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]

The Collaborative International Dictionary of English v.0.48:

Zymogen \Zym"o*gen\, n. [Zyme + -gen.] (Physiol. Chem.)
   A mother substance, or antecedent, of an enzyme or chemical
   ferment; -- applied to such substances as, not being
   themselves actual ferments, may by internal changes give rise
   to a ferment.
   [1913 Webster]

         The pancreas contains but little ready-made ferment,
         though there is present in it a body, zymogen, which
         gives birth to the ferment.              --Foster.
   [1913 Webster]

WordNet (r) 3.0 (2006):

zymogen
    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]