Toward the Mechanistic Understanding of Enzymatic CO₂ Reduction

Abstract

Reducing CO 2 is a challenging chemical transformation that biology solves easily, with high efficiency and specificity. In particular, formate dehydrogenases are of great interest since they reduce CO 2 to formate, a valuable chemical fuel and hydrogen storage compound. The metal-dependent formate dehydrogenases of prokaryotes can show high activity for CO 2 reduction. Here, we report an expression system to produce recombinant W/Sec-FdhAB from Desulfovibrio vulgaris Hildenborough fully loaded with cofactors, its catalytic characterization and crystal structures in oxidized and reduced states. The enzyme has very high activity for CO 2 reduction and displays remarkable oxygen stability. The crystal structure of the formate-reduced enzyme shows Sec still coordinating the tungsten, supporting a mechanism of stable metal coordination during catalysis. Comparison of the oxidized and reduced structures shows significant changes close to the active site. The DvFdhAB is an excellent model for studying catalytic CO 2 reduction and probing the mechanism of this conversion.

Carbon Dioxide Utilisation—The Formate Route

“…4, 5, 6 and 9, and references herein), show a stable hexa-coordination, with the cysteine or selenocysteine always bound to the molybdenum/tungsten ion. This is also the case of the recently solved structure of the formate-reduced D. vulgaris SeCys-W-FDH [132] and also of the NADH-reduced R. capsulatus Cys-Mo-FDH, whose structure was determined by cryo-electron microscopy [139].…”

“…Because the metal-dependent FDHs are involved in diverse metabolic pathways (energy and C1 metabolism), for which different "interfaces" are needed, this class is extraordinarily heterogeneous, comprising enzymes with diverse redox-active centres, such as iron-sulfur centres (Fe/S), haems and flavins, besides the characteristic molybdenum or tungsten active sites, organised in different subunit compositions and quaternary structures ( D. gigas [130] or D. vulgaris [129,131,132], with "only" four [4Fe-4S] centres and one tungsten centre ( Fig. 6) [133,134], or the more "complex" heteromeric (abc) Mo-FDH of D. desulfuricans [135][136][137] or D. vulgaris [129,131] that contains eight redox-active centres ([4Fe-4S] centres and c-type haems) in addition to the molybdenum centre.…”

“…6) [133,134], or the more "complex" heteromeric (abc) Mo-FDH of D. desulfuricans [135][136][137] or D. vulgaris [129,131] that contains eight redox-active centres ([4Fe-4S] centres and c-type haems) in addition to the molybdenum centre. Remarkably, the overall protein fold of the molybdenum -and tungsten-containing subunits, including the arrangement of Fe/S centre, is highly conserved 4 [116,117,119,130,132,[138][139][140].…”

Reducing CO2 to HCO2- at Mild Potentials: Lessons from Formate Dehydrogenase

“…38,39 While the mechanism of CO2 and HCO2 -conversion in tungsten and molybdenum-based formate dehydrogenase enzymes is still under debate, multiple mechanisms have been proposed. 38,[42][43][44] Of these, only one proposed mechanism invokes a classic metal hydride, 45 which is based on a computational study and is not consistent with the known chemistry of Mo/W enzymes. 38 Instead of a classic metal hydride intermediate, the major mechanistic proposals state that the electrons and protons are not co-located on the metal.…”

Understanding How the Rate of C–H Bond Cleavage Affects Formate Oxidation Catalysis by a Mo-Dependent Formate Dehydrogenase

“…In contrast, this suggestion has recently been challenged by the crystal structure of FDH-AB from D. vulgaris Hildenborough, in which the active-site Sec is coordinated to the W center in the reduced W(IV) state—albeit in the absence of any substrate or product in the active site. 24 …”