Spectroscopic and computational studies of reduction of the metal versus the tetrapyrrole ring of coenzyme F₄₃₀ from methyl-coenzyme M reductase

Methyl-coenzyme M reductase (MCR) catalyzes the final step in methane biosynthesis by methanogenic archaea and contains a redox-active nickel tetrahydrocorphin, coenzyme F₄₃₀, at its active site. Spectroscopic and computational methods have been used to study a novel form of the coenzyme, called F₃₃₀, which is obtained by reducing F₄₃₀ with sodium borohydride (NaBH₄). F₃₃₀ exhibits a prominent absorption peak at 330 nm, which is blue shifted by 100 nm relative to F ₄₃₀. Mass spectrometric studies demonstrate that the tetrapyrrole ring in F₃₃₀ has undergone reduction, on the basis of the incorporation of protium (or deuterium), upon treatment of F₄₃₀ with NaBH₄ (or NaBD₄). One- and two-dimensional NMR studies show that the site of reduction is the exocyclic ketone group of the tetrahydrocorphin. Resonance Raman studies indicate that elimination of this π-bond increases the overall π-bond order in the conjugative framework. X-ray absorption, magnetic circular dichroism, and computational results show that F₃₃₀ contains low-spin Ni(II). Thus, conversion of F ₄₃₀ to F₃₃₀ reduces the hydrocorphin ring but not the metal. Conversely, reduction of F₄₃₀ with Ti(III) citrate to generate F₃₈₀ (corresponding to the active MCR_red1 state) reduces the Ni(II) to Ni(I) but does not reduce the tetrapyrrole ring system, which is consistent with other studies [Piskorski, R., and Jaun, B. (2003) J. Am. Chem. Soc. 125, 13120-13125; Craft, J. L., et al. (2004) J. Biol. Inorg. Chem. 9, 77-89]. The distinct origins of the absorption band shifts associated with the formation of F₃₃₀ and F₃₈₀ are discussed within the framework of our computational results. These studies on the nature of the product(s) of reduction of F₄₃₀ are of interest in the context of the mechanism of methane formation by MCR and in relation to the chemistry of hydroporphinoid systems in general. The spectroscopic and time-dependent DFT calculations add important insight into the electronic structure of the nickel hydrocorphinate in its Ni(II) and Ni(I) valence states.