Reducing agents, enzyme

Many naturally occurring substances are epoxides You have seen two examples of such compounds already m disparlure the sex attractant of the gypsy moth (Section 6 18) and m the carcinogenic epoxydiol formed from benzo[a]pyrene (Section 118) In most cases epoxides are biosynthesized by the enzyme catalyzed transfer of one of the oxy gen atoms of an O2 molecule to an alkene Because only one of the atoms of O2 is trans ferred to the substrate the enzymes that catalyze such transfers are classified as monooxy genases A biological reducing agent usually the coenzyme NADH (Section 15 11) is required as well... 

Ammonia reacts with the ketone carbonyl group to give an mine (C=NH) which is then reduced to the amine function of the a ammo acid Both mine formation and reduc tion are enzyme catalyzed The reduced form of nicotinamide adenine diphosphonu cleotide (NADPH) is a coenzyme and acts as a reducing agent The step m which the mine is reduced is the one m which the chirality center is introduced and gives only L glutamic acid... 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

Fructose bisphosphate aldolase of animal muscle is a Class I aldolase, which forms a Schiff base or imme intermediate between the substrate (fructose-1,6-bisP or dihydroxyacetone-P) and a lysine amino group at the enzyme active site. The chemical evidence for this intermediate comes from studies with the aldolase and the reducing agent sodium borohydride, NaBH4. Incubation of fructose bisphosphate aldolase with dihydroxyacetone-P and NaBH4 inactivates the enzyme. Interestingly, no inactivation is observed if NaBH4 is added to the enzyme in the absence of substrate. 

The requirement for reduction prior to DNA alkylation and crosslinking was first demonstrated by Iyer and Szybalski in 1964 [29], and can be induced both by chemical reducing agents such as sodium dithionite and thiols in vitro and by various reductive enzymes such as DT-diaphorase (NAD(P)H-quinone oxidoreduc-tase) in vitro and in vivo [47]. Much work to characterize the mechanism of reductive activation and alkylation has been carried out, principally by the Tomasz and Kohn groups, and Figure 11.1 illustrates a generally accepted pathway for mitomycin C [16, 48-50] based on these experiments, which is very similar to the mechanism originally proposed by Iyer and Szybalski [29]. 

Proteolytic enzyme from the latex of Carica papaya with an approximate molecular weight of 27000. It is differentiated from papain in electrophoresis behavior, in solubility and in substrate specifity. Isolation by acidify of papaya-latex with HCl, salting out with NaCl and following chromatographic purification. The formulation contains L-cysteine as reducing agent. 

The packed bed reactor is particularly lu ul for reactions involving solid-phase reagents such as cati Psts, immobilized enzymes,and metallic reducing agents [641,642,646,647,658]. Since solutions are added in this isase, there are no... 

The final step after peroxide bleaching is to ensure that the goods do not contain residual peroxide. Reducing agents have been used traditionally for this purpose. However, the possibility of using environmentally friendly catalase enzymes should not be overlooked [87-89]. 

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. 

Morowitz et al. (1995) chose a completely different route. They were able to synthesize glutamic acid from a-ketoglutarate, ammonia and formic acid in an aqueous medium without using enzymes. It is noteworthy that the reducing agent, formic acid, is one of the main products in the Miller-Urey experiment. 

Most poly(HA) depolymerases are inhibited by reducing agents, e.g., dithio-erythritol (DTT), which indicates the presence of essential disulfide bonds, and by serine hydrolase inhibitors such as diisopropyl-fluoryl phosphate (DFP) or acylsulfonyl derivates. The latter compounds covalently bind to the active site serine of serine hydrolases and irreversibly inhibit enzyme activity [48]. 

When the enzyme alcohol dehydrogenase converts acetaldehyde to ethanol, NADH acts as a reducing agent by transferring a hydride from C4 of the nicotinamide ring to the carbonyl group of acetaldehyde. 

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