Stimulation of peroxidase activity by decamerization related to ionic strength:: AhpC protein from Amphibacillus xylanus

AhpC protein, purified from Amphibacillus xylanus with a molecular mass of 20.8 kDa, protects cells against oxidation damage. The enzyme catalyses the reduction of hydroperoxides in cooperation with the 55 kDa flavoprotein, A. xylanus NADH oxidase (NADH oxidase-AhpC system). A. xylanus AhpC has two disulfide linkages between monomers and can act in the homodimer form. Gel-filtration column chromatography and dynamic light scattering (DLS) suggest that A, xylanus AhpC also forms a large oligomeric assembly (10-12 mers), A. xylanus AhpC was crystallized and X-ray diffraction data were collected to 3.0 Angstrom. The self-rotation function revealed fivefold and twofold axes located perpendicularly to each other, suggesting that the molecular assembly of A. xylanus AhpC is composed of ten monomers. The oligomerization of A. xylanus AhpC is affected by ionic strength in the DLS measurements. The H2O2 reductase activity of the A. xylanus NADH oxidase-AhpC system is also affected by ionic strength, and it was found that the decamerization of AhpC might be required for the activation off the NADH oxidase-AhpC system.