MCAT Biology : Enzymes and Enzyme Inhibition
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Example Question #41 : Enzymes And Enzyme Inhibition
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?
Noncompetitive inhibition
Uncompetitive inhibition
Competitive inhibition
Irreversible inhibition
Feedback inhibition
Noncompetitive inhibition
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 #42 : Enzymes And Enzyme Inhibition
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?
PsiI and AgeI in buffer 2
ClaI and AgeI in buffer 2
ClaI and PsiI in buffer 4
ClaI and AgeI in a mixture of buffers 1 and 4
ClaI and PsiI in buffer 4
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 #82 : 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 
Which of the following reactions would you expect PDE5 to catalyze?
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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