Methane monooxygenases (MMOs) mediate the facile conversion of methane into methanol in methanotrophic bacteria with high efficiency under ambient conditions. Because the selective oxidation of methane is extremely challenging, there is considerable interest in understanding how these enzymes carry out this difficult chemistry. The impetus of these efforts is to learn from the microbes to develop a biomimetic catalyst to accomplish the same chemical transformation. Here, we review the progress made over the past two to three decades toward delineating the structures and functions of the catalytic sites in two MMOs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). sMMO is a water-soluble three-component protein complex consisting of a hydroxylase with a nonheme diiron catalytic site; pMMO is a membrane-bound metalloenzyme with a unique tricopper cluster as the site of hydroxylation. The metal cluster in each of these MMOs harnesses O to functionalize the C-H bond using different chemistry. We highlight some of the common basic principles that they share. Finally, the development of functional models of the catalytic sites of MMOs is described. These efforts have culminated in the first successful biomimetic catalyst capable of efficient methane oxidation without overoxidation at room temperature.
Several small molecules and ions, notably carbon monoxide, cyanide, cyanate, and hydrogen sulfide, are potent inhibitors of Ni-containing carbon monoxide dehydrogenases (Ni-CODH) that catalyze very rapid, efficient redox interconversions of CO2 and CO. Protein film electrochemistry, which probes the dependence of steady-state catalytic rate over a wide potential range, reveals how these inhibitors target particular oxidation levels of Ni-CODH relating to intermediates (Cox, Cred1, and Cred2) that have been established for the active site. The following properties are thus established: (1) CO suppresses CO2 reduction (CO is a product inhibitor), but its binding affinity decreases as the potential becomes more negative. (2) Cyanide totally inhibits CO oxidation, but its effect on CO2 reduction is limited to a narrow potential region (between −0.5 and −0.6 V), below which CO2 reduction activity is restored. (3) Cyanate is a strong inhibitor of CO2 reduction but inhibits CO oxidation only within a narrow potential range just above the CO2/CO thermodynamic potential—EPR spectra confirm that cyanate binds selectively to Cred2. (4) Hydrogen sulfide (H2S/HS−) inhibits CO oxidation but not CO2 reduction—the complex on/off characteristics are consistent with it binding at the same oxidation level as Cox and forming a modified version of this inactive state rather than reacting directly with Cred1. The results provide a new perspective on the properties of different catalytic intermediates of Ni-CODH—uniting and clarifying many previous investigations.
The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO2/CO and H+/H2 interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO2 or H+ reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO2 nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction.
Carbon monoxide dehydrogenases (CODH) catalyze the reversible conversion between CO and CO2. Several small molecules or ions are inhibitors and probes for different oxidation states of the unusual [Ni-4Fe-4S] cluster that forms the active site. The actions of these small probes on two enzymes, CODH ICh and CODH IICh, produced by Carboxydothermus hydrogenoformans have been studied by protein film voltammetry to compare their behavior and establish general characteristics. Whereas CODH ICh is, so far, the best studied of the two isozymes in terms of its electrocatalytic properties, it is CODH IICh which has been characterized by x-ray crystallography. The two isozymes, which share 58.3% sequence identity and 73.9% sequence similarity, show similar patterns of behavior with regard to selective inhibition of CO2 reduction by CO (product) and cyanate, potent and selective inhibition of CO oxidation by cyanide, and with regard to the action of sulfide, which promotes oxidative inactivation of the enzyme. For both isozymes, rates of binding of substrate analogues CN− (for CO) and NCO− (for CO2) are orders of magnitude lower than turnover, a feature that is clearly revealed through hysteresis of cyclic voltammetry. Inhibition by CN− and CO is much stronger for CODH IICh compared to CODH ICh, a property that has relevance for applying these enzymes as model catalysts in solar-driven CO2 reduction.
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