Reducing enzymes

Study of the temperature optimum of pectinesterase activity showed, that peak of pectinesterase activity was observed at the temperature equal to 45 °C. It is shown on Figure 1 that pectinesterase was stable at pH 4 — 5. At pH 2 activity of the enzyme reduced by 25% in 60 min., at pH 3 and pH 6 it decreased by 6 — 8%. At pH 8 the activity decreased by 90.7% during the same time. At pH 9 the enzyme activity was inactivated during 15 min. 

Guy, J., Ellis, E., Hope, G. and Rao, N.A. (1989b). Antioxidant enzymes reduce loss of blood-brain barrier integrity m experimental optic neuritis. Arch. Ophthalmol. 107, 1359-1363. 

Xanthine dehydrogenase from chicken liver reacts readily with NAD as acceptor (77) while that from Micrococcus lactilyticus is inactive towards this, reacting instead with ferredoxin (18). Both enzymes react only slowly with oxygen. It seems reasonable to assume, however, that for each member of this group of enzymes, reducing substrates all react via molybdenum, as in milk xanthine oxidase. Presumably, different... 

Note Most of the above enzymes reduce 02 directly in H20, in contrast with one-electron extracellular oxidases which produce radicals. 

A different approach is the combination of a Pt-carbonyl-cluster with a special dye, Safranine O (Saf 3,7-diamino-2,8-dimethyl-5-phenylphenazinium) in an aqueous/organic two-phase system [48]. The dye is reduced in the organic phase and subsequently, in a type of phase-transfer catalysis, it reduced the cofactor in the aqueous phase. In this example l-LDH is used as a production enzyme, reducing pyruvate to L-lactate (Scheme 43.6). Complete conversion was obtained within 48 h, the mixture containing pyruvate, NAD+ and the Pt-cluster catalyst in a 600 10 1 molar ratio. The TOF for NAD+ was 15 h-1. 

Enzymes reduce the activation energy of a reaction by providing an alternative path from reactants to products, one that may break up the reaction into smaller steps that are easier to overcome (Figure 3-1). 

Fluoroquinolones are inhibitors of bacterial topoisomerase 11 (DNA gyrase). This enzyme reduces the supercoiling of DNA in order to allow separation of the two strands of DNA that are required for replication and transcription. In the first stage, topoisomerase II cleaves the two strands of DNA, after repairing it, and restores the supercoiling of DNA. ... 

SSRI of choice based on intermediate half-life, linear pharmacokinetics, absence of appreciable age effect on clearance, substantially less effect on P450 enzymes, reducing potential for drug interactions... 

A very important application of ADH from I. kefir is the diastereoselective reduction of diketones. The enzyme reduces both oxo functions highly diastereoselec-tively so that (2R,5R) -hexanediol could be produced starting from 2,5-hexanedione in quantitative yields with ee > 99% and de > 99% [7] (Scheme 2.2.4.2). For this process, resting whole cells of L. kefir were used. 

Captopril, many others Inhibit angiotensin converting enzyme Reduce angiotensin II levels reduce vasoconstriction and aldosterone secretion increase bradykinin Hypertension heart failure, diabetes Oral Toxicity Cough, angioedema teratogenic... 

Enzyme inhibition. The enzymes of biotransformation may be inhibited by a single exposure to chemicals. This occurs by several mechanisms formation of a complex, competition between substrates, destruction of the enzyme, reduced synthesis of the enzyme, allosteric effects, and lack of cofactors. The consequences will depend on the role of metabolism in toxicity in the same way as induction (see above). 

The enteric bacterium Enterobacter cloacae produces a nitroreductase that reduces nitrofurans, nitroimidazoles, nitrobenzene derivatives, and quinones (Bryant DeLuca, 1991). This oxygen-insensitive enzyme has been purified and is known to require FMN to transfer reducing equivalents from NAD(P)H to the nitroaromatic compounds, TNT being the preferred substrate. Aerobically, this enzyme reduces nitrofurazone through the hydroxylamine intermediate, which then tautomerizes to yield an oxime end-product. Anaerobically, however, the reduction proceeds to the fully reduced amine adduct. When E. cloacae was grown in the presence of TNT, the nitroreductase activity increased five- to tenfold. 

Low-molecular-mass thiols such as coenzyme A and protein-bound thiol cofactors such as phospho-pantetheine are present in all cells. Their SH groups can also be oxidized to disulfides and it is of interest that in resting bacterial spores these compounds exist largely as disulfides or mixed disulfides. Upon germination of the spores special enzymes reduce the disulfides.136 Some proteins involved in control of protein synthesis contain SH groups that add covalently to C-6 atoms of a uracil ring in specific mRNA molecules. Control of their state of reduction may also be important.137... 

The aldehyde ferredoxin oxidoreductase from the hyperthermophile Pyrococcus furiosus was the first molybdopterin-dependent enzyme for which a three-dimensional structure became available.683,684 The tungstoenzyme resembles that of the related molybdo-enzyme (Fig. 16-31). A similar ferredoxin-dependent enzyme reduces glyceraldehyde-3-phosphate.685 Another member of the tungstoenzyme aldehyde oxidoreductase family is carboxylic acid reductase, an enzyme found in certain acetogenic clostridia. It is able to use reduced ferredoxin to convert unactivated carboxylic acids into aldehydes, even though E° for the acetaldehyde/acetate couple is -0.58 V.686... 

Reductive acetyl-CoA pathway 4-5 3 NAD(P)H, 2-3 ferredoxin, 1 H2 (in methanogens) Acetyl-CoA synthase/ CO dehydrogenase, formate dehydrogenase, pyruvate synthase C02 Acetyl-CoA, pyruvate Acetyl-CoA synthase/CO dehydrogenase, enzymes reducing C02 to methyltetrahydropterin... 

H ydroxypropionate / 4- hydroxybutyrate cycle 9 6 NAD(P)H Acetyl-CoA/propionyl-CoA carboxylase HCOJ Acetyl-CoA, succinyl-CoA Acetyl-CoA/propionyl-CoA carboxylase", enzymes reducing malonyl-CoA to propionyl-CoA, methylmalonyl-CoA rnutase, 4-hydroxybutyryl-CoA dehydratase... 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

Enzyme inhibitors Inhibitors are substances that lower the activity of enzymes, reducing the speed of their reactions. Some inhibitors can even completely stop enzyme-catalyzed reactions. They are found naturally, but are also produced artificially as drugs, pesticides, and other substances. The most successful inhibitors are those that have a structure very similar to that of a substrate, so that they can bind to enzyme active sites. If the active sites are occupied by an inhibitor, the enzyme is no longer available to bind its real substrate. Other inhibitors do not look like substrates. Instead, they bind on an enzyme so as to distort its shape the geometry of the active site is changed so that the substrate no longer fits. 

---