Multienzyme assemblies and dynamics in acetogenesis and methanogenesis

Abstract

The Wood-Ljungdahl pathway of acetogenesis allows for growth utilizing carbon dioxide as the sole carbon source. This process uses a series of metalloenzymes to catalyze the reduction of two molecules of carbon dioxide to acetyl-CoA. Key to this pathway are the bifunctional carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) and corrinoid iron-sulfur protein (CFeSP). CODH/ACS is a bifunctional enzyme, with the CODH subunit catalyzing the reduction of carbon dioxide to carbon monoxide. CFeSP is a cobalt-containing corrinoid-dependent methyltransferase. The active site of ACS, termed the A-cluster, is a nickel, iron, and sulfur-containing metallocluster which catalyzes the synthesis of acetyl-CoA from carbon monoxide provided by CODH, a methyl group provided by CFeSP, and CoA. Despite the unprecedented organometallic chemistry performed at the A-cluster much is unknown about this ACS activity. No substrate-bound structures of ACS have ever been reported and the molecular basis for ACS and CFeSP interactions are largely uncharacterized. Here we present the structure of the carbonylated A-cluster in CODH/ACS, highlighting the role of conformational dynamics in catalysis. We further characterize ACS dynamics using negative-stain electron microscopy, demonstrating a much larger extent of conformational flexibility than has been crystallographically observed. The basis for CFeSP:ACS interactions is further interrogated using a series of CFeSP domain constructs, showing the corrinoidbinding Rossmann fold to be sufficient for ACS interactions. Finally, we report preliminary structural characterization of acetyl-CoA decarbonylase/synthase (ACDS), a 2.2 MDa complex of CODH, ACS, and CFeSP found in archaeal methanogens. Together, this advances our understanding of the roles of structure and dynamics in the Wood-Ljungdahl pathway.