This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Enzymes, Subject name: Biochemistry
Zymogen activation by paial proteolysis is an example of 
Select and explain. 
A. Allosteric modification, B. Enzyme induction, C. Enzyme repression, D. Covalent modification
D: Covalent modification
Answer: D: Covalent modification 
Explanation: The activity of enzymes may be increased or decreased by covalent modification. It means, either the addition of a group to the enzyme protein by a covalent bond; or removal of a group by cleaving a covalent bond. Zymogen activation by paial proteolysis is an example of covalent activation. Addition or removal of a paicular group brings about covalent modification of enzyme protein. This is a reversible reactionRef: DM Vasudevan, Page no: 56