Alcohol dehydrogenase mechanism

A Zn2+ at the active site polarizes the carbonyl oxygen of acetaldehyde, allowing transfer of a hydride ion (red) from the reduced cofactor NADH. The reduced intermediate acquires a proton from the medium (blue) to form ethanol. Alcohol Dehydrogenase Mechanism... 

Zhang Y, Huang X, Li Y (2008) Negative effect of [bmim][PFJ on the catalytic activity of alcohol dehydrogenase mechanism and prevention. J Chem Technol Biotechnol 83 1230-1235... 

Uncovering of the three dimentional structure of catalytic groups at the active site of an enzyme allows to theorize the catalytic mechanism, and the theory accelerates the designing of model systems. Examples of such enzymes are zinc ion containing carboxypeptidase A 1-5) and carbonic anhydrase6-11. There are many other zinc enzymes with a variety of catalytic functions. For example, alcohol dehydrogenase is also a zinc enzyme and the subject of intensive model studies. However, the topics of this review will be confined to the model studies of the former hydrolytic metallo-enzymes. 

Ethanol is oxidized by alcohol dehydrogenase (in the presence of nicotinamide adenine dinucleotide [NAD]) or the microsomal ethanol oxidizing system (MEOS) (in the presence of reduced nicotinamide adenine dinucleotide phosphate [NADPH]). Acetaldehyde, the first product in ethanol oxidation, is metabolized to acetic acid by aldehyde dehydrogenase in the presence of NAD. Acetic acid is broken down through the citric acid cycle to carbon dioxide (CO2) and water (H2O). Impairment of the metabolism of acetaldehyde to acetic acid is the major mechanism of action of disulfiram for the treatment of alcoholism. 

Alcoholism leads to fat accumulation in the liver, hyperlipidemia, and ultimately cirrhosis. The exact mechanism of action of ethanol in the long term is stiU uncertain. Ethanol consumption over a long period leads to the accumulation of fatty acids in the liver that are derived from endogenous synthesis rather than from increased mobilization from adipose tissue. There is no impairment of hepatic synthesis of protein after ethanol ingestion. Oxidation of ethanol by alcohol dehydrogenase leads to excess production of NADH. 

The second mode of toxicity is postulated to involve the direct interaction of the epidithiodiketopiperazine motif with target proteins, forming mixed disulfides with cysteine residues in various proteins. Gliotoxin, for example, has been demonstrated to form a 1 1 covalent complex with alcohol dehydrogenase [13b, 17]. Epidithiodi-ketopiperazines can also catalyze the formation of disulfide bonds between proxi-mally located cysteine residues in proteins such as in creatine kinase [18]. Recently, epidithiodiketopiperazines have also been implicated in a zinc ejection mechanism, whereby the epidisulfide can shuffle disulfide bonds in the CHI domain of proteins, coordinate to the zinc atoms that are essential to the tertiary structure of that domain, and remove the metal cation [12d, 19],... 

The inactivation of enzymes containing the zinc-thiolate moieties by peroxynitrite may initiate an important pathophysiological process. In 1995, Crow et al. [129] showed that peroxynitrite disrupts the zinc-thiolate center of yeast alcohol dehydrogenase with the rate constant of 3.9 + 1.3 x 1051 mol-1 s-1, yielding the zinc release and enzyme inactivation. Later on, it has been shown [130] that only one zinc atom from the two present in the alcohol dehydrogenase monomer is released in the reaction with peroxynitrite. Recently, Zou et al. [131] reported the same reaction of peroxynitrite with endothelial NO synthase, which is accompanied by the zinc release from the zinc-thiolate cluster and probably the formation of disulfide bonds between enzyme monomers. The destruction of zinc-thiolate cluster resulted in a decrease in NO synthesis and an increase in superoxide production. It has been proposed that such a process might be the mechanism of vascular disease development, which is enhanced by diabetes mellitus. 

NADH. The enzymes are widely distributed in nature, being found in microorganisms, plants, and animals. Catalytic mechanism, specificity, and physical properties of the alcohol dehydrogenases have been reviewed in detail (91-93). 

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). 

Phosphorescence and ODMR are additional spectroscopies that can be used to investigate intramolecular interactions that affect tyrosine residues in proteins and polypeptides/215,216) An example is tyrosine and tyrosinate in horse liver alcohol dehydrogenase.(202) The same approach has been used to study the role of tyrosine in the mechanism of action of carboxypeptidase B.(21/,218) jn botli these proteins, as in other proteins which contain both... 

