Electron-transfer mechanisms through biological redox chains in multicenter enzymes

Area of Science:

• Biochemistry
• Electrochemistry
• Enzyme kinetics

Background:

• Multicenter enzymes facilitate intramolecular electron transfer (IET) through complex redox chains.
• Understanding IET mechanisms is crucial for enzyme function and bioenergetics.
• Fumarate reductases (FRs) are key enzymes in anaerobic respiration, involving IET to their active sites.

Purpose of the Study:

• To investigate and compare the IET mechanisms in two distinct fumarate reductases: Flavocytochrome c(3) (Fcc(3)) and the Escherichia coli fumarate reductase complex (FrdAB).
• To utilize square-wave voltammetry (SWV) as a kinetic tool to probe electron transfer pathways and rates.
• To determine if electron transfer occurs directly or via hopping through redox cofactors.

Main Methods:

• Adsorption of active fumarate reductases onto an electrode surface.
• Application of square-wave voltammetry (SWV) with large amplitudes to measure electron transfer kinetics.
• Kinetic modeling of electron transfer rates as a function of overpotential.
• Comparison of experimental data with direct and hopping electron transfer models.

Main Results:

• Both Fcc(3) and FrdAB exhibited electron transfer to/from their buried active sites.
• FrdAB's electron transfer kinetics were consistent with both direct and hopping mechanisms.
• Fcc(3)'s kinetics were best explained by a direct electron transfer mechanism, potentially involving superexchange.
• For FrdAB, the maximum electron transfer rate correlated with catalytic turnover, indicating electron transfer is rate-limiting.

Conclusions:

• Enzyme structure dictates the predominant electron transfer mechanism.
• Superexchange through redox chains may enhance electronic coupling in some enzymes like Fcc(3).
• Electron transfer can be the rate-limiting step in enzymatic catalysis, as observed in FrdAB.