Which enzyme initiates glycolysis by phosphorylating glucose to glucose-6-phosphate in most tissues?

• A. Glucokinase
• B. Hexokinase
• C. Aldolase
• D. Pyruvate kinase

Answer: (B)

The step that starts glycolysis is the phosphorylation of glucose to glucose-6-phosphate, which traps glucose inside the cell and commits it to further metabolism. In most tissues, this phosphorylation is carried out by hexokinase. It has a high affinity for glucose, so it works efficiently even at low glucose levels, and it is regulated by its product, glucose-6-phosphate, providing feedback inhibition to prevent unnecessary glycolysis when G6P accumulates.

A useful contrast is that glucokinase, found in the liver and pancreatic beta cells, has a much lower affinity for glucose (high Km) and is not inhibited by glucose-6-phosphate, allowing it to phosphorylate glucose mainly when levels are high, such as after a meal. Aldolase and pyruvate kinase act later in glycolysis, not at the initiating step, with aldolase splitting fructose-1,6-bisphosphate and pyruvate kinase driving the final formation of pyruvate.