Figure 22 Square scheme depicting the reversible binding of an exogenous ligand to a [4Fe-4S] cluster; represented as [Fe3Fe-4S]. Data are as determined by simulation for binding of 1-hydroxyethane-2-thiolate at the transformed cluster of Desulfovibrio africanus FdIII, at 0°C. Note that RS

In most Fe-S-containing enzymes, the cluster provides a relay site or “way station” for electrons. Using protein film voltammetry, it has now proved possible to observe Fe-S clusters in enzymes as discrete signals and to examine their role in catalytic electron transport. Examples that have been studied are fumarate reductase (Frd) from E. coli [90, 91] and the NiFe hydrogenase from Allochromatium vinosum [92], the structures of which are outlined in Figure 23 (see color plate) [93, 94]. In each case, the buried active sites-a covalently bound FAD in the case of Frd or a bis-cysteinato bridged NiFe center

in the case of hydrogenase-are “wired” to the protein surface by a series of three clusters each separated by 10-15 Å. An intriguing feature is that in both cases, the medial cluster has a reduction potential that is approximately 0.2 V lower (Frd) or 0.3 V higher (H2ase) than the distal and proximal clusters, thereby requiring a thermodynamically uphill step in the relay. Consideration of how this may affect electron-transport efficiency in enzymes has been presented in the article by Dutton and coworkers [18].