EPR and kinetic properties of NapA from Desulfovibrio desulfuricans ATCC 27774

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

Münster (Germany)

Simposio; International Symposium on Microbial Sulfur Metabolism.; 2006

Anaerobic respiration with sulphate as terminal electron acceptor is a central component of the global sulphur cycle and is exhibited exclusively by prokaryotes. Sulphate reducing bacteria (SRB) are thus of major functional and numerical importance in many ecosystems, such as, cyanobacterial microbial mats, marine sediments, oil fields environments, deep-sea hydrothermal vents and even in human diseases. SRB were assumed to be microorganisms using only a limited spectrum of organic substrates with sulphate as terminal electron acceptor. Currently, SRB appear to be the microorganisms that reduce the greatest number of electron acceptors, including inorganic sulphur compounds and various other organic and inorganic compounds like nitrate and nitrite. The dissimilatory reduction of nitrate and nitrite (also called ammonification) can function as the sole energy-conserving process in some sulphate-reducing eubacteria. Desulfovibrio desulfuricans (Dd) ATCC 27774 can use either inorganic sulphur or nitrogen compounds as terminal electron acceptors. The periplasmic nitrate reductase (Nap) catalyzes the conversion of nitrate to nitrite, which is straight reduced to ammonia in the periplasm by the ccNir membrane complex. EPR and kinetic studies has been made in the periplasmic nitrate reductase isolated from Dd ATCC 27774. This protein, belonging to the DMSO reductase family of mononuclear Mo-containing enzymes, is constituted by a single 80 kDa subunit and contains a Mo-bisMGD cofactor and a [4Fe-4S] cluster [3,4]. Potentiometric-EPR-monitored redox titrations and EPR studies in both catalytic and inhibiting conditions, reveal distinct types of Mo(V) EPRactive species. The possible structures and the role in catalysis of the distinct Mo(V)species detected by EPR are discussed.