Dehydrogenase-catalyzed oxidation reaction

This is the case with NAD(P)H-dependent dehydrogenases, where enzymes find applications in several synthetic processes (comprising the reduction of aldehydes, ketones, carboxylic acids, double, and triple carbon-carbon bonds), aimed at the preparation of chiral enantiopure bioactive compounds and of building blocks for fine chemicals and pharmaceutical products. Moreover, dehydrogenase-catalyzed oxidation reactions are gaining increasing interest as an environmentally friendly alternative to chemical oxidation processes, especially in those cases where a defined selectivity (either stereo-, regio-, or chemoselectivity) is required as well [1]. 

Figure 11-4. Mechanism of oxidation and reduction of nicotinamide coenzymes. There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AHj. One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H ) and is transferred to the 4 position, where it may be attached in either the A or the B position according to the specificity determined by the particular dehydrogenase catalyzing the reaction. The remaining hydrogen of the hydrogen pair removed from the substrate remains free as a hydrogen ion.

Alcohol dehydrogenases catalyze oxidation of alcohols in a reaction dependent on the pyridine nucleotide NAD+ [Eq. (5)]. Since the reaction is reversible, alcohol dehydrogenases also catalyze the reduction of aldehydes by... 

While most alkaloids do not contain aldehydes when they enter mammalian, microbial, or plant tissues, this functional group may become important when formed as a metabolite of alcohols (via alcohol dehydrogenase) or amines (via oxidative dealkylation and oxidative deamination). Aldehyde dehydrogenases catalyze oxidation of aldehydes to the corresponding carboxylic acids. The physical properties, catalytic mechanism, and specificity of this group of enzymes has been reviewed (99). The general reaction catalyzed by aldehyde dehydrogenase is seen in Eq. (9). 

This enzyme [EC 1.3.7.1], also referred to as 6-oxotetra-hydronicotinate dehydrogenase, catalyzes the reaction of l,4,5,6-tetrahydro-6-oxonicotinate with oxidized fer-redoxin to produce 6-hydroxynicotinate and the reduced ferredoxin. 

Further transfer of the acyl group to coenzyme A is catalyzed by the same enzyme. This displacement reaction produces reduced lipoic acid. A third enzyme, dihydrolipoyl dehydrogenase, catalyzes oxidation of this product back to the disulfide form. The electrons lost in that oxidation are transferred first to an enzyme-bound flavin (not shown in the figure) and then to NAD +. ... 

In fact, the a-ketoglutarate/glutamate dehydrogenase is a generally applicable method for the regeneration of NAD and NADP in laboratory scale productions. Both components involved are inexpensive and stable. Quite recently, a method for the oxidation of the reduced nicotinamide coenzymes based on bacterial NAD(P)H oxidase has been described [225], This enzyme oxidizes NADH as well as NADPH with low Km values. The product of this reaction is peroxide, which tends to deactivate enzymes, but it can be destroyed simultaneously by addition of catalase. The irreversible peroxide/catalase reaction favours the ADH catalyzed oxidation reaction, and complete conversions of this reaction type are described. 

The MTT group has also been implicated in the intolerance to alcohol a.ssociatcd with certain injectable cephalosporins ccfamandole, cefotcian. ccfmetazolc, and cefoperazone. Thus, disulfiram-like reactions, attributed to the accumulation of acetaldehyde and resulting from the inhibition of aldehyde dehydrogenase-catalyzed oxidation of ethanol by M lT-contuining cephalo.sporins. " may occur in patients who have consumed alcohol before, during, or shortly after the course of therapy. 

C and 1-E) that is either an aldehyde or a ketone. Note that reactions 1-C and 1-E act not only on metabolites, but also on xenobiotic alcohols, and are reversible (i.e., reactions 1-D and 1-F) because dehydrogenases catalyze the reactions in both directions. And whereas a ketone is seldom oxidized further, aldehydes are good substrates for aldehyde dehydrogenases or other enzymes and lead irreversibly to carboxylic acid metabolites (reaction 1-G). A classical example is that of ethanol, which in the body exists in redox equilibrium with acetaldehyde this metabolite is rapidly and irreversibly oxidized to acetic acid. 

Today, the utilization of a dehydrogenase-catalyzed reduction reaction is still the most widespread approach for the regeneration of oxidized NAD(P)+. Its principle is displayed in Fig. 16.2-2. 

Figure 16.2-3. Intrasequential regeneration of NAD(P) The strategy applied is the synthetic coupling of a dehydrogenase-catalyzed oxidation and a regeneration reaction yielding the final product and NAD(P) regeneration.

Reaction 4. In this oxidation reaction the hydroxyl group of the p-carbon is now dehydrogenated. NAD+ is reduced to form NADH that is subsequently used to produce three ATP molecules by oxidative phosphorylation. L- -Hydroxyacyl-CoA dehydrogenase catalyzes this reaction. 

NADH dehydrogenase Catalyzes oxidative phosphorylation reactions... 

