X-ray absorption spectroscopy of dimethylsulfoxide reductase from Rhodobacter capsulatus
- Cite this article as:
- Baugh, P., Garner, C., Charnock, J. et al. JBIC (1997) 2: 634. doi:10.1007/s007750050178
Mo K-edge X-ray absorption spectroscopy (XAS) has been used to probe the environment of Mo in dimethylsulfoxide (DMSO) reductase from Rhodobacter capsulatus in concert with protein crystallographic studies. The oxidised (MoVI) protein has been investigated in solution at 77 K; the Mo K-edge position (20006.4 eV) is consistent with the presence of MoVI and, in agreement with the protein crystallographic results, the extended X-ray absorption fine structure (EXAFS) is also consistent with a seven-coordinate site. The site is composed of one oxo-group (Mo=O 1.71 Å), four S atoms (considered to arise from the dithiolene groups of the two molybdopterins, two at 2.32 Å and two at 2.47 Å, and two O atoms, one at 1.92 Å (considered to be H-bonded to Trp 116) and one at 2.27 Å (considered to arise from Ser 147). The Mo K-edge XAS recorded for single crystals of oxidised (MoVI) DMSO reductase at 77 K showed a close correspondence to the data for the frozen solution but had an inferior signal:noise ratio. The dithionite-reduced form of the enzyme and a unique form of the enzyme produced by the addition of dimethylsulfide (DMS) to the oxidised (MoVI) enzyme have essentially identical energies for the Mo K-edge, at 20004.4 eV and 20004.5 eV, respectively; these values, together with the lack of a significant presence of MoV in the samples as monitored by EPR spectroscopy, are taken to indicate the presence of MoIV. For the dithionite-reduced sample, the Mo K-edge EXAFS indicates a coordination environment for Mo of two O atoms, one at 2.05 Å and one at 2.51 Å, and four S atoms at 2.36 Å. The coordination environment of the Mo in the DMS-reduced form of the enzyme involves three O atoms, one at 1.69 Å, one at 1.91 Å and one at 2.11 Å, plus four S atoms, two at 2.28 Å and two at 2.37 Å. The EXAFS and the protein crystallographic results for the DMS-reduced form of the enzyme are consistent with the formation of the substrate, DMSO, bound to MoIV with an Mo-O bond of length 1.92 Å.