![]() ![]() ![]() The hydrogenases are of particular interest as they tend to be more biased toward H 2 evolution than hydrogenases ( 8). Hydrogenases are able to catalyze both H 2 oxidation and H 2 evolution with minimal electrochemical overpotential (driving force) ( 4, 7), comparable to the 2H +/H 2 equilibrium established on platinum ( 5). Many hydrogenases have extremely high activities ( 3), a fact that has been emphasized most recently in studies by protein film electrochemistry ( 4– 6). ![]() The relatively slow attack by oxygen compared to carbon monoxide suggests that a very high level of discrimination can be achieved by subtle factors such as electronic effects (specific orbital overlap requirements) and steric constraints at the active site. These results therefore show that destruction of the cluster is initiated by binding and reduction of oxygen at the di-iron domain-a key step that is blocked by carbon monoxide. Carbon monoxide, a competitive inhibitor of CrHydA1 which binds to an Fe atom of the 2Fe H domain and is otherwise not known to attack FeS clusters in proteins, reacts nearly two orders of magnitude faster than oxygen and protects the enzyme against oxygen damage. By protein film electrochemistry we were able to determine the order of events leading up to this destruction. X-ray absorption spectroscopy shows that reaction with oxygen results in destruction of the domain of the active site H-cluster while leaving the di-iron domain (2Fe H) essentially intact. In this study, the mechanism of oxygen inactivation of the hydrogenase CrHydA1 from C. However, the extreme sensitivity of hydrogenases to oxygen presents a major challenge for exploiting these organisms to achieve sustainable photosynthetic hydrogen production. Green algae such as Chlamydomonas reinhardtii synthesize an hydrogenase that is highly active in hydrogen evolution. ![]()
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