Abstract
A Fe2S2 cluster with unprecedented CysSS(-) (cysteinepersulfide) coordination has been observed crystallographically in the AdoMet-dependent hydrogenase maturase enzyme HydE. Geometry-optimized density functional theory calculations are used to develop an electronic structure description of this unusual cluster. The results indicate that the CysSS(-) ligand is unique because it can act as a donor as well as an acceptor ligand. This is due to the presence of S-S π* (occupied) and S-S σ* (unoccupied) orbitals in this ligand. Extensive back-bonding is observed between the cluster and the S-S σ* orbital. The back-bonding is significantly higher in the reduced state, which is calculated to shift the reduction potential of this Fe2S2 cluster by +400 mV in the gas phase relative to a CysS(-)-coordinated Fe2S2 cluster model of BioB.
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