Alcohol dehydrogenase

Alcohol dehydrogenases are a group of closely related enzymes occurring in many organisms and facilitating the conversion between alcohols and aldehydes or ketones. They contain a zinc ion at their active site. The enzyme code of alcohol dehydrogenases is EC 1.1.1.1.

In humans the enzyme (human α alcohol dehydrogenase PDB 1HSO) is produced in the liver and catalyzes the oxidation of ethanol to acetaldehyde:

CH₃CH₂OH + NAD+ → CH₃CHO + NADH + H⁺

This allows the consumption of alcoholic beverages, but its original purpose is probably the breakdown of alcohols naturally contained in foods or produced by bacteria in the digestive tract. Note that acetaldehyde is even more toxic than ethanol and is responsible for many of the "hangover" effects, but it is quickly converted to acetate and other harmless molecules.

Alcohol dehydrogenase also oxidizes methanol to formaldehyde, and the dangerous effects of the latter are the main reason for the toxicity of the former.

Humans have 3 distinct genes for alcohol dehydrogenase, although the gene products show 93% sequence homology. These products may be expressed differently during development and may have different substrate specificities.

In yeast and many bacteria, alcohol dehydrogenase plays an important part in fermentation. It catalyzes the opposite reaction from the one described abovea: pyruvate resulting from glycolysis is converted to acetaldehyde and carbon dioxide, and the acetaldehyde is then reduced to ethanol by alcohol dehydrogenase. 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.