MCAT Biology : Enzymes and Enzyme Inhibition

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #86 : Proteins

An unknown molecule is added to an enzyme-catalyzed reaction, immediately decreasing its rate. If the addition of more substrate has no effect, but the addition of an antibody for the unknown molecule restores the initial reaction rate, what form of inhibition is most likely occurring?

Possible Answers:

Feedback inhibition

Uncompetitive inhibition

Irreversible inhibition

Competitive inhibition

Noncompetitive inhibition

Correct answer:

Noncompetitive inhibition

Explanation:

This question is referring specifically to the different modes of enzyme inhibition. The given fact that increasing substrate concentration does not restore enzyme function indicates that the inhibitor is binding to the enzyme at an allosteric site (eliminating competitive inhibition). The given fact that inhibitor-specific antibodies restored enzyme function indicates that the inhibition is reversible.

Uncompetitive inhibition is a specific type of noncompetitive inhibition in which the inhibitior binds to the enzyme-substrate complex. We are unable to conclude that this is the case based on the given information alone.

Example Question #87 : Proteins

A graduate student needs to cut a DNA plasmid using two different restriction enzymes in a buffer. He can use any two of the following enzymes in any of the four given buffers.

Enzyme

Efficiency in Buffer (%)

 

1

2

3

4

AgeI

100

50

10

75

ClaI

10

50

50

100

PsiI

10

100

10

100

Which two enzymes and buffer should the student choose for the digestion?

Possible Answers:

ClaI and AgeI in a mixture of buffers 1 and 4

ClaI and AgeI in buffer 2

ClaI and PsiI in buffer 4

PsiI and AgeI in buffer 2

Correct answer:

ClaI and PsiI in buffer 4

Explanation:

Because ClaI and PsiI both have 100% efficiency in buffer 4, that is the most ideal choice. A combination of buffers is never a valid option, as diluting the buffers fundamentally changes the composition of the reaction and the efficiency of the enzyme. In the absence of two enzymes that can function with 100% efficiency, it is best to find a combination with the greatest possible efficiency or to complete the digest sequentially in the appropriate buffers.

Example Question #88 : Proteins

Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males. 

Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.  

Erections occur when nitric oxide  is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions. 

Which of the following reactions would you expect PDE5 to catalyze? 

Possible Answers:

Correct answer:

Explanation:

For this problem, we need to know what phosphodiesterases do. Phosphodiesterases catalyze the breakdown of phosphodiester bonds via hydrolysis. Cyclic GMP contains an internal phosphodiester bond. Therefore its breakdown would result in the formation of GMP. The hydrolysis of cGMP should not yield GMP and a phosphate, since cyclic GMP only has one phosphate group. 

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