The Collaborative International Dictionary of English v.0.48:

proenzyme \pro"en*zyme\, n. [.] (Bioch.)
   Any one of a class of proteins that are converted, in the
   normal course of cellular metabolism, into one or more active
   enzymes; also called zymogen. The conversion usually is due
   to a specific cleavage of a peptide bond by another enzyme,
   or may be due to acid.
   [PJC]

WordNet (r) 3.0 (2006):

proenzyme
    n 1: any of a group of compounds that are inactive precursors of
         enzymes and require some change (such as the hydrolysis of
         a fragment that masks an active enzyme) to become active
         [syn: proenzyme, zymogen]