Reductases tmao reductase

Fig.l. Gene maps of recombinant plasmids pTOH, pTTOH, pEOH, and pETOH. Abbreviations Pnc, trc promoter Px7, T7 and lac hybrid promoter Tat, twin-arginme TorA signal sequence of TMAO reductase OPH, organophosphoms hydrolase gene Hisg, hexa-histidine affinity tag. 

Various bacteria grow anaerobically using trimethylamine-N-oxide (TMAO) as an alternative electron acceptor of a respiratory transport chain (31, 32). The energy-yielding reaction involves the conversion of TMAO to tetramethylamine (TMA) catalyzed by a TMAO reductase (TMAOR). 

L = OSer for DMSOR, TMAO reductase L = SCys for dissimilatory nitrate reductase L = SeCys for formate dehydrogenase... 

In these investigations, the Mo- and W-grown cells were found to contain the same amount of DMSOR. This result is in contrast to that of Santini et al, who observed that the amount of W-substituted TMAO reductase, produced from E. coli by genetic manipulation of the pathway for metal uptake, was ca. 15% of the level found with Mo. ... 

Members of the dimethylsulfoxide (DMSO) reductase family have the molybdenum ligated by two MPTs, one 0x0 group, and the donor atom from the side chain of an amino acid residue—O7 of a serinyl residue (DMSO reductase or trimethylamineoxide (TMAO) reductase), S7 of a cysteinyl residue (dissimilatory nitrate reductase), or Sc7 of selenocys-teinyl residue (formate dehydrogenase). 

In this context, the study by Buc et al. shall be specifically mentioned here as they were able to substitute molybdenum by tungsten in the TMAO-reductase of E. coliJ"° After purification, the isolated enzymes (wild type and... 

Interestingly, the tungsten-substituted TMAO-reductase has an almost twofold higher catalytic efficiency (8.53 x lo L moT s ) than the molybdenum one (3.75 X10 L mol s ), although the value was just half of the value for the molybdenum-containing enzyme (A eat" = 53.6 s" k ° = 94.7 s ). This is obviously caused by its much smaller Michaelis-Menten constant. 

OCh 114 is capable of anaerobic respiration. Trimethyl amine N-oxide (TMAO) is one of the terminal oxidant (7). We have found tha the electron transport chain of the TMAO respiration of this bacterium is differenct from that of the related facultative phototrophs at two points (7) cytochrome hrSji complex is probably involved in OCh 114 while it is bypassed in Rhodobacter capsulatus (8) TMAO reductase of OCh 114 does not reduce dimethylsulfoxide while a single enzyme reduces both TMAO and dimethylsulphoxide in Rhodobacter sphaeroides (9) and R. capsulatas (10). 

We did the same experiments using purified cytochrome 2 (trace b) in the presence of the soluble fraction, which contained TMAO reductase. The oxidation was very slow. We observed faster oxidation with less pure cytochrome 2 preparation, which contained small amount of other cytochrome(s) (data not shown). Probably, cytochrome 2 is not the direct electron donnor to TMAO reductase. Cytochrome with lower molecular weight, especially the one with the medium size, is possibly the electron donner to the reductase. 

Most marine fishes contain trimethylamine oxide (TMAO) this colorless, odorless, and flavorless compound is degraded to trimethylamine, which gives a fishy odor and causes consumer rejection. This compound is not present in land animals and freshwater species (except for Nile perch and tila-pia from Lake Victoria). TMAO reductase catalyzes the reaction and is found in several fish species (in the red muscle of scombroid fishes and in the white and red muscle of gadoids) and in certain microorganisms Enterobacteriaceae, She-tvanella putrefaciens). 

The conditions under which these function and their regulation depend on the organism. For example, in Escherichia coli, oxygen represses the synthesis of the other reductases, and under anaerobic conditions the reductases for fumarate, DMSO, and TMAO are repressed by nitrate. This does not apply to Wolinella succinogenes in which sulfur represses the synthesis of the more positive electron acceptors nitrate and fumarate (Lorenzen et al. 1993). The DMSO reductase from Escherichia coli (Weiner et al. 1988) has a broad substrate versatility, and is able to reduce a range of sulfoxides and A-oxides. Anaerobic sulfate reduction is not discussed here in detail. 

Metabolism of trimethylamine oxide in fish muscle involves an enzyme-catalyzed oxidation-reduction reaction. The enzyme responsible for the conversion of trimethylamine oxide to trimethylamine is known as trimethylamine-W-oxide reductase. This enzyme acts on nicotinamide adenine dinucleotide (NADH) and TMAO to produce NAD+, trimethylamine and water (Fig. 13.13.1). TMAO acts as the oxidizing agent and is reduced, while NADH undergoes oxidation as the reducing agent. 

Other members of this family that have been structurally determined by X-ray diffraction include formate dehydrogenase (FDH), trimethylamine oxidase (TMAO), dissimilatory nitrate reductase(NAP), and most recently, arsenite oxidase (AsO). Only the distinctive points of their structures will be briefly described here. 

The conditions under which these function and their regulation depend on the organism. For example, whereas in E. coli, oxygen represses the synthesis of the other reductases, and under anaerobic conditions the reductases for fumarate, DMSO, and TMAO are repressed by nitrate, this is not the case for Wolinella succinogenes... 

Almost all of the Mo and W enzymes show paramagnetic pentavalent forms that can be studied by methods such as EPR spectroscopy, and EPR in particular has been extensively used to study the enzymes. Beeause the pentavalent form is typieally only a fraction of the total metal present, few parallel studies using XAS have been reported. Rhodobacter DMSO reduetase, however, is one notable exeeption. In this case there are two different forms that show essentially 100% of Mo as the Mo(v) form. The first is an inhibited species, which forms with vicinal diols such as glycerol, and is a stable Mo(v) entity that is resistant to both oxidation and reduction. This species was studied in the first XAS report on DMSO reductase, and was fou nd to be a six-coordinate complex with four equivalent sulfurs and two Mo-0 bonds, probably from the vicinal diol coordinated to Mo(v). The stability of this spe-eies means that it has little catalytic relevance, but the second quantitatively Mo(v) DMSO reductase species is formed by reduction with the alternative substrate trimethylamine Af-oxide (TMAO). This substrate forms the most... 

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