A number of enzymes which catalyze oxidation reactions, including mammalian lysyl and plasma amine oxidases and bacterial alcohol dehydrogenases, have been determined to utilize pyrroloquinoline quinone (PQQ, methoxatin) as a cofactor (Duine et al., 1987). Substrates of the amine oxidases appear to be activated for a-proton abstraction by formation of a Schiff base with PQQ, fol-... 

In addition to the microsomal monooxygenases, other oxidases and dehydrogenases that catalyze oxidation reactions are present in the mitochondrial and soluble tractions of tissue homogenates. 

Figure 8. (A) Reaction progress curve for the yeast alcohol dehydrogenase-catalyzed oxidation of NADH with acetaldehyde and (B) the replot of the data from the progress curve according to the linear transformation of the rate equation (18.73) (Leskovac, 2000).

Malate dehydrogenase is the enzyme that catalyzes the oxidation of the secondary alcohol group of malate to a ketone. (We will see that it is one of the reactions in the catabolic pathway known as the citric acid cycle see Section 25.10.) The oxidizing agent in this reaction is NAD. Most enzymes that catalyze oxidation reactions are called dehydrogenases. Recall that the number of C—H bonds decreases in an oxidation reaction (Section 11.5). In other words, dehydrogenases remove hydrogen. 

In a study of the alcohol dehydrogenase catalyzed oxidation of ethanol, the molar concentration of ethanol decreased in a first-order reaction from 220 mmol dm" to 56.0 mmol dm" in 1.22 x 10 s. 

The NAD- and NADP-dependent dehydrogenases catalyze at least six different types of reactions simple hydride transfer, deamination of an amino acid to form an a-keto acid, oxidation of /3-hydroxy acids followed by decarboxylation of the /3-keto acid intermediate, oxidation of aldehydes, reduction of isolated double bonds, and the oxidation of carbon-nitrogen bonds (as with dihydrofolate reductase). 

The third reaction of this cycle is the oxidation of the hydroxyl group at the /3-position to produce a /3-ketoacyl-CoA derivative. This second oxidation reaction is catalyzed by L-hydroxyacyl-CoA dehydrogenase, an enzyme that requires NAD as a coenzyme. NADH produced in this reaction represents metabolic energy. Each NADH produced in mitochondria by this reaction drives the synthesis of 2.5 molecules of ATP in the electron transport pathway. L-Hydroxyacyl-... 

Generally, NAD-linked dehydrogenases catalyze ox-idoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochon-drial pathway of fatty acid synthesis and steroid synthesis—and also in the pentose phosphate pathway. 

Thiamin has a central role in energy-yielding metabo-hsm, and especially the metabohsm of carbohydrate (Figure 45-9). Thiamin diphosphate is the coenzyme for three multi-enzyme complexes that catalyze oxidative decarboxylation reactions pymvate dehydrogenase in carbohydrate metabolism a-ketoglutarate dehydro-... 

Many dehydrogenase enzymes catalyze oxidation/reduction reactions with the aid of nicotinamide cofactors. The electrochemical oxidation of nicotinamide adeniiw dinucleotide, NADH, has been studied in depthThe direct oxidation of NADH has been used to determine concentration of ethanol i s-isv, i62) lactate 157,160,162,163) pyTuvate 1 ), glucose-6-phosphate lactate dehydrogenase 159,161) alanine The direct oxidation often entails such complications as electrode surface pretreatment, interferences due to electrode operation at very positive potentials, and electrode fouling due to adsorption. Subsequent reaction of the NADH with peroxidase allows quantitation via the well established Clark electrode. 

Figure 17.19 A membianeless ethanol/02 enz3fme fuel cell. Alcohol dehydrogenase and aldehyde dehydrogenase catalyze a stepwise oxidation of ethanol to acetaldehyde and then to acetate, passing electrons to the anode via the mediator NAD+/NADH. At the carhon cathode, electrons are passed via the [Ru(2,2 -bipyridyl)3] and biUverdin/bilimbin couples to bilirubin oxidase, which catalyzes O2 reduction to H2O. (a) Schematic representation of the reactions occruring. (b) Power/cmrent response for the ceU operating in buffered solution at pH 7.15, containing 1 mM ethanol and 1 mM NAD. Panel (b) reprinted from Topcagic and Minteer [2006]. Copyright Elsevier, 2006.

CL reaction can be catalyzed by enzymes other than HRP (e.g., microperoxidase and catalase) and by other substances [hemoglobin, cytochrome c, Fe(III), and other metal complexes]. The presence of suitable molecules such as phenols (p-iodophenol), naphthols (l-bromo-2-naphthol), or amines (p-anisidine) increases the light production deriving from the HRP-catalyzed oxidation of luminol and produces glow-type kinetics [6, 7], The use of other enzymes, such as glucose-6-phosphate dehydrogenase [38-41], P-galactosidase [42], and xanthine oxidase [43-46], as CL labels has been reported. 

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