The Hydrogenase Activity of the Molybdenum/Copper-containing Carbon Monoxide Dehydrogenase of Oligotropha carboxidovorans

Abstract

Background: CO dehydrogenase oxidizes CO to CO 2 under aerobic conditions. Results: CO dehydrogenase also oxidizes H 2 , eliciting a new EPR signal. Conclusion: CO dehydrogenase is an effective hydrogenase, and the EPR signal observed provides new mechanistic information. Significance: We have gained a deeper understanding of the chemistry catalyzed by this enzyme and new information concerning the mechanism of H 2 oxidation.

The Mononuclear Molybdenum Enzymes

“…242 The enzyme is reduced by H 2 at a pH-independent rate constant of 5.3 s −1 (as compared to 51 s −1 with CO), and in the course of the reaction an EPR signal is observed arising from the binuclear center that exhibits strong coupling to two approximately equivalent protons with A 1,2,3 = 80, 20, and 130 MHz. Both g 1,2,3 (2.0127, 1.9676, 1.9594) and A 1,2,3 ( 63,65 Cu) = 170, 200, 170) are also significantly different from the parameters manifested by the CO-reduced enzyme, and are also distinct from the signal seen with dithionite-reduced enzyme (which also, however, exhibits proton hyperfine coupling).…”

“…The bound H 2 is expected to be polarized by interaction with the copper, 244 and the reaction has been proposed to proceed by deprotonation to a copper hydride, which subsequently deprotonates to populate the delocalized redox-active orbital of the binuclear cluster. 242 …”

Orbital contributions to CO oxidation in Mo–Cu carbon monoxide dehydrogenase

“…Glu 763 is only 3.7Å from the Cu(I) site, and may further activate a water molecule associated with Cu(I) for nucleophilic attack on the μ 2 -η 2 CO 2 carbon centre to yield the Mo-bound bicarbonate product species. Glu 763 may also play an important role in the coupled electron-proton transfer steps necessary to convert Mo(IV)-OH 2 to Mo(VI)=O in the electron transfer (oxidative) half reaction of the catalytic cycle 19 .…”

Electronic structure contributions to reactivity in xanthine oxidase family enzymes

“…Only the Mo center is redox active in the catalytic oxidation of CO to CO 2 , $$CO+H_{2}O\to C{O}_2+2H^{+}+2e^{-}$$ and the Cu ion remains in the +1 oxidation state throughout the catalytic cycle. CODH also possesses hydrogenase activity, oxidizing H 2 to protons[20]: $$H_2\to 2H^{+}+2e^{-}$$ Although the redox role of the Mo in CODH mediated catalysis is well established, the role of the Cu(I) ion in the catalytic cycle is less well understood. However, recent evidence suggests that it is likely the locus of substrate binding and initial activation, vide infra .…”

“…This is a direct result of the smaller number of Mo(V) species that have been generated for CODH [3, 20, 49], in addition to the inherent problems associated with additional chromophores common to all XO family enzymes. Chemical reduction of CODH by dithionite, CO, or H 2 yields a Mo(V) EPR signal that displays a nearly isotropic coupling to the diamagnetic 63,65 Cu nucleus.…”