Tunneling in Alcohol Dehydrogenases

Probably the most extensively studied enzymes are those from alcohol dehydrogenase family. One enzyme from this series which has been thoroughly examined both experimentally and theoretically is liver alcohol dehydrogenase (LADH). It catalyzes the reversible conversion of an alcohol to an aldehyde by transferring hydride from substrate to the cofactor (NAD+)  

Historically, the intervention of tunneling has usually been invoked when the observed KIE exceeded limits set by semi-classical theory. A recent example is the hydrogen atom transfer step in methylmalonyl-CoA mutase (MCM) catalyzed 

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