| Abstract
| - Raman spectroscopy has been used to investigate the structure of the molybdenum cofactor inDMSO reductase from Rhodobacter capsulatus. Three oxidized forms of the enzyme, designated ‘redoxcycled', ‘as prepared', and DMSORmodD, have been studied using 752 nm laser excitation. In addition,two reduced forms of DMSO reductase, prepared either anaerobically using DMS or using dithionite,have been characterized. The ‘redox cycled' form has a single band in the MoO stretching region at865 cm-1 consistent with other studies. This oxo ligand is found to be exchangeable directly with DMS18Oor by redox cycling. Furthermore, deuteration experiments demonstrate that the oxo ligand in the oxidizedenzyme has some hydroxo character, which is ascribed to a hydrogen bonding interaction with Trp 116.There is also evidence from the labeling studies for a modified dithiolene sulfur atom, which could bepresent as a sulfoxide. In addition to the 865 cm-1 band, an extra band at 818 cm-1 is observed in theMoO stretching region of the ‘as prepared' enzyme which is not present in the ‘redox cycled' enzyme.Based on the spectra of unlabeled and labeled DMS reduced enzyme, the band at 818 cm-1 is assignedto the SO stretch of a coordinated DMSO molecule. The DMSORmodD form, identified by its characteristicRaman spectrum, is also present in the ‘as prepared' enzyme preparation but not after redox cycling. Thecomplex mixture of forms identified in the ‘as prepared' enzyme reveals a substantial degree of activesite heterogeneity in DMSO reductase.
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