zymogens Most proteins become fully active as their synthesis is completed and they spontaneously fold into their native,three- dimensional conformations.Some proteins, however,are synthesized as inactive precursors,called zymogens or proenzymes,that only acquire full activity upon specific proteolytic cleavage of one or several of their peptide bonds.zymogens Most proteins become fully active as their synthesis is completed and they spontaneously fold into their native, three￾dimensional conformations. Some proteins, however, are synthesized as inactive precursors, called zymogens or proenzymes, that only acquire full activity upon specific proteolytic cleavage of one or several of their peptide bonds