Alcoholism enzymes

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

The first sequence is from the enzyme citrate synthase, residues 260-270, which form a buried helix the second sequence is from the enzyme alcohol dehydrogenase, residues 355-365, which form a partially exposed helix and the third sequence is from troponin-C, residues 87-97, which form a completely exposed helix. Charged residues are colored red, polar residues ate blue, and hydrophobic residues are green. 

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. 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

For instance, for the enzyme alcohol dehydrogenase the formal systematic name would be ... 

Alcohol metabolism (Figure 6.37) occurs mainly through oxidative pathways involving the enzymes alcohol dehydrogenase (ADH), acetaldehyde dehydrogenase (ALDH),... 

The donor types D3, D4, and D6 of Keilin and Nicholls (37) all reduce compound I of Type A enzymes directly to the ferric state in a two-electron process without detectable intermediates. Each of these donors is probably also able to bind in the heme pocket of the free enzyme. Alcohols (type D3) form complexes with free ferric Type A enzymes whose apparent affinities parallel the effectiveness of the same alcohols as compound I donors (39). Formate (type D3) reacts with mammalian ferric enzyme at a rate identical to the rate with which it reduces compound I to free enz5mie (22). Its oxidation by compound I may thus share an initial step analogous to its complex formation with ferric enzyme. Formate also catalyzes the reduction of compound II to ferric enzyme by endogenous donors in the enz5mie (40, 41). Both compound I and compound II may thus share with the free enzyme the ability to ligate formate in the heme pocket. Nitrite, which is oxidized to nitrate by a two-electron reaction with compoimd I (type D4), also forms a characteristic complex with free enzyme (42). In both cases the reaction involves the donor in its protonated (HNO2) form. 

Alcohols are oxidized to aldehydes by the liver enzyme alcohol dehydrogenase, and aldehydes to carboxylic acids by aldehyde dehydrogenase. In mammals, monooxygenases can be induced by plant secondary metabolites such as a-pinene, caffeine, or isobornyl acetate. Reduction is less common and plays a role with ketones that cannot be further oxidized. Hydrolysis, the degradation of a compound with addition of water, is also less common than oxidation. 

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. 

The best-known exception to exponential kinetics is the elimination of alcohol (ethanol), which obeys a linear time course (zero-order kinetics), at least at blood concentrations > 0.02 %. It does so because the rate-limiting enzyme, alcohol dehydrogenase, achieves half-saturation at very low substrate concentrations, i.e at about 80 mg/L (0.008 %). Thus, reaction velocity reaches a plateau at blood ethanol concentrations of about 0.02 %, and the amount of drug eliminated per unit of time remains constant at concentrations above this level. 

This phenomenon of saturation is seen with alcohol (ethanol) which rapidly saturates its first metabolic enzyme, alcohol dehydrogenase, and thereafter is eliminated at a constant rate, which approximates 10 ml per hour. And this is the figure you will find in many textbooks. However, as the Cp... 

NAD is one of Nature s most important oxidizing agents it can be considered as a biological equivalent of the chromium(VI) ion. NAD is shorthand for nicotinamide adenine dinucleotide it is a co-enzyme, which together with an enzyme is essential for several life-sustaining processes (Box 2.2). On reduction it forms the corresponding 1,4-dihydropyridine, NADH, The oxidation of ethanol to acetaldehyde (ethanal) is effected by the enzyme alcohol dehydrogenase and mediated by NAD (Scheme 2.31). 

Alcohol, taken orally, is absorbed in the stomach but mostly from the small intestine into the bloodstream. It first reaches the liver, where some alcohol is immediately metabolized (first-pass metabolism) by the enzyme alcohol dehydrogenase. Alcohol is a water-soluble chemical and distributes throughout body fluids but not readily into body fat. Females, who naturally have more lipid (fat) tissue per pound of mass, will have a higher BAG than males after consuming the same amount of alcohol. 

