Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 113

Figure 7 (a) The structure of the [FeFe] hydrogenase from Desulfovibrio desulfuricans , PDB
code 1HFE. This enzyme is analogous to the [NiFe] hydrogenase shown in Figure 6 as it contains
three iron sulfur clusters. Smaller [FeFe] hydrogenases are known which only require the 6Fe 'H-
cluster' for activity. This enzyme is also a heterodimer, although other [FeFe] hydrogenases have a
monomeric minimal functional unit. The large (dark grey ribbon) protein subunit co-ordinates all
the metal centers. Color code: yellow depicts sulfur; blue depicts nitrogen; bright red depicts
oxygen; green depicts hydrocarbon; orange-red ball and sticks depict iron. (b) The chemical
structure of the H-cluster active site.