RESONANCE RAMAN AS A DIRECT PROBE FOR THE CATALYTIC MECHANISM OF MOLYBDENUM OXOTRANSFERASES

Recent studies of human sulfite oxidase and Rhodobacter sphaeroides DM SO reductase have demonstrated the ability of resonance Raman to probe in detail the coordination environment of the Mo active sites in oxot ransferases via Mo=O, Mo-S(dithiolene), Mo-S(Cys) or Mo-O(Ser), dithio lene chelate ring and bound substrate vibrations. Furthermore, the abi lity to monitor the catalytically exchangeable oxo group via isotopic labeling affords direct mechanistic information and structures for the catalytically competent Mo(IV) and Mo(VI) species. The results clearl y demonstrate that sulfite oxidase cycles between cis-di-oxo-Mo(VI) an d mono-oxo-Mo(IV) states during catalytic turnover, whereas DMSO reduc tase cycles between mono-oxo-Mo(VI) and des-oxo-Mo(IV) states. In the case of DMSO reductase, O-18-labeling experiments have provided the fi rst direct evidence for an oxygen atom transfer mechanism involving an Mo=O species. Of particular importance is that the active-site struct ures and detailed mechanism of DMSO reductase in solution, as determin ed by resonance Raman spectroscopy, are quite different to those repor ted or deduced in the three X-ray crystallographic studies of DMSO red uctases from Rhodobacter species.