Fructose 2,6-bisphosphate is the most powerful activator of hepatic phosphofructokinase-1. Its concentration in the liver is controlled mainly by insulin and glucagon acting on:

• A Insulin-driven transcription of aldolase B, which degrades fructose 2,6-bisphosphate
• B Direct allosteric binding of glucagon to the PFK-1 regulatory subunit
• C The bifunctional enzyme PFK-2/fructose 2,6-bisphosphatase through phosphorylation state changes ✓
• D Glucagon-mediated depletion of ATP, removing the allosteric brake on PFK-1

Explanation

PFK-2/fructose 2,6-bisphosphatase is a single bifunctional protein with kinase and phosphatase domains. Glucagon via PKA phosphorylation shifts it toward the bisphosphatase domain, lowering fructose 2,6-bisphosphate and favouring gluconeogenesis, whereas insulin favours dephosphorylation and glycolysis. Glucagon receptors are not present on PFK-1 itself, and ATP levels rise, not fall, after glucagon action, eliminating options B and D.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

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