Alcohol dehydrogenase active site

Alcohol dehydrogenase activity (ADH) and peroxidase activity (POD) measurements performed during the process of LADH spin labeling, confirm the previously found evidence that both types of activities utilize the same enzyme active site (1). Actually, as shown in Pig. 1, the two catalytic activities decay at the same rate, suggesting a very similar effect of the spin label sensitive Cys-46 residue alkylation on both of them. 

The Protein Data Bank PDB ID 1A71 Colby T D Bahnson B J Chin J K Klinman J P Goldstein B M Active Site Modifications m a Double Mutant of Liver Alcohol Dehydrogenase Structural Studies of Two Enzyme Ligand Com plexes To be published... 

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. 

Step 3 of Figure 29.3 Alcohol Oxidation The /3-hydroxyacyl CoA from step 2 is oxidized to a /3-ketoacyl CoA in a reaction catalyzed by one of a family of L-3-hydroxyacyl-CoA dehydrogenases, which differ in substrate specificity according to the chain length of the acyl group. As in the oxidation of sn-glycerol 3-phosphate to dihydroxyacetone phosphate mentioned at the end of Section 29.2, this alcohol oxidation requires NAD+ as a coenzyme and yields reduced NADH/H+ as by-product. Deprotonation of the hydroxyl group is carried out by a histidine residue at the active site. 

The crystal structure of the HNL isolated from S. bicolor (SbHNL) was determined in a complex with the inhibitor benzoic acid." The folding pattern of SbHNL is similar to that of wheat serine carboxypeptidase (CP-WII)" and alcohol dehydrogenase." A unique two-amino acid deletion in SbHNL, however, is forcing the putative active site residues away from the hydrolase binding site toward a small hydrophobic cleft, thereby defining a completely different active site architecture where the triad of a carboxypeptidase is missing. 

Most inhibitors of aldehyde dehydrogenase are inhibitors because they react with the thiol group at the active site of the enzyme. Inhibitors such as disulfiram (Fig. 4.31) have been used in the treatment of alcoholism because if someone drinks alcohol while taking the inhibitor, there is a buildup of acetaldehyde, which causes many unpleasant symptoms such as flushing and nausea (77,78). However, if someone drinks a large amount of alcohol while taking disulfiram it can lead to a life-threatening reaction. 

Schneider, G., Eklund, H., Cedergren-Zeppezauer, E., and Zeppezauer, M. (1983). Crystal structures of the active site in specifically metal-depleted and cobalt-substituted horse liver alcohol dehydrogenase derivatives. Proc. Natl. Acad. Set. U.S.A. 80, 5289-5293. 

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. 

Alcohol dehydrogenases (ADH EC 1.1.1.1), for which several X-ray structures are available ", catalyze the biological oxidation of primary and secondary alcohols via the formal transfer of a hydride anion to the oxidized form of nicotinamide adenine dinucleotide (NAD ), coupled with the release of a proton. Liver alcohol dehydrogenase (LADH) consists of two similar subunits, each of which contains two zinc sites, but only one site within each subunit is catalytically active. The catalytic zinc is coordinated in a distorted tetrahedral manner to a histidine residue, two cysteine residues and a water molecule. The remaining zinc is coordinated tetrahedrally to four cysteine residues and plays only a structural role . 

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... 

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... 

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/ ... 

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