# Role of a disulfide cross-link in the conformational stability of a thermostable xylanase

Tatu, Utpal and Murthy, SK and Vithayathil, PJ (1990) Role of a disulfide cross-link in the conformational stability of a thermostable xylanase. In: Journal of Protein Chemistry, 9 (5). pp. 641-646.

The role of a S-S cross-link in the conformational stability of xylanase fromHumicola lanuginosa has been investigated using CD, UV absorption spectroscopy, and RIA displacement studies. Our studies show that reduction and carboxymethylation of the S-S cross-link in xylanase results in a gross conformational perturbation of the protein. The secondary structure analysis of the CD spectra indicates that the xylanase with an intact S-S contains 66% $\beta$-sheet structure and remaining random coil. Cleavage of the S-S bond results in a loss of 25% $\beta$-sheet structure. Thermal denaturation studies using CD spectroscopy and pH-dependent tyrosine ionization studies using UV spectroscopy show that the presence of disulfide cross-link offers resistance against unfolding by extremes of temperature and pH. Further, we demonstrate that the heat-induced changes in xylanase with intact S-S bond are almost totally reversible, while those in the S-S cleaved enzyme fail to show any significant reversal. Our studies support the present theory that S-S cross-links exert their stabilizing effect in proteins by destabilizing the unfolded state of the protein and forcing it back to a more folded state.