Direct oxygen atom transfer in the mechanism of action of Rhodobacter sphaeroides dimethyl sulfoxide reductase

Abstract

The forward and reverse oxygen atom (oxo) transfer reactions MoVIOzL" + X 5=* Mo^OLn + XO resulting in oxidation/ reduction of substrates X/XO have been amply demonstrated in this* 1•2 and other laboratories.lb'c•3•4 These observations are the basis of the oxo transfer hypothesis1 for the mechanism of action of oxomolybdenum enzymes,3•5•6 which catalyze the overall reaction X + HzO = XO + 2H+ + 2e". Under this hypothesis, depicted as scheme 1 (Figure 1) for enzymes lacking terminal Mo=S groups, substrate XO binds cis to the MoIV0 group. Atom transfer ensues to afford the Mo^Oz group and X; the enzyme site is returned to the Mo(IV) state by sequential electron and proton transfer. The reverse sequence applies to substrate oxidation. Existence of the Mo^Oz and Mo^O groups in fully oxidized and dithionite-reduced enzyme states, respec-

MOLYBDENUM-COFACTOR–CONTAINING ENZYMES: Structure and Mechanism

“…According to the proposed mechanism, an oxygen atom is ultimately transferred from the substrate into water, in agreement with 18 O labeling experiments (45). This finding is supported by the observed coordination geometry in R. sphaeroides DMSO reductase with only one oxo group bound to the Mo, which is the oxygen atom being transferred into water in the reductive halfcycle.…”

Towards the reaction mechanism of pyrogallol–phloroglucinol transhydroxylase of Pelobacter acidigallici

“…With this, the transhydroxylase di¡ers from all other molybdenum cofactor-containing enzymes which catalyze hydroxylations by oxygen transfer between water and an organic substrate [13,14]. These ¢ndings support the concept of a transhydroxylation reaction in which a hydroxyl group is transferred from a donor molecule (tetrahydroxybenzene) to an acceptor molecule (pyrogallol) [6].…”

“…All mass spectra contained a group of peaks with masses of 342, 343 and 344, which were due to trisilylated phloroglucinol. The dominant mass of 342 represents phloroglucinol containing only 16 O and mainly 12 C, whereas higher masses were caused by phloroglucinol containing some 13 C. Phloroglucinol produced from pyrogallol by the transhydroxylase reaction showed the same mass pattern as commercial phloroglucinol (Table 1). No shift towards higher masses was observed, meaning that the phloroglucinol derived from pyrogallol and tetrahydroxybenzene had not incorporated 18 O from the labeled water.…”

Mechanistic aspects of molybdenum-containing enzymes

“…5). It has been demonstrated that DMSO reductase is able to catalyze oxygen atom transfer between IV O-labeled DMSO and a watersoluble phosphine oxo acceptor via a catalytically labile oxygen site [18], and the label found to be present in the MoNO group on the basis of resonance Raman work [19]. Furthermore XAS results with both oxidized and reduced DMSO reductase [20] are consistent with the enzyme cycling as indicated in Fig.…”