Why is it essential that the regulatory mechanisms that activate glycogen synthesis also deactivate glycogen phosphorylase?

There are two pathways occur in the same cellular compartment, and, if both are on at the same time, a futile ATP hydrolysis cycle results. Using the same mechanism to turn them on/off or off/on is highly efficient.

- "Glycogen Phosphorylase" activity can be allosterically controlled ATP and G6P allosteric inhibitors, AMP allosteric activator as well as, controlled through covalent modification, phosphorylation and via hormones.

- Reaction Catalyzed by Glycogen Synthase:

The activity of glycogen synthase is subject to the same type of covalent modification as glycogen phosphorylase, however, the response is opposite. Glycogen Synthase is activated by G6P. It is also controlled via hormones.