Artificial Metalloproteins Containing Co₄O₄ Cubane Active Sites

Area of Science:

• Bioinorganic Chemistry
• Catalysis
• Biomimetic Chemistry

Background:

• Artificial metalloproteins (ArMs) are engineered proteins that mimic the function of natural metalloenzymes.
• The cobalt-tetracalcium-oxide (Co4O4) cubane core is structurally similar to the oxygen-evolving complex in photosystem II and water oxidation catalysts.

Purpose of the Study:

• To construct artificial metalloproteins with Co4O4 cubane active sites.
• To investigate the role of secondary sphere interactions, particularly hydrogen bonds, in modulating water oxidation reactivity.
• To bridge the gap between synthetic and biological water oxidation systems.

Main Methods:

• Biotin-streptavidin technology was used for immobilizing Co4O4 cubane active sites within proteins.
• X-ray crystallography was employed to determine the structural features of the ArMs, including hydrogen bonding interactions.
• Solution electrochemistry was utilized to study the oxidation potentials and pH-dependent reactivity of the ArMs.

Main Results:

• Crystallographic analysis revealed stabilized Co(III)-OH2 moieties through hydrogen bonds in immobilized cubane sites.
• Variants with Ser and Phe residues showed predominantly 1e-/1H+ chemistry until pH 8, becoming pH-independent thereafter.
• Introducing a Tyr residue proximal to the Co4O4 site, forming a single hydrogen bond, resulted in multi-e-/multi-H+ chemistry and a pH-dependent mechanistic shift at pH 9.5, attributed to Tyr deprotonation.

Conclusions:

• Secondary sphere interactions, specifically hydrogen bonds from tyrosine, are crucial for mediating multi-electron/multi-proton water oxidation in artificial metalloproteins.
• These findings highlight the importance of the protein environment in controlling the catalytic activity of metalloenzymes.
• The developed ArMs serve as valuable models for understanding both biological water oxidation and synthetic water oxidation catalysts.