Alcohol-dehydrogenase-nicotinamide active site

Figure 16.10 Active site of alcohol dehydrogenase. The active site contains a itne ion bound to two cysteine residues and one histidine residue. Notice that the zinc ion binds the acetaldehyde substrate through its oxygen atom, ipolarizing the substrate so that it more easily accepts a hydride from NADH. Only the nicotinamide ring of NADH is shown.

The substrate models concerned were fitted into the model of the active site of alcohol dehydrogenase-nicotinamide adenine dinucleotide (ADH-NAD) with VDW contacts, etc. not considered explicitly. 

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 . 

Cyclopropanone hydrate forms a stable thiohemiketal with the active-site thiol which is not oxidized by nicotinamide adenine dinucleotide (NAD). This hydrate has also been found to be a suicide inhibitor for horseradish peroxidase flavoenzyme alcohol oxidase and quinoprotein alcohol dehydrogenases ... 

The results of cysteine modification confirm the similarities in structure and function of the active sites of mammalian and yeast alcohol dehydrogenases (Section II,D). Minor differences are, however, observed. Thus, the nicotinamide-substituted imidazole dinucleotide (137) selectively alkylates one of the two cysteine ligands to the catalytic zinc atom, Cys-43, in the yeast enzyme. In the horse enzyme, on the other hand, the same reagent alkylates a different ligand to the same zinc atom, Cys-174. 

Fig. 2. Reasonable structural explanation for the stereospecilicity of hydrogen transfer catalyzed by horse liver alcohol dehydrogenase. The binary complexes were obtained by building ethanol into the X-ray structure of the holoenzyme, on the basis of the assumption that the ethanolic oxygen is directly coordinated to the active site zinc ion. (A) When the pro-(R) proton of ethanol is directed at C-4 of the nicotinamide ring, the methyl function is favorably positioned in the active site (B) when the pro-(S) proton is directed at C-4, the methyl function interacts stcrically with Phe-93. (From Ref. 30), with permission.]...

Alcohol dehydrogenase is one of the active enzymes in yeast. The active site in alcohol dehydrogenase contains a zinc ion, Zn, that is coordinated to the sulfur atoms of two cysteine residues of the enzyme. The hydride reducing reagent in alcohol dehydrogenase is nicotinamide adenine dinucleotide, NADH, which transfers a hydride ion to a carbonyl compound to yield an alcohol and NAD" ", in a mechanism that is related to the Cannizzaro reaction (Sec. 16.3). 

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