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Zymogens are also called proenzymes. They are inactive forms of enzymes used as reycling molecules, produced and secreted, many by the liver, then transported back to the plasma membrane via vesicles (endosomes) and transformed into its active form.
The existance of some structral features prevents the enzyme breaking down the cells inwhich they are made, e.g. hydrolase, which has extra inhibitory domains at the N-terminus. Once secreted the zymogen is converted to its active form which requires a biochemical change. In some cases this can be hydrolysis. In hydrolase this is achieved as endolysosomes (the fusion of pre existing lysomoes and endosomes), containing active hydrolases which digest the extra inhibitory domains [1].


  1. Alberts et al.,2008, Molecular biology of the cell, Fifth ed., Page 792/3, New York, Garland science
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