EPR and Redox properties of NapA from Desulfovibrio desulfuricans ATCC 27774.

J.J.G. MOURA; P.J. GONZÁLEZ; M.G. RIVAS; C.D. BRONDINO; I. MOURA

Ventura, CA (USA)

Conferencia; International Gordon Research Conference, Metals in Biology.; 2006

Nitrate reductases are enzymes that catalyze the conversion of nitrate to nitrite. We report here EPR studies in the periplasmic nitrate reductase isolated from the sulfate-reducing bacteria Desulfovibrio desulfuricans ATCC 27774. This protein, belonging to the DMSO reductase family of mononuclear Mo-containing enzymes, is constituted by a single 80 kDa unit and contains a Mo-bisMGD cofactor and a [4Fe-4S] cluster. Potentiometric-EPR-monitored redox titrations, carried out with and without nitrate in the potential range +200 / -500 mV, and EPR studies of the enzyme, in both catalytic and inhibited conditions, reveal distinct types of Mo(V) EPR active species, which indicates that the Mo site presents a high coordination flexibility. These studies show that nitrate modulates the redox properties of the Mo active site, but not that of the [4Fe-4S]. EPR studies of the enzyme in catalytic conditions suggest that the reaction mechanism initiates with the enzyme in the Mo(V) state. The possible structures and the role in catalysis of the distinct Mo(V) species detected by EPR are discussed.