In the case of horse liver alcohol dehydrogenase, a homodimeric enzyme, Subramanian et al.(202) used the relative phosphorescence of tyrosine and tryptophan to examine the effects of various ternary complexes known to selectively quench the fluorescence of the tryptophans of each subunit. One proposed quenching mechanism is the formation of a ground-state tyrosinate in a ternary complex at neutral pH.(201) This tyrosinate, by being a resonance... 

Cook, P.F., Oppenheimer, N.J. and Cleland, W.W. (1981). Secondary deuterium and nitrogen-15 isotope effects in enzyme-catalyzed reactions. Chemical mechanism of liver alcohol dehydrogenase. Biochemistry 20, 1817-1825... 

Klinman, J.P. (1981). Probes of mechanism and transition-state structure in the alcohol dehydrogenase reaction. CRC Crit. Rev. Biochem. 10, 39-78... 

Disulfiram (Antabuse). Disnlfiram is the only medication specifically approved by the FDA as an aversion therapy for snbstance abnse, specifically alcohol abnse or dependence. Disnlfiram s mechanism of action is qnite simple it is an inhibitor of alcohol dehydrogenase, the major enzyme responsible for the metabolism. Inhibiting this enzyme resnlts in the accnmnlation of acetylaldehyde. Acetylaldehyde is primarily responsible for many of the nnmistakable symptoms of a hangover, and when it accnmnlates in the presence of disnlfiram, it produces a constellation of very nncomfortable physical symptoms. 

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. 

Rate experiments that are typically carried out in the presence of different concentrations of an alternative product (or product analog) while using the normal substrates . This approach can be particularly useful when the normal product cannot be used because it is unstable, insoluble, or ineffective (the latter indicated by a very high Ki value). Moreover, the normal product may be consumed as an essential substrate in a coupled assay system for the primary enzyme. Fromm and Zewe used the alternative product inhibition approach in their study of hexokinase. Wratten and Cleland later applied this procedure to exclude the Theorell-Chance mechanism for liver alcohol dehydrogenase. See Abortive Complexes... 

A sequential enzyme-catalyzed reaction mechanism in which two substrates react to form two products and in which there is a preferred order in the binding of substrates and release of products. Several enzymes have been reported to have this type of binding mechanism, including alcohol dehydrogenase , carbamate kinase , lactate dehydrogenase , and ribitol dehydrogenase. ... 

An enzyme reaction mechanism involving A binding before B and followed with the random release of products. In the absence of products and abortive complexes, the steady-state rate expression is identical to the rate expression for the ordered Bi Bi mechanism . A random on-ordered off Bi Bi mechanism has been proposed for a mutant form of alcohol dehydrogenase. ... 

Mechanism of Action An alcohol dehydrogenase inhibitor that inhibits the enzyme that catalyzes the metabolism of ethanol, ethylene glycol, and methanol to their toxic metabolites. Therapeutic Effect Inhibits conversion of ethylene glycol and methanol into toxic metabolites. 

Oxidation by direct H transfer from the a-carbon of alcohols to the pyrroloquinoline quinone (PQQ) cofactor of alcohol dehydrogenases was studied using ab initio quantum mechanical methods <2001JCC1732>. Energies and geometries were calculated at the 6-31G(d,p) level of theory, results were compared to available structural and spectroscopic data, and the role of calcium in the enzymatic reaction was explored. Transition state searches at the semi-empirical and STO-3G(d) level of theory provided evidence that direct transfer from the alcohol to C-5 of PQQ is energetically feasible. 

Denkel, E., Sterzel, W. Eisenbrand, G. (1987) In-vivo activation of A-nitrosodiethanolamine and other A-nitroso-2-hydroxyalkylamines by alcohol dehydrogenase and sulfotransferase. In Bartsch, H., O Neill, I. Schulte-Hermann, R., eds, The Relevance of N-Nitroso Compounds to Human Cancer. Exposures and Mechanisms (lARC Scientific Publications No. 84), Lyon, lARCPrei i, pp. 83-86... 

FIGURE 10. Essential features of the mechanism of action of liver alcohol dehydrogenase. Adapted with permission from Reference 9. Copyright (2004) ACS... 

The mechanisms that underlie ethanol s teratogenic effects are unknown. Ethanol rapidly crosses the placenta and reaches concentrations in the fetus that are similar to those in maternal blood. The fetal liver has little or no alcohol dehydrogenase activity, so the fetus must rely on maternal and placental enzymes for elimination of alcohol. 

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