Question

Why might we make enzymes that work on the same substrate, but with different enzyme kinetics?

How do the different hexokinase isozymes of liver and muscle reflect the different roles of these organs in carbohydrate metabolism?

Why is the fact how hexokinase 4 is not inhibited by glucose 6-phosphate, instead being inhibited by reversible binding of a regulatory protein specific to liver be important in its function?

Answer 1

Enzymes working on same substrate with different enzyme kinetics are useful in different biological processes. Where high conversion rate to product or etc is needed the enzyme is more processive. Such variation is important for different location functions of enzyme.

The hexokinase of both liver and muscles need to function for different purposes. The muscles consume glucose , while liver maintains level of glucose in blood either by removing or making glucose. This , the enzymes reflect on their roles.

In liver hexokinase 4 inhibition must not be by glucose 6 phosphate as this will create problem and will not lead to conversion of blood glucose to glycogen. But his is regulated using another regulatory protein which on absence of glucose or its low level on blood binds with enzyme and makes it non functional temporarily and move it to nucleus. When more and ample amount of glucose is available for storing in as glycogen the enzyme is released from inhibitor and can work to convert glucose to glucose 6 po hosphate again for glycogen formation.