<P> In yeast and many bacteria, alcohol dehydrogenase plays an important part in fermentation: Pyruvate resulting from glycolysis is converted to acetaldehyde and carbon dioxide, and the acetaldehyde is then reduced to ethanol by an alcohol dehydrogenase called ADH1 . The purpose of this latter step is the regeneration of NAD, so that the energy - generating glycolysis can continue . Humans exploit this process to produce alcoholic beverages, by letting yeast ferment various fruits or grains . It is interesting to note that yeast can produce and consume their own alcohol . </P> <P> The main alcohol dehydrogenase in yeast is larger than the human one, consisting of four rather than just two subunits . It also contains zinc at its catalytic site . Together with the zinc - containing alcohol dehydrogenases of animals and humans, these enzymes from yeasts and many bacteria form the family of "long - chain" - alcohol dehydrogenases . </P> <P> Brewer's yeast also has another alcohol dehydrogenase, ADH2, which evolved out of a duplicate version of the chromosome containing the ADH1 gene . ADH2 is used by the yeast to convert ethanol back into acetaldehyde, and it is expressed only when sugar concentration is low . Having these two enzymes allows yeast to produce alcohol when sugar is plentiful (and this alcohol then kills off competing microbes), and then continue with the oxidation of the alcohol once the sugar, and competition, is gone . </P> <P> In plants, ADH catalyses the same reaction as in yeast and bacteria to ensure that there is a constant supply of NAD . Maize has two versions of ADH - ADH1 and ADH2, Arabidopsis thaliana contains only one ADH gene . The structure of Arabidopsis ADH is 47% - conserved, relative to ADH from horse liver . Structurally and functionally important residues, such as the seven residues that provide ligands for the catalytic and noncatalytic zinc atoms, however, are conserved, suggesting that the enzymes have a similar structure . ADH is constitutively expressed at low levels in the roots of young plants grown on agar . If the roots lack oxygen, the expression of ADH increases significantly . Its expression is also increased in response to dehydration, to low temperatures, and to abscisic acid, and it plays an important role in fruit ripening, seedlings development, and pollen development . Differences in the sequences of ADH in different species have been used to create phylogenies showing how closely related different species of plants are . It is an ideal gene to use due to its convenient size (2--3 kb in length with a ~ 1000 nucleotide coding sequence) and low copy number . </P>

Where is the enzyme alcohol dehydrogenase found in the body