INTRODUCTION
Alcohol dehydrogenase (ADH), is a sector of oxidoreductase family, it catalyzes the oxidation of alcohols, using NAD+ or NADP+ as the electron acceptor (White and White 1997). The reaction is ammendable and substrates can be a collection of primary or secondary alcohols, and hemiacetals. Alcohol dehydrogenases are active in major parts of organisms, also, it is the most active construction of the enzyme. Alcohol dehydrogenase is the toiler of the alcohol enzymes it delineate the most of the alcohol that proceeds the body of human. Researchers have name as many as 10 collection of the alcohol dehydrogenase molecule. Everything conducts the same chemical reaction the only alteration is that some collection of alcohol dehydrogenase work more easily than others.
Alcohol dehydrogenases oxidises alcohol towards aldehydes orketones. The reaction refers conenzyme NAD+ as hydrogen acceptor and has a wide specificity for alcohol substrates. They belong to a class of dehydrogenases that have a nucleotide-binding field. The action of all such NAD-dependent dehydrogenases is to familiarize the coenzyme and substrate on the enzyme bound, such as the C4 atom on the nicotinamide is manage at the responsive carbon of the substrate. The NAD-binding rules of all these dehydrogenases are highly homologous, but they have obvious unlike catalytic field.
THE STRUCTURE
Alcohol dehydrogenase is a homodimer. Every monomer has a total of 374 residues that gives a molecular weight of 74000 dalton. Two domains are present. The NAD+-binding domain is compost of a central beta-sheet of six strands bounded by alpha helices. The C-terminus binds with the NAD+ within the beta sheet. Alpha/beta structure is also present with the catalytic domain The semi-domain blend develops a cleft which contains the active catalytic site. The interface is made by two helices, one from each field bumping each other. There are two Zn++ cations every monomer, one at the catalytic site being required for catalysis. The alcohol substrate attaches inside the cleft where the Zn++ cation is, whilst the nicotinamide ring of the NAD finds its way pointing into the cleft. The dimer develops with the two NAD-binding domains bind together such as their two central beta sheets connect to form a 12-stranded beta sheet. The catalytic domains are located at both ends.
Figure 1. Structure of Alcohol Dehydrogenase (ADH5)
Figure 2 shows the NAD+ binding domain with helix in a cyan and some sheet in blue. The helix is magenta and the sheet is purple of the catalytic domain. The Nicotinamide adenine dinucleotide+ binding is shown cyan in helix and blue at the sheet. The catalytic domain consist helix in magenta and purple sheet. The substrate is dimethylsulphoside (DMSO) which is green. The active Zn++ ion which is brown, while the other is in white. C-terminus of beta sheet of the NAD+ binding domain is attached to to NAD+ which is in CPK coloring that consist of adenine ring. Its nicotinamide ring is applied into close similarity of the substrate and of the Zn++ ion.
Figure 2. 6ADH- Ribbon view of horse liver alcohol dehydrogenase monomer
Figure 3 shows van der Waals spaceballs in order to emphasize the intersections of helices center of the two domains. In stick model, shows NAD+Â and DMSO.
Figure 3. Crossing helices of Alcohol Dehydrogenase (ADH5)
Figure 4 show the active site of Alcohol dehydrogenase. The active site is composts of a zinc atom Histidine-67, Cysteine-174, Cysteine-46, Serine-48, Histidine-51, Ileucine-269, Valine-292, Alanine-317, and Phenylalanine-319. The zinc matched the substrate. The zinc is matched by Cysteine-46, Cysteine-174, and Histidine-67. Phenylalanin-319, Alanine-317, Histidine-51, Ileucine-269 and Valine-292 consolidate NAD+ by producing hydrogen bonds. Histidine-51 and Ileucine-269 produce hydrogen bonds joining alcohols on nicotinamide ribose.
