Electron-paramagnetic-resonance parameters of molybdenum(V) in sulphite oxidase from chicken liver

Abstract

A study has been made of e.p.r. signals due to Mo(V) in reduced sulphite oxidase (EC 1.8.3.1) from chicken liver. Reduction by SO3(2-), or photochemically in the presence of a deazaflavin derivative, produces spectra indistinguishable from one another. Three types of spectra from the enzyme were distingusihed and shown to correspond to single chemical species, since they could be simulated at both 9 and 35 GHz by using the same parameters. These were the low-pH form of the enzyme, with gav. 1.9805, the high-pH form, with gav. 1.9681 and a phosphate complex, with gav. 1.9741. The low-H form shows interaction with a single exchangeable proton, with A(1H)av. (hyperfine coupling constant) = 0.98 mT, probably in the form of an MoOH group. Parameters of the signals are compared with those for signals from xanthine oxidase and nitrate reductase. The signal from the phosphate complex of sulphite oxidase in unique among anion complexes of Mo-containing enzymes in showing no hyperfine coupling to protons. There is no evidence for additional weakly coupled protons or nitrogen nuclei in the sulphite oxidase signals. The possibility is considered that the enzymic mechanism involves abstraction of a proton and two electrons from HSO3- by a Mo = O group in the enzyme.

Sulfite:Cytochrome c Oxidoreductase fromThiobacillus novellus

“…At both pH values the signals obtained were very similar to the so-called "high pH" EPR signal that can be readily observed with avian and mammalian sulfite oxidoreductases (43)(44)(45). However, another characteristic EPR signal of these enzymes, the "low pH" form, was not observed with T. novellus SOR samples at pH 7.0 and could not be induced by the addition of 200 mM NaCl (Fig.…”

Modified Active Site Coordination in a Clinical Mutant of Sulfite Oxidase

“…50,51 At present neither of these two alternatives has been definitively proven. Anion-bound species such as the phosphate, sulfite (or sulfate), and arsenite compexes also show no hyperfine splitting from an exchangeable proton 22,37,[39][40][41][42] because in these cases the Mo−OH group is lacking. No splitting from coupled exchangeable protons is detected in any R160Q EPR signal at X-band microwave frequencies.…”

“…The high-pH signal occurs at low anion concentrations and at high-pH values, while the low-pH/Cl -signal occurs at low pH and high chloride. 37,38 A third category of signals are those from oxy-anion complexes, which tend to form at low pH and high oxy-anion concentrations. SO Mo V EPR signals arising from anion accessed by following the link in the citation at the bottom of the page.…”

MoV Electron Paramagnetic Resonance of Sulfite Oxidase Revisited: The Low-pH Chloride Signal

“…Recently, efforts have been focused on studying SO using advanced pulsed EPR techniques, such as electron-nuclear double resonance (ENDOR) and electron-spin envelope echo modulation (ESEEM), 9 but the conventional continuous-wave (CW) EPR has not been re-evaluated in nearly two decades. In this work, 10 we revisit the Mo V EPR of one of the major signals from this important enzyme, [11][12][13] and using the stable magnetic isotopes 35 Cl and 37 Cl, in conjunction with difference EPR spectroscopy, and analogous complexes with other halides, we present new evidence for halide molybdenum coordination of the Mo V low-pH species.…”