What is the function of glucokinase?

• A. Catalyze the breakdown of glucose to pyruvate
• B. Inhibits glycolysis
• C. Controls rate of glycolysis and gluconeogenesis
• D. Transports glucose into cells

Answer: (C)

Glucokinase acts as the regulatory gate for hepatic glucose use by phosphorylating glucose to glucose-6-phosphate. Because it has a relatively high Km, its activity rises when blood glucose is high, allowing the liver to rapidly trap and channel glucose into pathways like glycolysis and glycogen synthesis. This gating effect helps set the pace of glycolysis in liver (and in pancreatic beta cells as a glucose-sensing step) and, by controlling how much glucose is funneled into gluconeogenic processes, influences the balance between glycolysis and gluconeogenesis. It’s important to note that glucokinase is not the enzyme that breaks glucose down to pyruvate—that’s downstream glycolysis—and it doesn’t transport glucose into cells—that role is played by glucose transporters. So its key function is to regulate how quickly glycolysis proceeds and how hepatic glucose output is modulated, effectively controlling the rate of these pathways.