On the redox equilibrium between H2 and hydrogenase.

Area of Science:

• Biochemistry
• Bioinorganic Chemistry
• Enzymology

Background:

• Hydrogenases are crucial enzymes catalyzing hydrogen oxidation/production.
• Nickel-containing hydrogenases are a significant class, involved in microbial energy metabolism.
• Understanding the nickel ion's redox behavior is key to elucidating hydrogenase mechanisms.

Purpose of the Study:

• To investigate the redox behavior of nickel ions in hydrogenases from Methanobacterium thermoautotrophicum and Chromatium vinosum.
• To explore nickel redox properties without artificial redox mediators.
• To determine if nickel hydrogenases possess additional sites for H2 interaction.

Main Methods:

• Performed redox titrations of nickel ions in isolated hydrogenase enzymes.
• Varied hydrogen (H2) partial pressure to probe redox states.
• Utilized Electron Paramagnetic Resonance (EPR) spectroscopy to monitor nickel species.
• Conducted experiments on both purified enzymes and enzyme within intact cells.

Main Results:

• Nickel ion redox behavior differed significantly from studies using redox mediators.
• An EPR signal, previously attributed to monovalent nickel with bound hydrogen, acted as an n=2 redox component.
• This EPR signal persisted even after hydrogen removal, indicating stability.
• Similar nickel redox behavior was observed in intact cells of M. thermoautotrophicum.

Conclusions:

• Nickel hydrogenases exhibit unique redox properties not fully captured by previous studies.
• The findings suggest the presence of a second site for H2 reaction within nickel hydrogenases.
• This implies a more complex mechanism for H2 activation and turnover than previously understood.