Oxidoreductases and lyases

[JKetlerP]

I'm a little confused on the difference between them. According to the definitions I found online, oxidoreductases catalyze oxidation/reduction reactions and lyases break/form double bonds by adding/removing 'elements of water, ammonia, or CO2'.

But don't these definitions overlap? Look at lactate dehydrogenase. There is definitely a redox reaction happening here, right? But there is also a double bond being broken by the addition of O and H (certainly 'elements of water')!

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[Lawpy]

Lactate dehydrogenase is an oxidoreductase because the enzyme uses an electron carrier NADH to reduce pyruvate into lactate (it also catalyzes the reverse reaction by using NAD+ to oxidize lactate into pyruvate).

More generally, a dehydrogenase is a special type of oxidoreductase that uses an electron carrier to facilitate a redox reaction with a substrate molecule. Oxidoreductases can form/break chemical bonds by redox reactions, just like hydrolases can form/break chemical bonds by hydrolysis reactions. Lyases therefore form/break chemical bonds by means other than redox and hydrolysis reactions. An example of a lyase is a carboxylase/decarboxylase that adds/removes carboxyl groups from substrates. Take a look at the mechanism of histidine decarboxylase, an enzyme that catalyzes formation of histamine from histidine.

Electron carriers aren't involved, and the carboxyl group leaves the substrate as carbon dioxide, so histidine decarboxylase is a lyase.

Also note that while hydrolases form/break bonds via hydrolysis reactions (so water is a reactant), dehydratases form/break bonds by dehydration reactions (usually forms double/triple bonds by removal of water, so water is a product). Dehydratases are a special category of lyases.