Proton Transfer Mechanisms in Bimetallic Hydrogenases

Area of Science:

• Bioinorganic Chemistry
• Enzymology
• Spectroscopy

Background:

• Hydrogenases are crucial metalloenzymes catalyzing efficient proton reduction and H2 oxidation.
• They serve as model systems for understanding low-valent transition metals, hydride chemistry, and proton-coupled electron transfer.
• Investigating dynamic proton transfer mechanisms in hydrogenases is essential for bioinorganic chemistry.

Purpose of the Study:

• To utilize photochemistry and infrared difference spectroscopy to identify dynamic hydrogen-bonding changes in [NiFe]- and [FeFe]-hydrogenase.
• To elucidate the roles of specific amino acid residues and outer coordination spheres in facilitating proton transfer.
• To compare proton transfer mechanisms between [NiFe]- and [FeFe]-hydrogenase and guide the design of biomimetic catalysts.

Main Methods:

• Employing visible light to trigger specific redox-state transitions in hydrogenases.
• Recording sensitive "light-minus-dark" infrared difference spectra to monitor key amino acid residues.
• Utilizing photolysis of bridging hydride ligands and external redox dyes for state accumulation.

Main Results:

• Identified dynamic hydrogen-bonding changes involving cysteine, glutamic acid, and water molecules in [NiFe]-hydrogenase.
• Revealed hydrogen-bonding alterations involving glutamic acid and arginine residues in [FeFe]-hydrogenase.
• Established a precise molecular model for proton transfer in [FeFe]-hydrogenase, resolving discrepancies with crystallographic data.

Conclusions:

• The outer coordination sphere plays a critical role in the catalytic activity of bimetallic hydrogenases.
• Protonation of a nickel-ligating cysteine in [NiFe]-hydrogenase favors H2 oxidation.
• Proton transfer in [FeFe]-hydrogenase involves cysteine as a relay, promoting both H2 oxidation and proton reduction.