Alcohol dehydrogenase

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Alcohol dehydrogenases (ADH) (EC 1.1.1.1) are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of NAD+ to NADH. In humans and many other animals, they serve to break down alcohols which could otherwise be toxic, and they also participate in generation of useful aldehyde, ketone, or alcohol groups during biosynthesis of various metabolites. In yeast, plants and many bacteria, some alcohol dehydrogenases catalyze the opposite reaction as part of fermentation to ensure a constant supply of NAD+.

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Evolution

Genetic evidence from comparisons of multiple organisms, showed that a glutathione dependent formaldehyde dehydrogenase, identical to an ADH3, probably is the ancestral enzyme for the entire ADH family.[2][3] Early on in evolution, an effective method for eliminating endogenous and exogenous formaldehyde was important and this capacity has conserved the ancestral ADH3 through time. From genetic duplications of ADH3, followed by series of mutations, the other ADHs evolved.[2][3] The ability to produce ethanol from sugar is believed to have evolved in yeast. This feature is not rational from an energetic point of view, but by making alcohol in such high concentrations so that they were toxic to other organisms, yeast cells could effectively eliminate their competition. Since rotting fruit can contain more than 4% of ethanol, animals eating the fruit needed a system to metabolize exogenous ethanol. This can explain the need for an ethanol active ADH in other species than yeast.

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