Enzymes are synthesised as zymogens and then make a transition to active form when required, is this true for every enzyme or are there any exceptions to that?
No, not all protein synthesized on ribosomes are synthesized in zymogen form . This generally is true for the secretory proteins & it is considered as one way of regulation of the activity as it is required.
Those synthesized as zymogens are actually rarer than those that are not. For example, almost all enzymes synthesized for major metabolic pathways such as glycolysis and the TCA cycle do not require activation from a zymogen form.
It is true that not all enzymes are secreted as zymogens but a significant amount of proteins are synthesized as inactive precursors, those that are activated at a certain time and place, physiologically appropriate, to protect the cell that produces them. I agree with what Dr Hasan said, because the synthesis of enzymes in the form of zymogens is one of the "safety mechanisms" that the body has for its survival. For example, the synthesis of digestive enzymes in inactive form is a safety mechanism for gastric mucosal cells, since proteolytic enzymes synthesized as zymogens are not activated until they leave the cell.
Pepsin is synthesized as pepsinogen, trypsin as trypsinogen, chymotrypsin as chymotrypsinogen, carboxypeptidases as procarboxypeptidases, and all of these zymogens are activated - usually when an external factor, pH changes, another enzyme, etc. acts on them - only when they have been secreted in the gastrointestinal tract. Another good example of what happens when some zymogens are activated "ahead of time", inside the cells, is seen in acute pancreatitis, in which the premature activation of some of the pancreatic enzymes like trypsin, phospholipase A2 and elastase, produce the self digestion of the pancreatic tissue.
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