Reductive reactions alcohol dehydrogenases

The yeast-induced reduction of (5e) —> (6e), (R = substituted phenyls, R = Me) is one of the very first preparative-scale alcohol dehydrogenase catalyzed reactions to have been reported. While the transformations of (5a-j) demonstrate that broad structural mlerances are possible in the substrate ketones, the enzymes are sometimes very discriminating. For example, while the BY-catalyzed reductions of the 2-, 3- and 4-substituted pyridyl ketones (5g) proceed smoothly with high stereospecificitythe analogous furanyl and thiophenyl ketones give virtually racemic product alcohols, and 2-acetylpyrrole is not a substrate at all. ... 

ALCOHOL DEHYDROGENASE-CATALYZED REACTIONS Stereoselective reduction of aldehydes and ketones... 

Under anaerobic conditions, NADH produced in glycolysis builds up. This results in a reduction in the amount of NAD+ available to support continuation of glycolysis. Organisms have two pathways for regenerating NAD+ under anaerobic conditions. Animal cells and lactic acid bacteria use the process of lactic acid fermentation. Yeast convert pyruvate to acetaldehyde in a reaction catalyzed by the enzyme pyruvate decarboxylase. This is followed by reduction of acetaldehyde to ethanol catalyzed by alcohol dehydrogenase. The reaction uses NADH and releases NAD+, which is subsequently used in glycolysis. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

Figure 8.27 Reduction of aldehyde in SCCO2 by an isolated enzyme, horse liver alcohol dehydrogenase (HLADH) [20c] (a) Reaction scheme (b) fluorinated coenzyme soluble in CO2 and (c) effect of coenzyme on the reaction.

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.

Rhin(bpy)3]3+ and its derivatives are able to reduce selectively NAD+ to 1,4-NADH in aqueous buffer.48-50 It is likely that a rhodium-hydride intermediate, e.g., [Rhni(bpy)2(H20)(H)]2+, acts as a hydride transfer agent in this catalytic process. This system has been coupled internally to the enzymatic reduction of carbonyl compounds using an alcohol dehydrogenase (HLADH) as an NADH-dependent enzyme (Scheme 4). The [Rhin(bpy)3]3+ derivative containing 2,2 -bipyridine-5-sulfonic acid as ligand gave the best results in terms of turnover number (46 turnovers for the metal catalyst, 101 for the cofactor), but was handicapped by slow reaction kinetics, with a maximum of five turnovers per day.50... 

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... 

The same authors proposed a complex system for the electrochemically driven enzymatic reduction of carbon dioxide to form methanol. In this case, methyl viologen or the cofactor PQQ were used as mediators for the electroenzymatic reduction of carbon dioxide to formic acid catalyzed by formate dehydrogenase followed by the electrochemically driven enzymatic reduction of formate to methanol catalyzed by a PQQ-dependent alcohol dehydrogenase. With methyl viologen as mediator, the reaction goes through the intermediate formation of formaldehyde while with PQQ, methanol is formed directly [77],... 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

It is worth noting that finding a secondary a-deuterium KIE larger than the EIE is not unique. In fact, it has been found in several other reactions. For instance, Cleland and co-workers (Cook et al., 1980,1981 Cook and Cleland, 1981a,b) found unexpectedly large secondary a-deuterium KIEs in some enzymatic reactions for example, a secondary a-deuterium KIE of 1.22 for the reduction of acetone catalysed by yeast alcohol dehydrogenase and a KIE of 1.34 for the reduction of cyclohexanone catalysed by horse-liver dehydrogenase. 

Yeast alcohol dehydrogenase (YADH), catalysis of reduction by NADH of acetone formate dehydrogenase (FDH), oxidation by NAD of formate horse-liver alcohol dehydrogenase (HLAD), catalysis of reduction by NADH of cyclohexanone With label in NADH, the secondary KIE is 1.38 for reduction of acetone (YADH) with label in NAD, the secondary KIE is 1.22 for oxidation of formate (FDH) with label in NADH, the secondary KIE is 1.50 for reduction of cyclohexanone (HLAD). The exalted secondary isotope effects were suggested to originate in reaction-coordinate motion of the secondary center. 

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

This zinc metalloenzyme [EC 1.1.1.1 and EC 1.1.1.2] catalyzes the reversible oxidation of a broad spectrum of alcohol substrates and reduction of aldehyde substrates, usually with NAD+ as a coenzyme. The yeast and horse liver enzymes are probably the most extensively characterized oxidoreductases with respect to the reaction mechanism. Only one of two zinc ions is catalytically important, and the general mechanistic properties of the yeast and liver enzymes are similar, but not identical. Alcohol dehydrogenase can be regarded as a model enzyme system for the exploration of hydrogen kinetic isotope effects. 

This enzyme [EC 1.1.99.8], also referred to as alcohol dehydrogenase (acceptor) and methanol dehydrogenase, catalyzes the oxidation-reduction reaction of a primary alcohol with an acceptor to generate an aldehyde and the reduced acceptor. The cofactor for this enzyme is pyrroloquinoline qutnone (PQQ). A wide variety of primary alcohols can act as the substrate. See also Alcohol Dehydrogenase... 

A major class of enzymes that catalyze oxidation-reduction reactions. This class includes dehydrogenases, reductases, oxygenases, peroxidases, and a few synthases. Examples include alcohol dehydrogenase (EC 1.1.1.1), aldehyde oxidase (EC 1.2.3.1), orotate reductase (EC 1.3.1.14), glutamate synthase (EC 1.4.1.14), NAD(P) transhydrogenase (EC 1.6.1.1), and glutathione peroxidase (EC 1.11.1.9). 

As with adults, the primary organ responsible for drug metabolism in children is the liver. Although the cytochrome P450 system is fully developed at birth, it functions more slowly than in adults. Phase I oxidation reactions and demethylation enzyme systems are significantly reduced at birth. However, the reductive enzyme systems approach adult levels and the methylation pathways are enhanced at birth. This often contributes to the production of different metabolites in newborns from those in adults. For example, newborns metabolize approximately 30% of theophylline to caffeine rather than to uric acid derivatives, as occurs in adults. While most phase I enzymes have reached adult levels by 6 months of age, alcohol dehydrogenase activity appears around 2 months of age and approaches adult levels only by age 5 years. 

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