The protein portion of an enzyme as contrasted with the nonprotein portion, coenzyme, or prosthetic portion (if present in the intact protein).
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apo·en·zyme .ap-ō-'en-.zīm n a protein that forms an active enzyme system by combination with a coenzyme and determines the specificity of this system for a substrate
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apo·en·zyme (ap″o-enґzīm) the protein component of an enzyme that is separable from the prosthetic group (cofactor or coenzyme) but that requires the presence of the prosthetic group to form the functioning compound (holoenzyme).Medical dictionary. 2011.