4286 6.1. Oxidized Inactive States of the Ni−Fe Site and Radiation Effects 4286 6.2. Oxidative Damage of FeS Clusters in [FeFe]-Hydrogenases 4288 6.3. Hydrophobic Tunnels in [NiFe]-Hydrogenases 4288 6.4. Hydrophobic Tunnels in [FeFe]-Hydrogenase 4291 6.5. Hydrogen Sensors Related to [NiFe]-Hydrogenases 4292 6.6. Oxygen-Insensitive [NiFe]-Hydrogenases from Ralstonia eutropha 4294 7. Evolutionary Relationships of Hydrogenases to Other Proteins 4294 7.1. Comparison of [NiFe]-Hydrogenase with Complex I 4294 7.2. Comparison of [FeFe]-Hydrogenase to Narf-like Proteins 4296 8. Substrate Binding and Catalysis 4297 8.1. [NiFe]-Hydrogenases 4297 8.2. [FeFe]-Hydrogenases 4297 8.3. Comparison of Active Redox States in [NiFe]and [FeFe]-Hydrogenases 4299 9. Concluding Remarks 4299 10. Acknowledgments 4300 11. Note Added after ASAP Publication 4300 12. References 4300