Final answer:
Kinases and phosphatases are the principal enzymes involved in controlling the phosphorylation status of covalently modulated enzymes, facilitating activation or inactivation through reversible phosphorylation.
Step-by-step explanation:
The enzymes that play the largest role in controlling the phosphorylation status of covalently modulated enzymes are kinases and phosphatases. Kinases catalyze the addition of phosphate groups (phosphorylation), while phosphatases remove these groups (dephosphorylation). Reversible covalent modification allows for the dynamic regulation of enzyme activity, necessary for appropriate cellular responses to changing conditions. Proteins like phosphofructokinase and pyruvate kinase in glycolysis, or pyruvate dehydrogenase in the citric acid cycle, are examples of enzymes whose activities are regulated by phosphorylation, guided by the energy needs of the cell as indicated by levels of ATP, ADP, and AMP.
Post-translational modification, such as phosphorylation, enables rapid changes in enzyme activity. The phosphorylation state of an enzyme can either activate or inactivate it, depending on the specific site of the modification. For instance, enzyme X might be activated by phosphorylation at one serine residue but deactivated by phosphorylation at another. The exact outcome is determined by the specific kinase or phosphatase that interacts with the enzyme under certain cellular conditions.