n. a protein that acts as a biological catalyst, accelerating the rate of a biochemical reaction without itself becoming permanently altered. Many enzymes require other organic molecules (coenzymes) or inorganic ions (cofactors) to function normally. Most enzymes are named according to the type of reaction they catalyze; for example, glucosidases convert glucosides to glucose; acetylcholinesterase (see cholinesterase) splits and inactivates molecules of the neurotransmitter acetylcholine.