Oxalate decarboxylase uses electron hole hopping for catalysis

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Oxalate decarboxylase (EC 4.1.1.2) is a hexameric enzyme from Bacillus subtilis that catalyzes oxalate decarboxylation under low-pH stress.
• Enzyme catalysis relies on manganese (Mn) ions within two cupin domains, specifically Mn(III) at the N-terminal site.

Purpose of the Study:

• To elucidate the mechanistic role of a second, C-terminal Mn ion in oxalate decarboxylase activity.
• To investigate the contribution of a tryptophan pair (W96/W274) in mediating inter-subunit electron hole hopping between Mn ions.

Main Methods:

• Theoretical analysis of electron hole-hopping pathways.
• Site-directed mutagenesis of key tryptophan residues (W96, W274).
• X-ray crystallography to determine structural integrity of mutants.

Main Results:

• A tryptophan pair (W96/W274) facilitates inter-subunit electron hole hopping between C-terminal and N-terminal Mn ions, reducing the Mn-Mn distance.
• Mutations of W96 or W274 to phenylalanine significantly decreased catalytic efficiency, while tyrosine substitutions largely restored activity.
• Identified additional electron hole-hopping networks to the protein surface, potentially preventing enzyme overoxidation.

Conclusions:

• Multistep electron hole hopping between two Mn ions, mediated by tryptophan residues, is essential for oxalate decarboxylase function.
• Aromatic residues serve as critical hopping stations in protein electron transfer mechanisms.
• The enzyme possesses protective pathways against high manganese oxidation states during catalysis.