Chemically Distinct Ni Sites in the A-Cluster in Subunit β of the Acetyl-CoA Decarbonylase/Synthase Complex from Methanosarcina t hermophila:  Ni L-Edge Absorption and X-ray Magnetic Circular Dichroism Analyses

The 5-subunit-containing acetyl-CoA decarbonylase/synthase (ACDS) complex plays an important role in methanogenic Archaea that convert acetate to methane, by catalyzing the central reaction of acetate C−C bond cleavage in which acetyl-CoA serves as the acetyl donor substrate reacting at the ACDS β subunit active site. The properties of Ni in the active site A-cluster in the ACDS β subunit from Methanosarcina thermophila were investigated. A recombinant, C-terminally truncated form of the β subunit was employed, which mimics the native subunit previously isolated from the ACDS complex, and contains an A-cluster composed of an [Fe4S4] center bridged to a binuclear Ni−Ni site. The electronic structures of these two Ni were studied using L-edge absorption and X-ray magnetic circular dichroism (XMCD) spectroscopy. The L-edge absorption data provided evidence for two distinct Ni species in the as-isolated enzyme, one with low-spin Ni(II) and the other with high-spin Ni(II). XMCD spectroscopy confirmed that the species producing the high-spin signal was paramagnetic. Upon treatment with Ti3+ citrate, an additional Ni species emerged, which was assigned to Ni(I). By contrast, CO treatment of the reduced enzyme converted nearly all of the Ni in the sample to low-spin Ni(II). The results implicate reaction of a high-spin tetrahedral Ni site with CO to form an enzyme-CO adduct transformed to a low-spin Ni(II) state. These findings are discussed in relation to the mechanism of C−C bond activation, in connection with the model of the β subunit A-cluster developed from companion Ni and Fe K edge, XANES, and EXAFS studies.