Alcohol dehydrogenase , zinc enzyme

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.)...

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. 

As you can judge from Table A, transition metal cations are frequently found in enzymes. The Zn2+ ion alone is known to be a component of at least 70 different enzymes. One of these, referred to as "alcohol dehydrogenase," is concentrated in the liver, where it acts to break down alcohols. Another zinc-containing enzyme is involved in the normal functioning of oil glands in the skin, which accounts for the use of Zn2+ compounds in the treatment of acne. 

Zinc, see also Enzyme cofactors as cofactor for alcohol dehydrogenase, 205 as cofactor for carbonic anhydrase, 197-200... 

Bluish, shimmering, brittle, relatively reactive metal. Is guite guickly covered with a protective oxide layer, which is why iron is treated with zinc With copper, forms the popular alloy brass, which was already known in antiquity. Used in batteries and as a stabilizer in plastics. Zinc oxide is used as a white pigment Zinc ions are essential to all life forms, e.g., as a component of alcohol dehydrogenase and many other enzymes. Hence human beings (70 kg) carry about 2.3 g (half as much as iron). 

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. 

Alcohol dehydrogenases are a class of zinc enzymes, which catalyse the oxidation of primary and secondary alcohols to the corresponding aldehyde or ketone by the transfer of a hydride anion to NAD+ with release of a proton ... 

Zinc is the metal constituent of a number of very important enzymes including carbonic anhydrase, carboxypeptidase, thermolysin and alcohol dehydrogenase, number of... 

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. 

Another zinc-utilizing enzyme is carbonate/dehydratase C (Kannan et al., 1972). Here, the zinc is firmly bound by three histidyl side chains and a water molecule or a hydroxyl ion (Fig. 27). The coordination is that of a distorted tetrahedron. Metals such as Cu(II), Co(Il), and Mn(ll) bind at the same site as zinc. Hg(II) also binds near, but not precisely at, this site (Kannan et al., 1972). Horse liver alcohol dehydrogenase (Schneider et al., 1983) contains two zinc sites, one catalytic and one noncatalytic. X-Ray studies showed that the catalytic Zn(II), bound tetrahedrally to two cysteines, one histidine, and water (or hydroxyl), can be replaced by Co(II) and that the tetrahedral geometry is maintained. This is also true with Ni(Il). Insulin also binds zinc (Adams etai, 1969 Bordas etal., 1983) and forms rhombohedral 2Zn insulin crystals. The coordination of the zinc consists of three symmetry-related histidines (from BIO) and three symmetry-related water molecules. These give an octahedral complex... 

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 most important chemical function of Zn2+ in enzymes is probably that of a Lewis acid providing a concentrated center of positive charge at a nucleophilic site on the substrate/ This role for Zn2+ is discussed for carboxypeptidases (Fig.12-16) and thermolysin, alkaline phosphatase (Fig. 12-23),h RNA polymerases, DNA polymerases, carbonic anhydrase (Fig. 13-1),1 class II aldolases (Fig. 13-7), some alcohol dehydrogenases (Fig. 15-5), and superoxide dismutases (Fig.16-22). Zinc ions in enzymes can often be replaced by Mn2+, Co2+, and other ions with substantial retention of catalytic activity/ ... 

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. 

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