Alcohol dehydrogenase reversal

Many biological processes involve oxidation of alcohols to carbonyl compounds or the reverse process reduction of carbonyl compounds to alcohols Ethanol for example is metabolized m the liver to acetaldehyde Such processes are catalyzed by enzymes the enzyme that catalyzes the oxidation of ethanol is called alcohol dehydrogenase... 

The reverse reaction also occurs m living systems NADH reduces acetaldehyde to ethanol m the presence of alcohol dehydrogenase In this process NADH serves as a hydride donor and is oxidized to NAD" while acetaldehyde is reduced... 

The reaction is reversible and when the relative concentration of ethanol is high, alcohol dehydrogenase carries out the oxidation of ethanol => alcohol dehydrogenase is important in detoxication. 

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... 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

Probably the most extensively studied enzymes are those from alcohol dehydrogenase family. One enzyme from this series which has been thoroughly examined both experimentally and theoretically is liver alcohol dehydrogenase (LADH). It catalyzes the reversible conversion of an alcohol to an aldehyde by transferring hydride from substrate to the cofactor (NAD+) ... 

The stereospecificity depends upon the enzyme in question. Let us consider the enzyme alcohol dehydrogenase, which is involved in the ethanol to acetaldehyde interconversion. It has been deduced that the hydrogen transferred from ethanol is directed to the Re face of NAD+, giving NADH with the AR configuration, hi the reverse reaction, it is the 4-pro-R hydrogen of NADH that is transferred to acetaldehyde. 

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. 

Low y and Wj values correspond to small spherical reversed micelles. In these structures the enzymes are forced into the interfacial film between the oil and the water. In contrast to lipases which are activated by interfaces, alcohol dehydrogenases show much lower activity under these circumstances. With increasing water concentration (increasing Wj) the reversed micelles grow. As a result, the influence of the interface on the enzyme decreases, enhancing the activity of an ADH, as shown in Fig. 4. 

Nicotinamide-dependent enzymes operate in a highly stereospecific manner. This phenomenon was first demonstrated for alcohol dehydrogenase which catalyzes the direct and stereospecific transfer of the pro-(R) hydrogen at C-l of ethanol to the re face of NAD+, or, in the reverse direction, the pro-(R) hydrogen of NADH to the re face of acetaldehyde (equation 2) (B-71MI11001, B-79MI11000). Many other nicotinamide-dependent... 

The mechanism for synthesis of alcohols and aldehydes from amino acids has been discussed in a review by Morgan (1976). Both S. lactis and its malty variant can reversibly form keto acids from the amino acids valine, leucine, isoleucine, methionine, and phenylalanine. However, unlike S. lactis, S. lactis var. maltigenes can decarboxylate these keto acids to form aldehydes and reduce the aldehydes to their corresponding alcohols through the action of alcohol dehydrogenase in the presence of NADH. 

When ethanol is oxidized by the action of alcohol dehydrogenase (Eq. 9-73), only the pro-R hydrogen atom is removed. If the reaction is reversed in such a way that deuterium is introduced into ethanol from the reduced coenzyme the optically active R-2-deuterio-ethanol is formed. The ability of an enzyme to... 

Notice that addition of an atom of 2H to the re face of acetaldehyde would give R-deuteroethanol (Eq. 9-75 reverse of Eq. 9-73). This is the reaction catalyzed by alcohol dehydrogenase. Addition of 4H to the re face places the entering hydrogen in the pro-R position. Addition to the si face would place it in the pro-S... 

The reactions of Eq. 15-19 occur nonenzymatically only under the influence of strong base but dehydrogenases often catalyze similar condensations relatively rapidly and reversibly. Pyruvate inhibits lactate dehydrogenase, 2-oxoglutarate inhibits glutamate dehydrogenase, and ketones inhibit a short-chain alcohol dehydrogenase in this manner.133,693... 

