low and high Km?

[Edison]

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my biochem is very weak, but exam's coming in 2 days and I don't have time to check the textbook now; i hope u guys can help me out and check if the followings sound right. thanks a bunch!

so GLUT2 and glucokinase in liver and pancreas have high Km, meaning they have low affinity for glucose, right? does it mean that in order for blood glucose to enter liver and pancreas and be phosphorylated, the [glucose] in blood must be very high? similarly, since GLUT4 & hexokinase in muscle, adipose and heart muscle have low Km (i.e. high glucose affinity), does it mean even a low blood [glucose] can enter these tissues?

if the above sounds right, why is it that my professor said "enzymes with a high Km can respond to a wide range of glucose levels"??? i thought the reverse is true?

 

[Miami_med]

A high Km means that a greater concentration of substrate is required in order to reach 1/2 of the maximum rate. However, this doesn't speak to the absolute rate. It is only a ratio of the max. So, a low Km suggests that the maximum rate of catalysis is achieved at a low conentration due to first order kinetics. A high Km achieves that rate at a higher concentration.

To put it together. An enzyme with a low Km reaches maximum rate at a low concentration, after which no greater rate can be achieved. Thus, it stops responding to higher concentrations. An enzyme with a high Km achieves maximum rate at a higher concentration. Thus, it will continue to respond to higher concentrations by increasing its rate. It thus "responds" to a greater range of concentrations.

In your example (And I'm taking your word for the receptors because I haven't looked at this in about 1 and 1/2 years), GLUT4 will respond in a major way to low concentrations of Glucose, while GLUT2 will respond, but at a lower ratio of its maximum rate. When Glucose is high (say after a meal), GLUT4 will max out, but GLUT2 will continue to increase its rate. Low Glucose = both respond (but GLUT4 responds at a higher ratio of its max) High Glucose = GLUT2 responds only, because GLUT4 is maxed out and can't change its rate. The absolute rate however, is dependent on enzyme concentration.

Hope that helped

 

[boen]

A high Km means that a greater concentration of substrate is required in order to reach 1/2 of the maximum rate. However, this doesn't speak to the absolute rate. It is only a ratio of the max. So, a low Km suggests that the maximum rate of catalysis is achieved at a low conentration due to first order kinetics. A high Km achieves that rate at a higher concentration.

To put it together. An enzyme with a low Km reaches maximum rate at a low concentration, after which no greater rate can be achieved. Thus, it stops responding to higher concentrations. An enzyme with a high Km achieves maximum rate at a higher concentration. Thus, it will continue to respond to higher concentrations by increasing its rate. It thus "responds" to a greater range of concentrations.

In your example (And I'm taking your word for the receptors because I haven't looked at this in about 1 and 1/2 years), GLUT4 will respond in a major way to low concentrations of Glucose, while GLUT2 will respond, but at a lower ratio of its maximum rate. When Glucose is high (say after a meal), GLUT4 will max out, but GLUT2 will continue to increase its rate. Low Glucose = both respond (but GLUT4 responds at a higher ratio of its max) High Glucose = GLUT2 responds only, because GLUT4 is maxed out and can't change its rate. The absolute rate however, is dependent on enzyme concentration.

Hope that helped

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