Chlorate reductase

Chlorate can serve as electron acceptor under anaerobic conditions (Thorell et al. 2003 Coates et al. 1999), and chlorate reductase has been found both in organisms such as Proteus mirabilis that can reduce chlorate but is unable to use to couple this to growth, and in true chlorate-respiring organisms. 

Chlorate reductase has been characterized in strain GR-1 where it was found in the periplasm. It is oxygen-sensitive and contains molybdenum and [3Fe-4S] and [4Fe-4S] clusters (Kengen et al. 1999). 

Kengen SWM, GB Rikken, WR Hagen, CG van Ginkel, ALM Stams (1999) Purification and characterization of (per)chlorate reductase from the chlorate-respiring strain GR-1. J Bacteriol 181 6706-6711. 

Wolterink AFWM, E Schiltz, P-L Hagedoorn, WR Hagen, SWM Kengen, AIM Stams (2003) Characterization of the chlorate reductase from Pseudomonas chloritidismutans. J Bacterial 185 3210-3213. Ziegler K, K Braun, A Bdckler, G Fuchs (1987) Studies on the anaerobic degradation of benzoic acid and 2-aminobenzoic acid by a denitrifying Pseudomonas strain. Arch Microbiol 149 62-69. 

The chlorate reductase has been characterized in strain GR-1 where it was found in the periplasm, is oxygen-sensitive, and contains molybdenum, and both [3Fe-4S] and [4Fe-4S] clusters (Kengen et al. 1999). The arsenate reductase from Chrysiogenes arsenatis contains Mo, Fe, and acid-labile S (Krafft and Macy 1998), and the reductase from Thauera selenatis that is specific for selenate, is located in the periplasmic space, and contains Mo, Fe, acid-labile S, and cytochrome b (Schroeder et al. 1997). In contrast, the membrane-bound selenate reductase from Enterobacter cloacae SLDla-1 that cannot function as an electron acceptor under anaerobic conditions contains Mo and Fe and is distinct from nitrate reductase (Ridley et al. 2006). 

Nicotinic acid hydroxylase Chlorate reductase Clostridium barkerii Proteus mirabilis 300000 2 Fe/S, FAD, Se... 

The above schemes work reasonably well for certain enzyme reactions, especially for substrates where oxygen addition/loss occurs at a main group element (e.g., N, S, Se, Cl, see Table I). In addition to SO and nitrate reductase, key examples are DMSOR, trimethylamine oxide reductase, chlorate reductase, and selenate reductase. In the case of enzymes catalyzing C-based redox reactions of organic molecules, notably XDH and aldehyde oxidase, a direct OAT step is unlikely and is replaced by mechanistic steps typical of hydro-xylation (2). The essential features of the mechanism are shown in Fig. 10 for xanthine dehydrogenase/oxidase. 

The selenate reductase from Enterobacter cloacae SLDla-1 functions only under aerobic conditions, and is not able to serve as an electron acceptor for anaerobic growth, in contrast to the periplasmic enzyme from Thauera selenatis (Schroder et al. 1997). In E. cloacae there are separate nitrate and selenate reductases, both of which are membrane-bound. The selenate reductase is able to reduce chlorate and bromate though not nitrate, contains Mo, heme and nonheme iron, and consists of three subunits in an a3p3y3 configuration. 

An organism such as E. coli has at least five molybdoenzymes. It will be of great interest to look at the synthesis, availability and cellular distribution of the molybdenum cofactor and its relationship to the function of these molybdoenzymes at different stages of the cell cycle. The study of chlorate-resistant1057 and nitrate reductase-deficient1058 mutants of E. coli, which are... 

Included in Table I are molybdenum enzymes that are as yet unclassified due to their partial characterization (46—49, 58). These enzymes includes polysulfide reductase that accomplish sulfur reduction to sulfide (46), underlining its role in the global sulfur cycling. Chlorate and selenate reductase are examples of relatively rare enzymes using simple oxyanions of third-row elements as substrates (47 19, 58). 

Figure 4-12. Catalytic voltammetry of Paracoccus pantotrophus nitrate reductase (NarGH) adsorbed as a film on a PGE electrode at pH 6. (A) Increasing the electrode rotation rate from 0 to 3000 rpm removes the mass transport limitation of the catalytic response in 50 pM NO3 . (B) The enzyme s greater rate of chlorate reduction compared to nitrate reduction is reflected in greater distortion of the waveform through dispersion of sluggish interfacial electron transfer rates (see also Fig. 4-4C). Scan rate 10 mV s. Adapted from ref. 64. with permission.

Prieto, R. Fernandez, E. (1993) Toxicity of and mutagenesis by chlorate are independent of nitrate reductase activity in Chlamydomonas reinhardtii. Mol. Gen. Genet 237,429-438. 

Certain bacteria can utilize nitrate nitrogen as the sole nitrogen source for the synthesis of all nitrogen containing compounds of the cell (Payne, 1973). This nitrate assimilation can occur under both aerobic and anaerobic conditions. In other instances (Payne, 1973) nitrate serves as a terminal hydrogen acceptor under anaerobic conditions and this process is called nitrate respiration. In both cases the product of nitrate reduction is nitrite. The nitrate reductases from bacteria have been differentiated by Pichinoty and Piechaud (1968) into nitrate reductase A which is membrane bound and can reduce chlorate in addition to nitrate as a substrate and nitrate reductase B which is... 

At least 12 genes are involved in the formation of nitrate reductase in various Enterobacteriaceae, 5 nar genes have been identified in Pseudomonas aeruginosa and 13 Chi mutations have been characterized in Bacillus licheniformis (Stouthamer, 1976). The physiological properties of chlorate-resistant mutants have been characterized and their location on the circular chromosome determined. Chi mutations have a pleotropic affect such characteristics as dehydrogenase activity, cytochrome distribution, and membrane protein composition may be influenced. The different Cfi/ mutants are able to synthesize various components of the complex nitrate reductase molecule. It is possible, in some instances, to form active enzyme by mixing components extracted from the appropriate mutants (Stouthamer, 1976). 

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