Sulphur shuttling across a chaperone during molybdenum cofactor maturation

Formate dehydrogenases (FDHs) are of interest as they are natural catalysts that sequester atmospheric CO 2 , generating reduced carbon compounds with possible uses as fuel. FDHs activity in Escherichia coli strictly requires the sulphurtransferase Ec FdhD, which likely transfers sulphur from IscS to the molybdenum cofactor (Mo- bis PGD) of FDHs. Here we show that Ec FdhD binds Mo- bis PGD in vivo and has submicromolar affinity for GDP—used as a surrogate of the molybdenum cofactor’s nucleotide moieties. The crystal structure of Ec FdhD in complex with GDP shows two symmetrical binding sites located on the same face of the dimer. These binding sites are connected via a tunnel-like cavity to the opposite face of the dimer where two dynamic loops, each harbouring two functionally important cysteine residues, are present. On the basis of structure-guided mutagenesis, we propose a model for the sulphuration mechanism of Mo- bis PGD where the sulphur atom shuttles across the chaperone dimer.