Inter-domain electron transfer in cellobiose dehydrogenase: modulation by pH and divalent cations

Area of Science:

• Biochemistry
• Enzymology
• Fungal Biology

Background:

• Cellobiose dehydrogenase (CDH) is a unique extracellular enzyme from wood-decomposing fungi.
• It functions as an electron transfer protein, mediating electron transfer from FADH2 to haem b.
• Its role in activating lytic polysaccharide mono-oxygenase for cellulose degradation is crucial but not fully understood, especially regarding pH optima and inter-domain electron transfer (IET) rates.

Purpose of the Study:

• To investigate the factors influencing inter-domain electron transfer (IET) in CDH, specifically pH and ion effects.
• To elucidate the mechanism behind varying pH optima and IET rates in different CDHs.
• To understand the role of the cytochrome domain in CDH function and its interaction with the flavodehydrogenase domain.

Main Methods:

• Kinetic techniques were employed to study CDH domain interactions and IET.
• Molecular docking was used to assess domain interactions at the molecular level.
• The influence of pH and various ions on IET rates was systematically analyzed.

Main Results:

• Elimination of IET at neutral or alkaline pH is attributed to electrostatic repulsion preventing a necessary closed conformation.
• Divalent alkali earth metal cations, at concentrations above 3 mM, neutralize this repulsion via a bridging effect, enhancing IET rates.
• This cation effect is mediated by charged amino acid clusters and haem b propionate groups at the domain interface, rather than specific binding sites.

Conclusions:

• A closed conformation of both CDH domains is essential for efficient IET.
• Electrostatic repulsion at neutral/alkaline pH hinders this closed conformation, reducing IET.
• Divalent cations can overcome this repulsion, facilitating IET and potentially increasing FAD reduction rates through an electron pulling effect in the closed conformation.