Alcohol dehydrogenase , zinc enzyme reactions

One of the most important metals with regard to its role in enzyme chemistry is zinc. There are several significant enzymes that contain the metal, among which are carboxypeptidase A and B, alkaline phosphatase, alcohol dehydrogenase, aldolase, and carbonic anhydrase. Although most of these enzymes are involved in catalyzing biochemical reactions, carbonic anhydrase is involved in a process that is inorganic in nature. That reaction can be shown as... 

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. 

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. 

Ethanol is metabolized primarily in the liver by at least two enzyme systems. The best-studied and most important enzyme is zinc dependent alcohol dehydrogenase. Salient features of the reaction can be seen in Fig. 35.1. The rate of metabolism catalyzed by alcohol dehydrogenase is generally linear with time except at low ethanol concentrations and is relatively independent of the ethanol concentration (i.e., zero-order kinetics). The rate of metabolism after ingestion of different amounts of ethanol is illustrated in Fig. 35.2. 

Zinc is an essential trace element. More than 300 enzymes that require zinc ions for activity are known. Most catalyze hydrolysis reactions, but zinc-containing representatives of aU enzyme classes are known, such as, for instance, alcohol dehydrogenase (an oxidoreductase), famesyl-Zgeranyl transferase (a transferase), -lactamase (a hydrolase), carbonic anhydrase (a lyase) and phosphomannose isomerase. 

Although zinc itself is not redox-active, some class I enzymes containing zinc in their active sites are known. The most prominent are probably alcohol dehydrogenase and copper-zinc superoxide dismutase (Cu,Zn-SOD). AU have in common that the redox-active agent is another transition-metal ion (copper in Cu,Zn-SOD) or a cofactor such as nicotinamide adenine dinucleotide (NAD+/NADH). The Zn(II) ion affects the redox reaction only in an indirect manner, but is nevCTtheless essential and cannot be regarded simply as a structural factor. 

The latest proposed mechanisms1462 for several zinc-containing metalloenzymes combine elements from both types of mechanism by suggesting that the substrate binds to the enzyme through the C—O group, but that in the process the metal-bound water molecule is not displaced, so that the reaction proceeds via a five-coordinate intermediate. This hybrid mechanism is discussed below in greater detail for alcohol dehydrogenase. 

We shall now briefly outline some of the features of the zinc metalloenzymes which have attracted most research effort several reviews are available, these are indicated under the particular enzyme, and for more detailed information the reader is referred to these. Attention is focussed here, albeit briefly, on carbonic anhydrases,1241,1262,1268 carboxypeptidases, leucine amino peptidase,1241,1262 alkaline phosphatases and the RNA and DNA polymerases.1241,1262,1462 Finally, we examine alcohol dehydrogenases in rather more detail to illustrate the use of the many elegant techniques now available. These enzymes have also attracted much effort from modellers of the enzymic reaction and such studies, which reveal much interesting coordination chemistry and often new catalytic properties in their own right—and often little about the enzyme system itself (except to indicate possibilities), will be mentioned in the next section of this chapter. 

These enzymes, which mainly catalyze hydrolytic reactions, have the zinc ions at their active sites. However, Zn ions also appear necessary in some cases for stabilization of the protein structure, e.g. in Cu/Zn SOD, insulin, liver alcohol dehydrogenase and alkaline phosphatases. 

The NAD+-dependent alcohol dehydrogenase from horse liver contains one catalytically essential zinc ion at each of its two active sites. An essential feature of the enzymic catalysis appears to involve direct coordination of the enzyme-bound zinc by the carbonyl and hydroxyl groups of the aldehyde and alcohol substrates. Polarization of the carbonyl group by the metal ion should assist nucleophilic attack by hydride ion. A number of studies have confirmed this view. Zinc(II) catalyzes the reduction of l,10-phenanthroline-2-carbaldehyde by lV-propyl-l,4-dihy-dronicotinamide in acetonitrile,526 and provides an interesting model reaction for alcohol dehydrogenase (Scheme 45). The model reaction proceeds by direct hydrogen transfer and is absolutely dependent on the presence of zinc(II). The zinc(II) ion also catalyzes the reduction of 2- and 4-pyridinecarbaldehyde by Et4N BH4-.526 The zinc complex of the 2-aldehyde is reduced at least 7 x 105 times faster than the free aldehyde, whereas the zinc complex of the 4-aldehyde is reduced only 102 times faster than the free aldehyde. A direct interaction of zinc(II) with the carbonyl function is clearly required for marked catalytic effects to be observed. 

Figure 3-24. A zinc(ii) complex which acts as a functional model for the hydride transfer reaction which occurs at the active site of the enzyme liver alcohol dehydrogenase.

Studies on the various zinc-activated dehydrogenases continue apace. The reduction of tra s-4-iViV-dimethylaminocinnamaldehyde (A) by liver alcohol dehydrogenase (LADH) is reported to involve the zinc at the active site of the enzyme acting as a Lewis acid and co-ordinating the substrate via the aldehyde oxygen.235 The kinetics of the reaction show that (A) 4- LADH -f NADH form a stable intermediate at pH 9, the overall reaction sequence being ... 

Hard electrophiles like Mg(C104)2 are used to activate abiotic systems. In the enzyme liver alcohol dehydrogenase (LAD) a considerably different catalytic apparatus is present a zinc ion coordinated to two cysteines and a histidine serves as a coordinating site for the carbonyl compound/alcoholate, as illustrated in equation (10). This zinc ion has amphoteric properties consistent with the capacity to activate the reaction in both directions without being consumed, in other words to act as a catalyst. Synthetic models of this catalytically active zinc have been shown to possess some catalytic activity in analogy to the enzyme (see Section L3.3.5.1iii). 

The fact that enzymes employ dynamics, should in no way be surprising -evolution knows nothing of quantum mechanics, classical mechanics, or vibration-ally enhanced tunneling. Rates of reaction are optimized for living systems using all physical and chemical mechanisms available. It is also important to point out that such protein dynamics are far from the only contributor to the catalytic effect. In fact in an enzyme such as alcohol dehydrogenase, transfer of a proton from the alcohol to the coordinated zinc atom is critical to the possibility of the reaction. The specific modulation of the chemical barrier to reaction via backbone protein dynamics is now seen to be part of the chemical armamentarium employed by enzymes to catalyze reactions. 

Several of the monatomic cations play important roles in our bodies. For example, we need calcium ions in our diet for making bones and teeth. Iron(ll) ions are found in hemoglobin molecules in red blood cells that carry oxygen from our lungs to the tissues of our bodies. Potassium, sodium, and chloride ions play a crucial role in the transfer of information between nerve cells. Enzymes (chemicals in the body that increase the speed of chemical reactions) ofren contain metallic cations, such as manganese(II) ions, iron(III) ions, copper(II) ions, and zinc ions. For example, Zn " " ions are in the center of the enzyme alcohol dehydrogenase, which is the enzyme in our livers that accelerates the breakdown of the ethanol consumed in alcoholic beverages. 

These general concepts can be exemplified by liver alcohol dehydrogenases (LADH), dimeric zinc enzymes of 80 kDa that catalyze the following class of reactions using the NADH/NAD system as coenzyme (or, really, as cosubstrate) ... 

Figure 28. Alcohol dehydrogenase, (a) Surroundings of one zinc ion. (b) the reaction catalyzed by this enzyme.

Alcohol dehydrogenases are zinc enzymes that use NAD as coenzyme according to the reaction ... 

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