THE CXXC MOTIF - A RHEOSTAT IN THE ACTIVE-SITE

The active-site CXXC motif of thiol:disulfide oxidoreductases is essen tial for their catalysis of redox reactions. Changing the XX residues can perturb the reduction potential of the active-site disulfide bond of the Escherichia coli enzymes thioredoxin (Trx; CGPC) and DsbA (CPHC ). The reduction potential is correlated with the acidity of the N-ter minal cysteine residue of the CXXC motif. As the pK(a) is lowered, the disulfide bond becomes more easy to reduce. A change in pK, can accou nt fully for a change in reduction potential in well-characterized CXX C motifs of DsbA but not of Trx. Formal analysis of the Nernst equatio n reveals that reduction potential contains both pH-dependent and pH-i ndependent components. Indeed, the difference between the reduction po tentials of wild-type Trx and wild-type DsbA cannot be explained solel y by differences in thiol pK, values. Structural data for thiol:disulf ide oxidoreductases reveal no single factor that determines the pi-I-i ndependent component of the reduction potential. In addition, the pH-d ependent component is complex when the redox state of the CXXC motif a ffects the titration of residues other than the thiols. These intricac ies enable CXXC motifs to vary widely in their capacity to assist elec tron flow, and thereby engender a family of thiol:disulfide oxidoreduc tases that play diverse roles in biochemistry.