An Unprecedented [Cu-4Fe-4S] Cluster in a Methanotroph Acetol Dehydrogenase

Biocatalytic metal-containing clusters are nanometer-sized chemical reactors that enable enzymes to perform chemistry far beyond what would be possible with amino acids alone ^1,2 . These clusters display a remarkable variability ³ and elucidating their exact mechanisms is often difficult to determine, requiring input from a multitude of techniques ranging from spectroscopic to structural and theoretical methods. Thus, for unknown clusters, it is essential to accumulate, collate, and interpret as much information as possible from every relevant technique available. Here we report the discovery and in-depth, interdisciplinary characterization of an entirely novel, [Cu-4Fe-4S] cluster in the protein acetol dehydrogenase (AceDH). AceDH, isolated directly from Methylacidiphilum fumariolicum SolV cells, catalyzed the oxidation of acetol to methylglyoxal, proving its role in the 2-propanol/acetone metabolism of methanotrophs ^4–6 . Structural, spectroscopic and electrochemical analyses reveal the [Cu-4Fe-4S] cluster has a unique three-dimensional- and electronic structure involving electronic coupling between the copper and one of the iron atoms, likely contributing to its high, +275 mV, redox potential. The binding site for the novel cluster is composed of two protein subunits and involves a novel motif. These findings expand the known repertoire of biological metal cofactors and provide insight into how heterometallic clusters are adapted for biological processes.