After a carbohydrate-rich meal, hepatic glucokinase continues phosphorylating glucose even at concentrations well above those that completely saturate hexokinase I. The kinetic property responsible is:

• A Glucokinase has a much lower Km than hexokinase I
• B Glucokinase is competitively inhibited by glucose-6-phosphate
• C Glucokinase has a much higher Km than hexokinase I ✓
• D Glucokinase has sigmoidal kinetics with a Hill coefficient of 1

Explanation

Hexokinase I has a low Km (about 0.05 mM) and saturates at fasting glucose levels, whereas hepatic glucokinase (hexokinase IV) has a high Km (about 10 mM), close to postprandial portal glucose concentration. Because it is unsaturated at normal glucose levels, its velocity rises almost linearly with rising glucose after a meal, allowing the liver to buffer blood glucose. Glucokinase is NOT product-inhibited by glucose-6-phosphate, unlike hexokinase I, but the property asked here is the high Km.

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

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