The enzyme alcohol dehydrogenase, E.C. 1.1.1.1, functions to catalyze the reversible reaction of alcohol oxidation and aldehyde or ketone reduction. 

After oral administration, abacavir is rapidly absorbed, and its bioavailability is about 83%. Food does not interfere with its absorption, and it is metabolized by alcohol dehydrogenase to 5 -carboxylic acid derivative and to S -glucuronidc by glucuronidation. Abacavir does not affect the cytochrome P-450 system. In combination with other antiretroviral drugs, abacavir is indicated for the treatment of HIV-1 infection. It is more potent than other nucleoside reverse transcriptase inhibitors in reducing HIV plasma concentration and increasing CD4+ count. 

The alcohol dehydrogenases are zinc metalloenzymes which can oxidize a wide variety of alcohols to their corresponding aldehydes or ketones using nicotinamide adenine dinucleotide (NAD+) as coenzyme. These reactions are readily reversible so that carbonyl compounds may be reduced by NADH. 

The results of the temperature dependence of the reaction rates of the enantiomers of secondary alcohols with a secondary alcohol dehydrogenase (SADE1) from the thermophilic bacterium Thermoanaerobacter ethanolicus demonstrated a temperature-dependent reversal of stereospecificity (Pham, 1990) (Figure 5.16). At T < 26°C, (S)-2-butanol was a better substrate than (i )-2-butanol on the basis of kCSLt/KM values however, at T> 26°C, (R)-2-butanol was a better substrate than (S)-2-butanol. (S)-2-Pentanol was the preferred substrate at T < 60°C however, the data predict that (i )-2-pentanol would be preferred at T > 70°C. (S)-2-Elexanol was predicted to be the preferred enantiomer only at T > 240°C. Therefore, the concept of isoinversion temperature is as valid for enzyme reactions as for others only the range of catalytically accessible temperatures is smaller. 

This reaction should not be confused with the monooxygenation of ethanol by CYP that occurs in the microsomes. The alcohol dehydrogenase reaction is reversible, with the carbonyl compounds being reduced to alcohols. 

Ketone and Aldehyde Reduction. In addition to the reduction of aldehyde and ketones through the reverse reaction of alcohol dehydrogenase, a family of aldehyde reductases also reduces these compounds. These reductases are NADPH-dependent, cytoplasmic enzymes of low molecular weight and have been found in liver, brain, kidney, and other tissues. 

Scheme 17 Reversible photostimulation of alcohol dehydrogenase (AlcDH) by the application of the photoisomerizable azobenzene-modified NAD+ (32a) in the presence ofthe Z1 HOI monoclonal antibody. Disphorase (Dl) is used to regenerate the oxidized cofactor.

A common step in the metabolism of alcohols is carried out by alcohol dehydrogenase enzymes that produce aldehydes from primary alcohols that have the -OH group on an end carbon and produce ketones from secondary alcohols that have the -OH group on a middle carbon, as shown by the examples in Reactions 7.3.6 and 7.3.7. As indicated by the double arrows in these reactions, the reactions are reversible and the aldehydes and ketones can be converted back to alcohols. The oxidation of aldehydes to carboxylic acids occurs readily (Reaction 7.3.8). This is an important detoxication process because aldehydes are lipid soluble and relatively toxic, whereas carboxylic acids are more water soluble and undergo phase n reactions leading to their elimination. 

The steady-state kinetic studies of liver alcohol dehydrogenase (12.5 nM) are performed. The initial rates (v in /rM/rnin) with varying substrate concentrations in both directions (forward for ethanol oxidation and reverse for ethanal reduction) are given below. Evaluate their kinetic parameters and equilibrium constant. 

The reduction of NAD+ (and NADP) is reversible, and NADH is itself a reducing agent. We will first look at one of its reactions a typical reduction of a ketone. The ketone is pyruvic acid and the reduction product lactic acid, two important metabolites. The reaction is catalysed by the enzyme liver alcohol dehydrogenase. 

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