Two Separate One-Electron Steps in the Reductive Activation of the A Cluster in Subunit β of the ACDS Complex in Methanosarcina thermophila†

Acetyl-CoA decarbonylase/synthase (ACDS) is a multienzyme complex found in methanogens and certain other Archaea that carries out the overall synthesis and cleavage of the acetyl C−C and C−S bonds of acetyl-CoA. The reaction is involved both in the autotrophic fixation of carbon and in the process of methanogenesis from acetate, and takes place at a unique active site metal center known as the A cluster, located on the beta subunit of the ACDS complex and composed of a binuclear Ni−Ni site bridged by a cysteine thiolate to an Fe4S4 center. In this work, a high rate of acetyl-CoA synthesis was achieved with the recombinant ACDS beta subunit by use of methylcobinamide as an appropriate mimic of the physiological base-off corrinoid substrate. The redox dependence of acetyl-CoA synthesis exhibited one-electron Nernst behavior, and the effects of pH on the observed midpoint potential indicated that reductive activation of the A cluster also involves protonation. Initial burst kinetic studies indicated the form...