Glucokinase (in liver) and hexokinase (in most cell types) both catalyze the pho

Question

Glucokinase (in liver) and hexokinase (in most cell types) both catalyze the phosphorylation of glucose following its transport into the cytosol from the bloodstream. The K_m of hexokinase for glucose is 30 mu M, and the K_m of glucokinase for glucose is 10 mM. Which one of the following statements regarding these enzymes is most accurate when the cytosolic glucose concentration is 5 mM? Hexokinase is operating near maximal velocity. Both hexokinase and glucokinase are operating near maximal velocity. Glucokinase is operating near maximal velocity. Neither hexokinase nor glucokinase is operating near maximal velocity. Glucokinase is inactive and hexokinase is active.

Explanation / Answer

The Km of hexokinase for glucose is 30uM (0.03mM) and the Km of glucokinase for glucose is 10mM. Cytosolic glucose concentration is 5mM.

A small Km indicates that the enzyme requires only a small amount of substrate to become saturated. Hence, the maximum velocity is reached at relatively low substrate concentration. So, glucose will be taken up easily for the given concentration. Thus, Hexokinase will operate near maximal velocity.

High Km for Glucokinase indicates the need for high substrate concentration to achieve maximum velocity.

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