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The enzyme DMSOase, Dimethylsulfoxide Reductase, is found in many bacteria and fungi. This enzyme catalyzes the reaction
O=S(CH3)2 + 2e- + 2H+ S(CH3)2 + H2O
The conversion of water soluble DMSO to the insoluble and volatile dimethylsulfide generates energy for bacteria by stripping electrons from organic matter. The crystal structure of the oxidized enzyme is shown here in wireframe display.
This is the oxidized form of DMSOase. The α-helices are coloured in blue and the beta-sheets are coloured in red. The active site (ball and stick) containing the molybdenum (light blue) is found at the center of a globular protein. The active site can be seen more clearly if you zoom in by right-clicking on the display at left and holding down the control key while dragging (for Windows).
This is the active site. The molybdenum is held to the protein chain by an oxygen from the side chain of Ser 147 (labelled). In this oxidized form, the other atoms coordinated to Mo are 3 sulfurs at 2.5 Å from two pterin cofactor molecules and an oxo group. A 4th sulfur atom has a long 3.3 Å partial bond to Mo and partial disulfide bond to a neighbouring sulfur and possibly a hydrogen bond to the oxygen of Ser 147. Each pterin moiety is attached to one of the nucleotides, in this case GMP. All Mo-containing enzymes except nitrogenase are found to contain at least one pterin cofactor.
The overall structure around Mo is very distorted trigonal prismatic (Mo(O)(S)3(O-ser)). The geometry of this site is poorly resolved and more work needs to be done to determine the exact geometry. The oxidation state of the Mo is ambiguous because of the partial bonds. There also appears to be partial bonding between the oxo group and a neighbouring sulfur at 2.2 Å. The enzyme is reduced when electrons and protons travel to the molybdenum and the oxo ligand is converted to water. The electrons are supplied by an electron transport protein.
This is a display of the reduced enzyme which has the same structure as the oxidized one except that the oxo group is missing at the molybdenum. Therefore, an oxygen in the square base of the Mo(O)(S)3(O-ser) structure has been removed to produce the distorted site Mo(S)3(O-ser). This ligand deficient metal could probably only exist in the protected confines of a protein where other ligands are prevented from coordinating. DMSO can coordinate at this vacant site via the oxygen. The oxo group is abstracted from the substrate during oxidation and becomes attached to molybdenum in an exothermic reaction.
This is the oxidized form of dimethylsulfoxide for you to explore further.