Class III hybrid cluster protein homodimeric architecture shows evolutionary relationship with Ni, Fe-carbon monoxide dehydrogenases.

Hybrid cluster proteins (HCPs) are Fe-S-O cluster-containing metalloenzymes in three distinct classes (class I and II: monomer, III: homodimer), all of which structurally related to homodimeric Ni, Fe-carbon monoxide dehydrogenases (CODHs). Here we show X-ray crystal structure of class III HCP from Methanothermobacter marburgensis (Mm HCP), demonstrating its homodimeric architecture structurally resembles those of CODHs. Also, despite the different architectures of class III and I/II HCPs, [4Fe-4S] and hybrid clusters are found in equivalent positions in all HCPs. Structural comparison of Mm HCP and CODHs unveils some distinct features such as the environments of their homodimeric interfaces and the active site metalloclusters. Furthermore, structural analysis of Mm HCP C67Y and characterization of several Mm HCP variants with a Cys67 mutation reveal the significance of Cys67 in protein structure, metallocluster binding and hydroxylamine reductase activity. Structure-based bioinformatics analysis of HCPs and CODHs provides insights into the structural evolution of the HCP/CODH superfamily.