Electron paramagnetic resonance studies on the molybdenum center of assimilatory NADH:nitrate reductase from Chlorella vulgaris.
The assimilatory nitrate reductase from Chlorella contains flavin, heme, and molybdenum as prosthetic groups. The molybdenum in assimilatory nitrate reductase is associated with a pterin moiety (molybdopterin) as evidenced by the ability of the enzyme to donate active molybdenum cofactor to the Neurospora nitrate reductase mutant nit-1 and by the oxidative conversion of the pterin to two well characterized fluorescent derivatives. The properties of the molybdenum center have been examined by EPR spectroscopy. A molybdenum V signal, absent in the resting enzyme, is elicited upon reduction with NADH and abolished upon reoxidation with nitrate. Reaction of the reduced enzyme with cyanide also abolishes the molybdenum V signal. The line shape and g values of the signal show pH dependence analogous to those observed previously with hepatic sulfite oxidase. The gav for molybdenum V at pH 7.0 was 1.977 and at pH 9.0, 1.961. The signal observed at pH 7.0 exhibits interaction with a single exchangeable proton. Potentiometric titration of the molybdenum center at pH 7.0 indicates that the oxidation-reduction potentials of the molybdenum VI/V and molybdenum V/IV couples are -34 and -54 mV, respectively. These potentials are significantly different from the potentials of the molybdenum center of respiratory-type nitrate reductase and in fact quite closely resemble those of hepatic sulfite oxidase. The oxidized enzyme exhibits the EPR signal of a low spin ferric heme which is abolished upon reduction with NADH.