studied using 351 disulphide bridges in 131 non-homologous single-chain protein structures. Free cysteines have more contacts to non-polar residues and fewer contacts to polar/charged residues than half-cystines. Histidine and methionine are preferentially seen nearby free cysteines. Tryptophan is preferentially nearby half-cystines.

Left-handed and right-handed disulphide bridges display different conformational parameters. The present work [5] may provide important information for engineering the protein environment around cysteines.

The results presented is essential for the design of putative sites in a protein that might accommodate for an extra disulphide bond in order to improve its stability. In addition, for evaluation of the structural importance of a particular disulphide bond the results might be important. The new data provides better parameters to which the geometry of a particular disulphide bond can be compared.

[1] Betz, S.F., Protein Sci., 2, 1551-1558, 1993.

[2] Wells, J.A., Powers, D.B., J. Biol. Chem., 261, 6564-6570, 1986.

[3] Matsumura, M., Becktel, W.J., Levitt, M., Matthews, B.W., Proc. Natl. Acad. Sci. U.S.A., 86, 6562-6566, 1989.

[4] Creighton, T.E., Bioessays, 8, 57-63, 1988.

[5] Petersen, M.T.N., Jonson, P.H., Petersen, S.B. Protein Eng., 12, 535-548, 1999.