Organometallic iron: the key to biological hydrogen metabolism.

X-ray crystallography of iron-hydrogenases reveals that the active-site H-cluster contains an unmistakably organometallic dinuclear iron subcluster. The nickel-hydrogenases, which in general play different metabolic roles, have a distinct but related active-site structure. The new structural definition, combined with chemical analogs and theoretical treatment, points toward mechanistic understanding of iron-hydrogenases and the possibility of a unified mechanism for all hydrogenases.