Figure 4. Active site of ADH
Mechanisms of Catalysis, Kinetics of Reaction and Mode of Regulation
In the oxidation mechanism, ADH is briefly affiliate with nicontinamide adenine dinucleotide (NAD+), which works as a cosubstrate. Through this reaction, ADH5 uses mostly zinc and NAD to assist the reaction. The duty of zinc is to put the alcohol group on the ethanol in an embodiment that gives consent for the oxidation to occur. NAD then functions as the cosubstrate and then accomplished the oxidation.
Figure 5. Mechanism of Alcohol Dehydrogenase
The reaction of Alcohol Dehydrogenase is shown at figure 6. . In this mechanism, Histidine 51 subjects to deprotonation and triggered by a base catalyst. This mechanism allows histidine to obtain a proton from NAD, which draws a proton Threonine 48. Turned out to be the transferring of proton, the Threonine was able to obtain a proton coming from alcohol substrate. While it accepts the proton, hydride transferring to NAD occured. The whole activity can be summed up as the alcohol oxidation to an aldehyde in accordance with the hydride transferring to NAD.
The Mechanism for ADH comes after a fickle bisubstrate mechanism. In the mechanism, the NAD+ and alcohol joined to the enzyme, the enzyme is now connected to the two subtrates. Hydrogen is transferred while connected starting from the alcohol to NAD, forming the products of NADH and a ketone or sometimes, an aldehyde. The products are both released the reaction was catalyzed in the enzyme.
Figure 6. The reaction of ADH
Catalyzation of Aldehyde NADH reaction in alcohol dehydrogenase exhibits kinetics consistency with mechanism of random order,and the rate-limiting step is the separation of the product enzyme- NAD+ complex. It is more adequate for smaller substrates and it becomes less adequate as the size of the substrate increases. One study showed how ADH immobilized in tresyl-chloride-activate agarose. It was illustrated that in the Michaelis-Menten model could not approved all the components activated by the inactiveness on the enzyme properties but it shows that Theorell-Chance model was more reliable.
Substrate amount is a regulator, where more substrates prevent alcohol dehydrogenase. Pyrazoles also happened to be inhibitors of Alcohol Dehydrogenase. Heavy metals, purine, pyrimidine derivatives, and chloro and flouroethanol are other inhibitors. Mercaptoethanol, Sulfhydryl activating reagents, cysteine and dithiothreitol are known as activators.
Associated Diseases and Importance of Alcohol Dehydrogenase to Human Health
Our own human body creates almost nine different ADH. Each has its own difference form of properties. Mostly are found in the liver. The sigma form, can be found at the stomach lining. Two subunits forms each enzyme, and more remarkably, between this different forms, subunits can be mixed and matched, creating an active mixed dimers. Not only the ethanol is the aim of this enzyme, also, they make remarkable modifications to fatty acids, steroids and some retinols. The extent of the different classifications of ADH, see to it that there is always that one that is exquisite to the present task.
Alcohol dehydrogenase is prone to alcoholism. Studies show that Alcohol dehydrogenase put a major influence on the reliance of ethanol metabolism particularly to the alcoholics. Few genes were detected by researchers that have associated with alcoholism. A high risk of alcoholism is there if the genes encrypt was slower metabolizing forms of ADH2 and ADH3.
Parkinson’s disease (PD) can also acquire by ADH modification. It is because the genes may be attracted with its diagnosis, because of the remarkable part this enzymes acts in dopamine and also retinoid metabolism and/or aldehyde detoxification. The area of some ADH genes in a array on chromosome 4 yield more support to ADH genes as entrant for this disorder, cause lately, a form of autosomal-dominant parkinsonism has been mapped to some area. They guided the promoter and some regions of the human class IV ADH gene in 10 patients with PD. Different polymorphisms were acquired. These polymorphisms can be appointed to 4 alleles (A1-A4). Then the concluded the frequencies of each four alleles and 78 patients with PD has the wild type allelle and one hundred thirty control subjects and found a remarkable alliance of the A1 allele with PD. In congenital cases, the alliance was strongest. Also, the data shows that the detected polymorphisms alone is not adequate to cause signs. Some genetic and/or environmental component must be involved.
