Covalent modification by phosphorylation is a major regulatory mechanism for enzymes. Glycogen synthase is INACTIVATED by phosphorylation, whereas glycogen phosphorylase is ACTIVATED by phosphorylation. This reciprocal regulation ensures:

• A Both enzymes are active simultaneously to maintain glycogen turnover
• B Glycogen synthase phosphorylation increases its affinity for UDP-glucose
• C When glycogenolysis is stimulated (e.g., by epinephrine/glucagon), glycogen synthesis is simultaneously suppressed, preventing a futile cycle ✓
• D Glycogen phosphorylase phosphorylation decreases its activity in the liver only

Explanation

PKA (activated by cAMP from glucagon or epinephrine) simultaneously phosphorylates glycogen synthase (inactivating it) and phosphorylase kinase (activating glycogen phosphorylase); this reciprocal regulation ensures net glycogen breakdown without futile resynthesis when glucose is needed. This is a textbook example of coordinated phosphorylation-based metabolic switching. Glycogen phosphorylase is activated — not inactivated — by phosphorylation in both liver and muscle.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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