Enzymatic Reduction Reactions

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

S Enzymatic Reduction Reaction Substrate Suitable microbe (a)... 

Drugs can also Interfere with laboratory results by negating certain nonspecific oxidation and reduction reactions essential for the chemical assay. Penicillin, streptomycin and ascorbic acid are known to react with cupric Ion thus, false positive results for glucose may occur If a copper reduction method Is used. If the specific enzymatic glucose-oxidase method Is employed, ascorbic acid can cause a false negative result by preventing the oxidation of a specific chromogen In the reaction. 

As already mentioned, macular zeaxanthin comprises two stereoisomers, the normal dietary (3/(,37()-/caxanthin and (3f ,3 S)-zeaxanthin(=(meyo)-zeaxanthin), of which the latter is not normally a dietary component (Bone et al. 1993) and is not found in any other compartment of the body except in the retina. The concentration of (tneso)-zeaxanthin in the retina decreases from a maximum within the central fovea to a minimum in the peripheral retina, similar to the situation with (3/ ,37 )-zeaxanthin. This distribution inversely reflects the relative concentration of lutein in the retina and gave rise to a hypothesis (Bone et al. 1997) that (meso)-zeaxanthin is formed in the retina from lutein. This was confirmed by an experiment in which xanthophyll-depleted monkeys had been supplemented with chemically pure lutein or (3/ ,37 )-zeaxanthin (Johnson et al. 2005). (Meyo)-Zeaxanthin was exclusively detected in the retina of lutein-fed monkeys but not in retinas of zeaxanthin-fed animals, demonstrating that it is a retina-specific metabolite of lutein only. The mechanism of its formation has not been established but may involve oxidation-reduction reactions that are mediated photochemically, enzymatically, or both. Thus, (meso)-zeaxanthin is a metabolite unique to the primate macula. 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

Fig. 13.12 General reaction scheme of NAD(P) (H)-linked enzymatic oxidation/reduction reactions. 

The reduction in enzymatic activity that results from the formation of nonproductive enzyme complexes at high substrate concentration. The most straightforward explanation for substrate inhibition is that a second set of lower affinity binding sites exists for a substrate, and occupancy of these sites ties up the enzyme in nonproductive or catalytically inefficient forms. Other explanations include (a) the removal of an essential active site metal ion or other cofactor from the enzyme by high concentrations of substrate, (b) an excess of unchelated substrate (such as ATP" , relative to the metal ion-substrate complex (such as CaATP or MgATP ) which is the true substrate and (c) the binding of a second molecule of substrate at a subsite of the normally occupied substrate binding pocket, such that neither substrate molecule can attain the catalytically active conformation". For multisubstrate enzymes, nonproductive dead-end complexes can also result in substrate inhibition in the presence of one of the reaction... 

Enzymatic and Model Carboxylation and Reduction Reactions for Carbon Dioxide Utilization -, Eds. Aresta, M. Schloss, J. V. Kluwer, 1990. 

Several factors may limit the overall rate of enzymatic reductive reactions. First, the electron transfer to the reactive metal (e.g., Co, Fe, or Ni) may be limiting. It is also possible that access of the organic substrates to the reduced metals contained within enzyme microenvironments may be limited. Mass transfer limitation is even more important in intact bacterial cells. For example, Castro et al. (1985) found that rates of heme-catalyzed reductive dehalogenations were independent of the heme content of the cells. 

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