GENERAL MICROBIOLOGY - T.P. Pirog - 2004

20. METABOLIC REGULATION

20.4. MECHANISMS OF ENZYME ACTIVITY REGULATION

20.4.2. Covalent modification of enzymes

In recent years, researchers have increasingly concluded that yet another mechanism plays a crucial role in regulating enzyme activity: the alteration of enzyme activity resulting from the Covalent Modification of their Structure. Such covalent Modification of the Enzyme Structure is driven by Other Enzymes, known as modifying enzymes. Under METABOLISM/18.html">The Influence of modifying enzymes, the active form transitions into the inactive form and vice versa. These two forms differ in that in one state the enzyme carries a covalently bound group, while in the other it does not. The first enzyme shown to exist in these two forms was discovered by the American biochemist G.T. Cori and his coworkers. It was Muscle Glycogen phosphorylase, which was modified via phosphorylation-dephosphorylation. It should be noted that in prokaryotes this enzyme is not regulated by covalent modification, and generally the modification of enzyme systems via phosphorylation-dephosphorylation is more prevalent in eukaryotes. In prokaryotes, two systems of covalent modification have been identified: The activity of Glutamine Synthetase is regulated by adenylylation-deadenylylation, and the activity of citrate lyase by Acetylation-deacetylation.