Metabolism of ethylene glycol and methanol to their toxic products can be blocked by inhibiting the enzyme alcohol dehydrogenase with a competing drug, such as fomepizole (4-methylpyrazole). Ethanol is also an effective antidote, but it can be difficult to achieve a safe and effective blood level. 

Methyl alcohol is poisonous and is commonly used to denature ethyl alcohol. Methanol poisoning results from ingestion, inhalation of methanol vapors, or absorption through the skin. Methanol is transformed in the body to formaldehyde (H2CO) by the enzyme alcohol dehydrogenase. The formaldehyde is then metabolized to formic acid (HCOOH)... 

The mechanism of toxicity of ethylene glycol involves metabolism, but unlike previous examples, this does not involve metabolic activation to a reactive metabolite. Thus, ethylene glycol is metabolized by several oxidation steps eventually to yield oxalic acid (Fig. 7.84). The first step is catalyzed by the enzyme alcohol dehydrogenase, and herein lies the key to treatment of poisoning. The result of each of the metabolic steps is the production of NADH. The imbalance in the level of this in the body is adjusted by oxidation to NAD coupled to the production of lactate. There is thus an increase in the level of lactate, and lactic acidosis may result. Also, the intermediate metabolites of ethylene glycol have metabolic effects such as the inhibition of oxidative phosphorylation, glucose metabolism, Krebs cycle, protein synthesis, RNA synthesis, and DNA replication. 

TA medical therapy based on competition at the active site is used to treat patients who have ingested methanol, a solvent found in gas-line antifreeze. The liver enzyme alcohol dehydrogenase converts methanol to formaldehyde, which is damaging to many tissues. Blindness is a common result of methanol ingestion, because... 

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

The steady state and stopped-flow kinetic studies on the horse liver enzyme are now considered classic experiments. They have shown that the oxidation of alcohols is an ordered mechanism, with the coenzyme binding first and the dissociation of the enzyme-NADH complex being rate-determining.15,26,27 Both the transient state and steady state methods have detected that the initially formed enzyme-NAD+ complex isomerizes to a second complex 27,28 In the reverse reaction, the reduction of aromatic aldehydes involves rate-determining dissociation of the enzyme-alcohol complex,27,29 whereas the reduction of acetaldehyde is... 

B 9. Describe how you would use affinity elution to remove the enzyme alcohol dehydrogenase bound to a Cibracron blue-agarose column. 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

Ester + enzyme phosphoryl enzyme + alcohol (leaving group) U II... 

Also, by the introduction of the enzyme ferredoxin reductase, a photochemical NADPH regeneration cycle has been established. The NADPH has been utilized in the reduction of ketones to alcohols in the presence of a secondary enzyme, alcohol dehydrogenase. 

Disulfiram (Antabuse) inhibits aldehyde dehydrogenase irreversibly, causing an increase in the level of acetaldehyde, formed from ethanol by the enzyme alcohol dehydrogenase. This results in nausea, vomiting, and other symptoms in the human—hence its use as a deterrent in alcoholism. Inhibition by disulfiram appears to be irreversible, the level returning to normal only as a result of protein synthesis. 

Ethylene glycol is used as a freezing-point depressant in automotive antifreeze. It is highly toxic because the enzyme alcohol dehydrogenase and the coenzyme nicotinamide adenine dinucleotide (NAD) oxidize ethylene glycol to much more liver-toxic compounds like glyoxal, hydroxyacetaldehyde, glyoxylic... 

Index Entries Biosensors gasohol immobilized enzymes alcohol oxidase horseradish peroxidase. 

Although enzymes have the advantage of selectivity and efficiency, they are, as previously mentioned, sometimes not very fast. To increase the total reaction time in microspace, Muller et al. [428] constructed a circulating continuous flow enzyme membrane reactor and investigated the reduction of acetophenone to (S)-phenylethanol in the presence of the enzyme alcohol dehydrogenase (ADH)... 

Acetaldehyde is then reduced to ethanol (ethyl alcohol) via the Zn-containing enzyme alcohol dehydrogenase ... 

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