Regulation of enzyme activity by covalent modification is seen in all except ?

Ans. is 'c' i.e., Aspaate transcarboxylase Covalent modificationThe enzymes regulated by covalent modification can exist in two forms.The two forms of enzyme can be conveed into each other by a covalent modification of the enzyme molecule.The most common covalent modification is the addition or removal of a phosphate group.The covalent attachment of phosphate group conves the enzyme into phophorylated form whereas removal of phosphate group conves the same enzyme into dephosphorylated form.One of the forms of the enzyme, either phosphorylated or dephosphorylated, is active and the other is inactive.Phosphorylation of the enzyme is catalyzed by a protein kinase that transfers a phosphate group from ATP to the enzymes.Dephosphorylation of the enzyme is catalyzed by a protein phosphatase that removes the phosphate group from the enzyme.Whether the enzyme will be active or inactive at a given time depends upon the relative activities of protein kinase and protein phosphatase which are usually controlled by hormones through second messengers.An example is glycogen synthase, which is active in dephosphorylated state and inactive in phosphorylate state.Glucagon stimulates formation of cAMP (a second messenger) which inturn stimulates protein kinase.Protein kinase catalyze the phosphorylation of glycogen synthase.Therefore, glucagon inactivate glycogen synthase.