Dramatic Conformational Flexibility of Carbon Monoxide Dehydrogenase/Acetyl‐CoA Synthase Revealed by Electron Microscopy

Abstract

Carbon monoxide dehydrogenase/acetyl‐CoA synthase (CODH/ACS) is a 310‐kDa bifunctional enzyme involved in anaerobic fixation of atmospheric carbon (CO2) by acetogenic bacteria. The CODH subunit reduces CO2 to CO, which passes through a 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 transfers the acetyl product to coenzyme A (CoA). Structures of CODH/ACS previously observed by X‐ray crystallography indicate that ACS must open to accommodate methyl transfer from CFeSP, but the full extent of that motion was not visualized. To examine the conformational flexibility of CODH/ACS, we performed single‐particle electron microscopy (EM) on anaerobically prepared samples. The resulting structures reveal never before seen conformations of the ACS subunit, suggesting that dramatic movements are involved in the methyl transfer from CFeSP to the ACS subunit of CODH/ACS.