Computational Studies on the A Cluster of Acetyl-Coenzyme A Synthase: Geometric and Electronic Properties of the NiFeC Species and Mechanistic Implications

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Abstract
DFT computational studies on the A cluster of acetyl-coenzyme A synthase are presented and discussed. They aim at evaluating possible A cluster models to settle the ongoing controversy about the nature of the proximal metal site in the catalytically active form of the cluster, recently proposed to be either Ni or Cu. Two possible models for the NiFeC species are considered, [Fe4S4]2+−Ni+CO−Ni2+ and [Fe4S4]2+−Cu+CO−Ni+. While for the former the computed 57Fe, 61Ni, and 13C hyperfine coupling parameters agree reasonably well with corresponding experimental values, for the latter model this agreement is very poor because the actual charge distribution is [Fe4S4]+−Cu+CO−Ni2+. Together, our results provide compelling evidence that the catalytically active A cluster contains Ni rather than Cu at the proximal metal site. Computations on the Ared2 state proposed to be part of the catalytic cycle (Darnault, C.; Volbeda, A.; Kim, E. J.; Legrand, P.; Vernède, X.; Lindahl, P. A.; Fontecilla-Camps, J. C. Nat. Struct. Biol. 2003, 10, 271−279) yield [Fe4S4]+−Ni+−Ni2+, hinting toward a Ni+/Ni3+ redox couple being involved in the methylation reaction.