Enzyme activated by a decrease in insulin: glucagon ration is:

Correct Answer: A. Glucose 6-phosphatase

Glucose 6-phosphatase (G6Pase) is the terminal enzyme of both gluconeogenesis and glycogenolysis, catalyzing the final step: glucose-6-phosphate → free glucose. This enzyme is exquisitely sensitive to the insulin:glucagon ratio. When this ratio decreases (i.e., glucagon rises relative to insulin, as in fasting or hypoglycemia), glucagon activates adenylyl cyclase → cAMP → PKA, which phosphorylates and activates CREB transcription factor. This upregulates G6Pase expression and activity, enabling hepatic glucose output to restore blood glucose. Conversely, high insulin suppresses G6Pase via dephosphorylation and reduced gene transcription. G6Pase is found exclusively in liver and kidney (not muscle), making it the rate-limiting step for hepatic glucose release. This is why patients with glycogen storage disease type I (Von Gierke disease) — who lack G6Pase — present with severe fasting hypoglycemia and hepatomegaly despite normal glycogen breakdown. The enzyme's regulation by the insulin:glucagon ratio is a cardinal feature of metabolic switching between fed (anabolic) and fasted (catabolic) states, as taught in Harrison and KD Tripathi biochemistry chapters on glucose homeostasis.

Why the other options are wrong

B. Glucokinase — Glucokinase is a glucose-trapping enzyme in liver and pancreatic β-cells, activated by high insulin:glucagon ratio (fed state), not decreased ratio. It phosphorylates glucose → G6P for glycolysis and glycogen synthesis. It is inhibited by glucagon signaling, making it the opposite of G6Pase in terms of hormonal regulation. NBE may trap students who confuse 'glucose metabolism' with 'glucose output.' C. Hexokinase — Hexokinase is a glucose-phosphorylating enzyme present in all tissues, activated in the fed state (high insulin) to trap glucose for glycolysis. It is inhibited by its product G6P, not regulated by insulin:glucagon ratio directly. Unlike G6Pase, hexokinase does not release free glucose and is not involved in hepatic glucose output during fasting. D. Phosphofructokinase — Phosphofructokinase (PFK) catalyzes the committed step of glycolysis (F6P → F1,6BP) and is activated by high insulin:glucagon ratio (fed state), not decreased ratio. It is inhibited by glucagon-mediated increases in ATP and citrate. PFK is a glycolytic enzyme, not involved in glucose output or gluconeogenesis.

High-Yield Facts

Mnemonics

G6Pase = Glucose OUT (Glucagon-Operated Unloading Terminal) When glucagon is high (low insulin:glucagon ratio), G6Pase releases glucose OUT of the liver. All other enzymes trap glucose IN (glucokinase, hexokinase, PFK). Fed vs. Fasted Enzyme Flip Fed (high insulin): Glucokinase ↑, PFK ↑, G6Pase ↓ (store glucose). Fasted (low insulin): G6Pase ↑, Glucokinase ↓, PFK ↓ (release glucose).

NBE Trap

NBE pairs "enzyme activated by decreased insulin:glucagon ratio" with glucokinase (option B) to trap students who remember "glucokinase is important in glucose metabolism" but confuse its direction of regulation — glucokinase is actually activated by HIGH insulin, not low. The trap exploits superficial hormone-enzyme association without testing mechanistic understanding of metabolic switching.

Clinical Pearl

In Indian clinical practice, understanding G6Pase regulation is critical for managing neonatal hypoglycemia and glycogen storage diseases — conditions where fasting glucose cannot be mobilized. Patients with Von Gierke disease require frequent feeds or continuous nasogastric glucose infusion because their livers cannot release glucose despite normal glycogen stores, illustrating why G6Pase is the gatekeeper of hepatic glucose output.

_Reference: KD Tripathi Pharmacology & Biochemistry Ch. 12 (Carbohydrate Metabolism); Harrison Principles of Internal Medicine Ch. 395 (Diabetes Mellitus); Robbins Pathology Ch. 7 (Genetic Disorders)_