The enzymes and what they do

Dehydrogenases (or oxidoreductases) constitute the first of six main divisions in the Enzyme Commission classification [ 1 ]. About 300 dehydrogenases that utilize nicotinamide coenzymes are known, and this chapter deals with some of them. The recommended name and EC number designate not a single enzyme protein, but a group of proteins with the same catalytic property [1]. 

Proteins with the same catalytic property do not always have closely similar structures. Alcohol dehydrogenases (EC 1.1.1.1) from yeast [2] and fruit-flies [3,4], for example, have strikingly different primary structures. 

It is also the case that enzymes showing sequence similarities do not necessarily catalyse the same reactions. Sheep liver sorbitol dehydrogenase (EC 1.1.1.14) does not utilize ethanol, though in primary structure it resembles both yeast and horse liver alcohol dehydrogenases [5,6]. 

The EE and SS isozymes of horse liver alcohol dehydrogenase have very similar primary structures [7,8]. Ethanol is a substrate for both, but both have a wide specificity, and ethanol is not the best substrate for either. The SS isozyme has 3/3-hydroxysteroid dehydrogenase activity [9,10], which the EE isozyme does not have, and which is thought to depend upon a single amino acid replacement [11], It remains to be established whether these different isozymes have different roles in vivo. 

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