Abstract
Methyl-coenzyme M reductase (MCR) catalyzes the final step of methanogenesis in which coenzyme B and methyl-coenzyme M are converted to methane and the heterodisulfide, CoMS-SCoB. MCR also appears to initiate anaerobic methane oxidation (reverse methanogenesis). At the active site of MCR is coenzyme F430, a nickel tetrapyrrole. This paper describes the reaction of the active MCR(red1) state with the potent inhibitor, 3-bromopropanesulfonate (BPS; I50 = 50 nM) by UV-visible and EPR spectroscopy and by steady-state and rapid kinetics. BPS was shown to be an alternative substrate of MCR in an ionic reaction that is coenzyme B-independent and leads to debromination of BPS and formation of a distinct state ("MCR(PS)") with an EPR signal that was assigned to a Ni(III)-propylsulfonate species (Hinderberger, D., Piskorski, R. P., Goenrich, M., Thauer, R. K., Schweiger, A., Harmer, J., and Jaun, B. (2006) Angew. Chem. Int. Ed. Engl. 45, 3602-3607). A similar EPR signal was generated by reacting MCR(red1) with several halogenated sulfonate and carboxylate substrates. In rapid chemical quench experiments, the propylsulfonate ligand was identified by NMR spectroscopy and high performance liquid chromatography as propanesulfonic acid after protonolysis of the MCR(PS) complex. Propanesulfonate formation was also observed in steady-state reactions in the presence of Ti(III) citrate. Reaction of the alkylnickel intermediate with thiols regenerates the active MCR(red1) state and eliminates the propylsulfonate group, presumably as the thioether. MCR(PS) is catalytically competent in both the generation of propanesulfonate and reformation of MCR(red1). These results provide evidence for the intermediacy of an alkylnickel species in the final step in anaerobic methane oxidation and in the initial step of methanogenesis.
Highlights
MCR can exist in several states that differ in their nickel oxidation and/or coordination states (Fig. 1)
When MCRred1 is reacted with BPS, a unique EPR signal called “MCRBPS” is observed, which, because of its air sensitivity and its similarity to the MCRred1 spectrum, was assigned as an Ni(I) state [42]
A more recent study based on EPR data suggests that when MCRred1 reacts with BPS, bromide is presumably released, and a species is formed that can be described as an Ni(III)-propylsulfonate or a high spin Ni(II) with an alkylsulfonate radical [29]
Summary
MCRred reacts with the methyl group of methyl-SCoM, forming a methyl-Ni(III) or methyl-Ni(II). The corresponding methyl-nickel species has not been observed on the enzyme, methane formation from the reaction of Ni(I)-derivatives of. F430 and activated methyl donors like methylsulfonium ions has been observed [24], and a methyl-Ni(II). F430 has been characterized by NMR methods [25]. Reacting Ni(I)-octaethylisobacteriochlorin (a structural cousin of F430) with various alkyl halides generates alkyl-Ni(III) species that undergo reduction to the alkyl-Ni(II) followed by protonolysis to yield the corresponding alkane [26].4
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