Spectroscopic and Electronic Structure Studies of a Dimethyl Sulfoxide Reductase Catalytic Intermediate: Implications for Electron- and Atom-Transfer Reactivity

Abstract

The electronic structure of a genuine paramagnetic des-oxo Mo(V) catalytic intermediate in the reaction of dimethyl sulfoxide reductase (DMSOR) with (CH3)3NO has been probed by EPR, electronic absorption and MCD spectroscopies. EPR spectroscopy reveals rhombic g- and A-tensors that indicate a low-symmetry geometry for this intermediate and a singly occupied molecular orbital (SOMO) that is dominantly metal centered. The excited state spectroscopic data were interpreted in the context of electronic structure calculations, and this has resulted in a full assignment of the observed magnetic circular dichroism (MCD) and electronic absorption bands, a detailed understanding of the metal-ligand bonding scheme, and an evaluation of the Mo(V) coordination geometry and Mo(V)-Sdithiolene covalency as it pertains to the stability of the intermediate and electron transfer regeneration. Finally, the relationship between des-oxo Mo(V) and des-oxo Mo(IV) geometric and electronic structures is discussed relative to the reaction coordinate in members of the DMSOR enzyme family.

Pyranopterin conformation defines the function of molybdenum and tungsten enzymes

“…A second line of experimental evidence arises from the application of Resonance Raman spectroscopy, which demonstrates that the two pyranopterins of the R. sphaeroides dimethyl sulfoxide reductase and E. coli biotin sulfoxide reductase are not equivalent, with vibrational modes assigned to two dithiolene-metal units, one of which has a higher C═C double bond character (32,33), rendering it similar to the single dithiolene-metal vibrational mode observed in human sulfite oxidase (34). Although these observations may simply reflect differences in the Mo-dithiolene bonding and ligand geometry (8), they are consistent with the presence of non-equivalent proximal and distal pyranopterins, with a facet of this being differential accessibility to the tetrahydro and dihydro redox states.…”

“…The reason for the presence of a second pyranopterin in the bis-pyranopterincontaining enzymes remains unclear. It could function to orient the metal center for optimization of substrate binding and catalysis (8), but this can also be achieved without a second pyranopterin (e.g., nitrate reductases exist in both the DMSOR and SUOX families). An attractive hypothesis is that the second pyranopterin enables additional facets of redox tuning to facilitate catalysis, thus contributing to the diversity of reactions catalyzed by these enzymes.…”

The Mononuclear Molybdenum Enzymes

“…382 The Mo hyperfine from the 25% of naturally occurring 95,97 Mo having I = 5/2 is A 1,2,3 = 14.55, 34.1, 53.20 (×10 −4 cm −1 ), which includes an isotropic A iso of 34.22 × 10 −4 cm −1 and an anisotropic component of [−20.9, +3.11, +17.8] × 10 −4 cm −1 that is close to the fully anisotropic limit of [− A aniso , 0, + A aniso ]. 389 This reflects a very low symmetry for the signal-giving species, neither trigonal prismatic as seen in the fully oxidized enzyme nor square pyramidal as seen in the reduced. The good agreement between calculated g and 95,97 Mo A tensors from a geometry-optimized model indicates that the signal-giving species is in a relaxed rather than entatic configuration, 389 consistent with its observed accumulation in the course of turnover.…”

“…389 This reflects a very low symmetry for the signal-giving species, neither trigonal prismatic as seen in the fully oxidized enzyme nor square pyramidal as seen in the reduced. The good agreement between calculated g and 95,97 Mo A tensors from a geometry-optimized model indicates that the signal-giving species is in a relaxed rather than entatic configuration, 389 consistent with its observed accumulation in the course of turnover. The essentially quantitative accumulation of the paramagnetic Mo(V) species in the course of the turnover with TMAO, in conjunction with the absence of other chromophores in the protein, has also made it possible to examine the molybdenum center of the R. sphaeroides DMSO reductase by magnetic circular dichroism 389 and XAS.…”

Unexpected Reaction Promoted by NH+···O=Mo Hydrogen Bonds in Nonpolar Solvents

“…[2,4,43,44] In the reduction process, the NH proton of the conserved tryptophan residue interacts with the terminal oxo ligand in the oxidised state. Although its real structure [9,45] and true role in the related enzymes [46,47] are still under discussion, the OH group should be coordinated to the Mo V species. On the basis of our results, a mechanism is herein proposed.…”