Conformational changes of the carbon‐fixing enzyme CODH/ACS revealed by electron microscopy

Abstract

Carbon monoxide dehydrogenase/acetyl‐CoA synthase (CODH/ACS) is a 310 kDa bifunctional enzyme involved in the anaerobic fixation of atmospheric carbon (CO2) by the acetogenic bacterium Moorella thermoacetica. The CODH subunit reduces CO2 to CO, which transfers through a central hydrophobic tunnel to the active site of ACS. ACS contains a dinickel iron‐sulfur site where CO combines with a methyl group supplied by corrinoid iron‐sulfur protein (CFeSP) and coenzyme A (CoA). Conformational changes of ACS have been hypothesized as the structural mechanism regulating access of methylated‐CFeSP and CoA to the proximal nickel in the active site. However, the conformations of CODH/ACS that have been observed crystallographically are insufficient to accommodate methyltransfer from CFeSP to ACS. To examine the conformations of CODH/ACS we used single‐particle electron microscopy (EM). Because CODH/ACS contains O2 sensitive metallocofactors, we developed a method for preparing negatively stained EM specimens in an anaerobic chamber. The resulting structures of CODH/ACS reveal a dramatic hinge‐like opening of the ACS subunit that can accommodate methyl transfer from CFeSP to CODH/ACS and gives new insight into the catalytic mechanism of this environmentally relevant enzyme.This work is supported by funding from the NIH. Samuel Thompson is supported by the MIT Undergraduate Research Opportunities Program.