Vanadium-dependent Nitrate Reductases

There are two reports on partially characterised vanadium-containing, molybdenum-free nitrate reductases, isolated from the bacteria Thioalkalivibrio nitratireducens and Pseudomonas isachenkoviiV T. nitratireducens is a facultative anaerobic, alkali- and halophilic bacterium living in soda lakes. Its nitrate reductase has a molecular mass of 

The molybdenum environment in the oxidised form of the molybdopterin cofactor of E. coli nitrate reductase A (66), and a model compound (67) for the vanadium centre in a putative vanadium-containing nitrate reductase containing an analogous cofactor. 

195 kDa. The enzyme, which consists of four subunits, contains vanadium and iron in a 1 3 ratio and a haeme-c type cofactor. The molybdopterine cofactor is absent, i.e. vanadium is not coordinated to a dithiolene moiety. The enzyme reduces nitrate (and other substrates such as nitrite, bromate and selenate) under anaerobic or micro-oxic conditions, using thiosulphate as electron donor. Interestingly, it also exhibits peroxidase and haloperoxidase activity in that it oxidises, by H2O2, t -dianisidine and halogenates monochlorodimedone. 

The periplasmatic vanadium-containing nitrate reductase from P. isachenkovii has a molecular mass of 220 kDa (four subunits). The pterin cofactor is again absent. In media supplemented with vanadate and nitrate, vanadate is first reduced by a membrane-bound reductase using NADH as electron donor. This dissimilatory reduction was followed by nitrate consumption.I ] 

Although the question of whether or not vanadium-dependent nitrate reductases actually exist has still to be settled, the ability of bacteria to use vanadate(V) as a primary electron acceptor is well established by now and will be addressed in the next section. 

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