Hexokinase Converts Free Sugars to Hexose Phosphates

Hexokinase Converts Free Sugars to Hexose Phosphates 

Glucose obtained by mammals from the diet through intestinal hydrolysis of lactose, sucrose, glycogen, or starch is brought into the hexose phosphate pool through the action 

The standard free energy change is about —5 kcal/mol, and the equilibrium constant is about 5,700. Thus, equilibrium considerations indicate a potential for this reaction to occur. The potential can be converted to reality only by an enzyme with appropriate kinetic properties. Hexokinases purified from various tissues typically have a Michaelis constant for glucose between 10 and 20 /am. Thus, by the expenditure of ATP, hexokinase can convert glucose in the micromolar 

The reaction catalyzed by hexokinase. Attack on the terminal phosphorus atom of ATP is probably facilitated by proton removal by a negatively charged group in the catalytic site of the enzyme ( B in the figure). It is also facilitated by the fact that the terminal phosphate of ATP is an excellent leaving group. 

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