Anisotropic covalency contributions to superexchange pathways in type one copper active sites

Area of Science:

• Biochemistry
• Bioinorganic Chemistry
• Enzyme Catalysis

Background:

• Type 1 (T1) copper sites are crucial for electron transfer (ET) in enzymes like nitrite reductases (NiRs) and multicopper oxidases (MCOs).
• These T1 sites connect to catalytic centers via a cysteine-histidine (Cys-His) bridge, offering two potential ET pathways (P1 and P2).

Purpose of the Study:

• To investigate how the electronic structure of T1 copper sites influences intramolecular electron transfer.
• To elucidate the role of T1 copper-sulfur bond covalency in activating ET pathways.
• To understand how enzyme-specific environments modulate these ET pathways for efficient catalysis.

Main Methods:

• Computational analysis of electronic coupling and superexchange mechanisms.
• Comparison of T1 copper site structures and their corresponding ET pathways in NiRs and MCOs.

Main Results:

• High covalency of the T1 copper-sulfur bond activates hole superexchange via the Cys-His bridge.
• This covalency-activated electronic coupling (HDA) facilitates long-range ET through both P1 (protein backbone) and P2 (H-bond) pathways.
• Blue (π-type) T1 sites in NiRs primarily use P1, while green (σ-type) T1 sites use the more efficient P2 pathway.
• In MCOs, the surrounding protein environment alters the Cys-His pathway conformation, favoring HDA for blue π sites and efficient catalysis.

Conclusions:

• The anisotropic covalency of metal-ligand bonds dictates the preferred ET pathways and electronic coupling strengths.
• Protein environments dynamically tune these pathways, enabling selective activation of superexchange for optimal enzyme function.
• The Cys-His bridge is a versatile mediator of ET, adaptable to different catalytic demands through modulation of electronic couplings.