Spectroscopic and functional characterization of Cu-containing nitrite reductase from Hyphomicrobium denitrificans...

The Cu-containing nitrite reductase from Hyphomicrobium denitrificans (HydNIR) has been spectroscopically and functionally characterized. The visible absorption spectrum implies that the enzyme has two type 1 Cu ions in one subunit (ca. 50 kDa). The electron paramagnetic resonance (EPR) spectrum of HydNIR is simulated assuming the sum of three distinct S = 1/2 systems: two type 1 Cu signals (axial and rhombic symmetries) and one type 2 Cu signal. The intramolecular electron transfer reaction from the type 1 Cu to the type 2 Cu at pH 6.0 does not occur in the absence of nitrite, but a very slow electron transfer reaction is observed in the presence of nitrite. The apparent first-order rate constants for the intramolecular electron transfer reactions (k(ET(intra))) in the presence of nitrite and also the apparent catalytic rate constants (k(cat)) of HydNIR decrease gradually with increasing pH in the range of pH 4.5-7.5. These pH profiles are substantially similar to each other, suggesting that the intramolecular electron transfer process is linked to the subsequent nitrite reduction process.