Biochemistry : Vmax and Km

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Vmax And Km

There are at least four types of glucose transporter in the body. GLUT1 and GLUT3 are located in most tissues including the brain and the red blood cells. These glucose transporters rapidly take up glucose from the blood but have the lowest  value. GLUT2 is commonly found in the liver and the pancreas. GLUT2 has a lower affinity for glucose but has the highest  value. GLUT4 is common in skeletal tissues and in adipose tissues. This transporter is normally not active for uptake unless stimulated by insulin or during exercise.  

What might be a consequence of a disease that replaces all GLUT2 with GLUT1? 

Possible Answers:

All of these

Hyperinsulinemia

Hypoglycemia

Hyperglycemia

None of these

Correct answer:

Hypoglycemia

Explanation:

GLUT2 have a lower affinity for glucose and is essential for the liver and the pancreas to regulate the blood glucose level. With a lower affinity transporter, glucose will not be taken up immediately by the liver/pancreas while reserving for the high glucose demanding organs (e.g brain, neurons, red blood cells). Therefore, if one were to inherit such disease, extreme hypoglycemia may occur.   

Example Question #2 : Vmax And Km

There are at least four types of glucose transporter in the body. GLUT1 and GLUT3 are located in most tissues including the brain and the red blood cells. These glucose transporters rapidly take up glucose from the blood but have the lowest  value. GLUT2 is commonly found in the liver and the pancreas. GLUT2 has a lower affinity for glucose but has the highest  value. GLUT4 is common in skeletal tissues and in adipose tissues. This transporter is normally not active for uptake unless stimulated by insulin or during exercise.  

Sam is performing an experiment and he wants to use the glucose transporter with the lowest  value. Which type of glucose transporter should he use? 

I. GLUT1

II. GLUT2

III. GLUT3

IV. GLUT4

Possible Answers:

I only

II only

I and II

III only

I and III

Correct answer:

I and III

Explanation:

GLUT1 and GLUT3 have the highest affinity for glucose. The Michaelis constant,  is the concentration of the substrate required to reach . The lower the , the higher the affinity is for the substrate.  

Example Question #2 : Vmax And Km

You are measuring the activity of an enzyme in solution and notice that enzyme activity increases with increasing substrate concentration to a certain point, after which enzyme activity does not increase even if you add more substrate. What is this called and what can be done to increase activity?

Possible Answers:

This is called the  and adding more enzyme concentration will increase activity.

This is called the rate constant and increasing the concentration of the product will increase activity.

This is called the maximal velocity and increasing the enzyme concentration will increase activity.

This is called the rate constant and adding more enzyme will increase activity.

This is called the maximal velocity and decreasing the concentration of the production will increase activity.

Correct answer:

This is called the maximal velocity and increasing the enzyme concentration will increase activity.

Explanation:

The enzyme in this question is said to be saturated with substrate. That means that at any given moment, all of the enzyme in solution is bound to a substrate so that adding more substrate will have no effect. This is referred to as the maximal velocity because the enzymes are working as fast as possible and will not respond to the addition of more substrate. By adding more enzyme, you a providing more active site to bind to more substrate molecules, effectively increasing the velocity of the overall reaction.

Example Question #3 : Vmax And Km

What does a small  indicate?

Possible Answers:

That high substrate concentrations are needed to achieve maximum reaction velocity

That the enzyme requires a large amount of substrate to become saturated

The enzyme has a low affinity for its substrate

That the enzyme requires only a small amount of substrate to become saturated

Correct answer:

That the enzyme requires only a small amount of substrate to become saturated

Explanation:

In enzyme kinetics,  is the concentration of substrate which allows the enzyme to reach  (maximum reaction velocity). A small  indicates that only a small amount of substrate is needed for the enzyme to become saturated and thus for the reaction to reach maximum velocity. This also indicates that the enzyme and substrate have high affinity for one another. A large  indicates that a large amount of substrate is needed for the enzyme to become saturated and thus for the reaction to reach maximum velocity.

Example Question #4 : Vmax And Km

The Michaelis constant, , occurs at what value on an enzyme kinetics graph?

Possible Answers:

Not enough information is given to predict 

Correct answer:

Explanation:

The Michaelis constant, , is a frequently used value in enzyme kinetics used to essentially describe how much substrate is needed to speed up a reaction. More specifically, it is the substrate concentration required to get a reaction to half of its  (the correct answer is ). The  itself is given as a specific substrate concentration, and in order to find it, one would draw a straight line across on an enzyme kinetics graph at  until reaching the curve. The substrate concentration that is found at that point is the .

Example Question #6 : Vmax And Km

Which of these are representations of ?

I. 

II. 

III. Y-intercept on a Lineweaver-Burk plot

Possible Answers:

II and III

I, II, III

I and II

I and III

I only

Correct answer:

I and II

Explanation:

 can be defined as  or as .

However, on a Lineweaver-Burk plot, the Y-intercept actually represents . It is the X-intercept that  can be derived from. The X-intercept is equivalent to .

Example Question #3 : Vmax And Km

When an enzyme is in the presence of a competitive inhibitor, which of the following will happen to the enzyme?

Possible Answers:

 will decrease.

 will decrease.

 will increase.

 will increase.

Correct answer:

 will increase.

Explanation:

Competitive inhibitors will block the active site of the enzyme. The presence of the competitive inhibitor increases the amount of substrate required to get the enzyme to half of its maximum velocity. As a result, the  of the enzyme will increase. Note that a competitive inhibitor will not affect the maximum velocity of the enzyme.

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