Evidence the Isc iron-sulfur cluster biogenesis machinery is the source of iron for [NiFe]-cofactor biosynthesis in Escherichia coli

Abstract

[NiFe]-hydrogenases have a bimetallic NiFe(CN)₂CO cofactor in their large, catalytic subunit. The 136 Da Fe(CN)₂CO group of this cofactor is preassembled on a distinct HypC-HypD scaffold complex, but the intracellular source of the iron ion is unresolved. Native mass spectrometric analysis of HypCD complexes defined the [4Fe-4S] cluster associated with HypD and identified + 26 to 28 Da and + 136 Da modifications specifically associated with HypC. A HypC_C2A variant without the essential conserved N-terminal cysteine residue dissociated from its complex with native HypD lacked all modifications. Native HypC dissociated from HypCD complexes isolated from Escherichia coli strains deleted for the iscS or iscU genes, encoding core components of the Isc iron-sulfur cluster biogenesis machinery, specifically lacked the + 136 Da modification, but this was retained on HypC from suf mutants. The presence or absence of the + 136 Da modification on the HypCD complex correlated with the hydrogenase enzyme activity profiles of the respective mutant strains. Notably, the [4Fe-4S] cluster on HypD was identified in all HypCD complexes analyzed. These results suggest that the iron of the Fe(CN)₂CO group on HypCD derives from the Isc machinery, while either the Isc or the Suf machinery can deliver the [4Fe-4S] cluster to HypD.