The Role of Selenocysteine in Ni, Fe Hydrogenases: Biophysical and Synthetic Model Studies

The active site of Ni,Fe hydrogenases contains a heterodinuclear cluster where the Ni and Fe centers are bridged by two cysteine thiolate S-donor atoms. Two terminal cysteine thiolates are also present in the Ni coordination environment. In a few of the enzymes, one of the terminal thiolates is substituted by a selenocysteine. This substitution has an effect on the redox chemistry of the enzyme and on the catalytic mechanism. The presence of selenocysteine provides a spectroscopic probe of a specific Ni ligand. X-ray absorption spectroscopy was used to probe the Ni and Se environments in Desulfomicrobium baculatum hydrogenase in the oxidized and reduced states. These studies reveal that the Se environment is sensitive to the redox poise of the enzyme, while no changes in the Ni environment were observed. Crystallographically characterized model Ni complexes featuring alkylthiolate and alkylselenolate ligation are used explore the changes in structure and redox properties that might arise in the biological systems.