An enzyme is a particular form of biological protein that specifically catalyses a biochemical reaction. Unlike conventional chemical catalysts, they are highly specific and selective of their target molecule (substrate).
An example is alcohol dehydrogenase (most enzymes can be identified with the suffix -ase), which is responsible for the breakdown of alcohol groups (mostly from ethanol) in the body. Different organisms tend to have similar enzymes to perform similar tasks, with evolution causing differences than can range from the minute and neutral to differences that alter the efficiency of the enzyme.
Of course, some creationists see enzymes as "intelligent design", without realizing that something better than most enzymes could be put together in a week by skilled engineers. Don't believe me? https://www.youtube.com/watch?v=W3KxU63gcF4 (warning, it's from CMI)
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