Cosubstrates

The E. coli enzyme accepts substitution on either cosubstrate propanal, acetone or 1-fluoro-2-propanone can replace the donor and a variety of aldehydes can replace the acceptor moiety 3. Shortcomings are the relatively low conversion rates obtained for any substrate analog and the as yet unidentified level of relative stereocontrol induced upon substitution at the nucleophilic carbon. 

All NOS isoforms utilize L-arginine as the substrate, and molecular oxygen and reduced nicotinamide adenine dinucleotide phosphate (NADPH) as cosubstrates. Flavin adenine dinucleotide (FMN), flavin mononucleotide (FAD), and (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) are cofactors of the enzyme. All NOS isoforms contain heme and bind calmodulin. In nNOS and eNOS,... 

Interestingly, the E. coli enzyme s relaxed acceptor specificity allows for substitution of both cosubstrates, albeit at strongly reduced (<1% of v, catalytic rates. Propanal, acetone, or fluoroacetone can replace ethanal as the donor in the synthesis of variously substituted 3-hydroxyketones such as (112) or (113) (Figure 10.41)... 

In Figure 3 the merits of the two processes for p-xylene oxidation are compared. The main disadvantages of the Eastman Kodak/Toray cooxidation method are the need for a cosubstrate (acetaldehyde of methylethylketone) with concomitant formation of a coproduct (0.21 ton of acetic acid per ton product) and high catalyst concentration. The Amoco MC process, on the other hand, has no coproduct and much lower catalyst concentrations but has the disadvantage that the bromide-containing reaction mixture is highly corrosive, necessitating the use of a titanium-lined reactor. 

The other oxygen atom is reduced to water, an additional electron donor or cosubstrate (Z) being necessary for this purpose. 

Aerobic biodegradation of trichloroethylene occurs by cometabolism with aromatie eompounds (Ensley 1991) and thus requires a cosubstrate such as phenol (Nelson et al. 1987, 1988) or toluene (Fan and Scow 1993). Trichloroethylene degradation by toluene-degrading baeteria has been demonstrated in the presence, but not absence, of toluene (Mu and Scow 1994). Isoprene, a structural analog of trichloroethylene, has also been used as a cosubstrate for triehloroethylene oxidation by some bacteria (Ewers et al. 1990). One source of inhibition of degradation in the absence of cosubstrate may be the toxieity of triehloroethylene itself to indigenous bacteria. 

The complexity introduced by exposure of an established mixed culture growing with a single substrate to an alternative cosubstrate is illustrated by the following. A stable mixed culture of Pseudomonas putida mt-2, P. putida FI, P. putida GJ31, and Burkholderia cepacia G4 growing with limited concentrations of toluene was established. Exposure to TCE for a month resulted in the loss of viability of the last three organisms, and resulted in a culture dominated by P. putida mt-2 from which mutants had fortuitously arisen (Mars et al. 1998). 

The importance of the nature of the cosubstrate was shown by Todini and Hulshoff Pol,9 who... 

Engineered systems may also incorporate the addition of a cosubstrate to help establish an active microbial community and thereby accelerate the biodegradation of oxygenates. Hydrocarbon... 

A mutant E. coli strain LS5218 (fadR atoC) was employed for the synthesis of P(3HB-co-3HV) copolymer since this mutant strain constitutively expresses the enzymes involved in the transport and utilization of short chain fatty acids [58, 59]. P(3HB-co-3HV) could be synthesized by a recombinant E. coli strain LS5218 harboring the R. eutropha PHA biosynthesis genes when propionic acid or valeric acid was added as a cosubstrate [58,60]. The P(3HB-co-3HV) copolymer consisting of up to 40 mol% of 3HV could be produced. An alternative method that allowed synthesis of P(3HB-co-3HV) using propionic acid or valeric... 

The class II secreted fungal heme peroxidases include the LMPs LiP, MnP and VP [70]. All of these enzymes are extracellular and contain protoporphyrin IX (heme) as prosthetic group. They use H2O2 or organic hydroperoxides as electron accepting cosubstrates during the oxidation of diverse compounds. They are secreted as glycosilated, 35-38 kDa size proteins. 

For luciferin, a firefly luciferase cosubstrate, another method of retention has been evaluated which consisted of incorporating the substrate in acrylic microspheres during their formation, these last being then confined in a polymeric matrix31. Using the suitable co-immobilized enzymes (adenylate kinase and creatine kinase), the three adenylic nucleotides (ATP, ADP and AMP) could be assayed continuously and reproducibly with a selfcontainment working time of 3 h. 

Acid Orange 7 Granular activated carbon-biofilm configured packed column With initial 500 mg L-1 dye concentration, a complete decolorization was achieved in all runs although the cosubstrates added into the BGAC-packed column system reduced until to zero [177]... 

The decreased level of free inositol leads to a reduction in the rate of Ptdlns resynthesis and to the subsequent accumulation of cytidine mo-nophosphorylphosphatidate (CMP-PA), the cosubstrate for the resynthesis, and of the other lipid metabolites, phosphatidic acid (PA) and DAG... 

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