A medical student compares the two enzymes that phosphorylate glucose in human cells. Which property distinguishes glucokinase (hexokinase IV) of the liver from hexokinase I of muscle?
- A Glucokinase has a low Km for glucose and is strongly inhibited by glucose-6-phosphate
- B Glucokinase has a high Km for glucose and is not inhibited by its product, glucose-6-phosphate ✓
- C Glucokinase uses ATP as phosphate donor whereas hexokinase uses UTP
- D Glucokinase is expressed constitutively in all tissues regardless of hormonal status
Correct answer: B. Glucokinase has a high Km for glucose and is not inhibited by its product, glucose-6-phosphate
Explanation
Hepatic glucokinase has a high Km (about 10 mmol/L), so it operates efficiently only at postprandial glucose concentrations, and it lacks product inhibition by glucose-6-phosphate, allowing the liver to buffer blood glucose. Muscle hexokinase I has a low Km (about 0.05 mmol/L) and is inhibited by glucose-6-phosphate. Both enzymes use ATP as the phosphate donor, so option C is false.
Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.