MCAT Biology : Proteins

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #241 : Organic Chemistry, Biochemistry, And Metabolism

Which of the following is not an example of positive feedback?

Possible Answers:

During childbirth, oxytocin creates a stimulus which causes the hypothalamus to release more oxytocin.

As more buffalo begin to run in a herd, the overall level of panic increases. This results in even more buffalo running.

A forest fire slowly expands outward, which provides it with even more fuel to burn.

As blood calcium levels increase, parathyroid hormone (PTH) is reduced.

Correct answer:

As blood calcium levels increase, parathyroid hormone (PTH) is reduced.

Explanation:

Negative feedback provides the body with a method for shutting down a reaction once sufficient product has been created. Parathyroid hormone (PTH) is responsible for increasing blood calcium levels, but once the level is sufficient, the parathyroid glands detect the sufficient calcium level and no longer produce PTH. PTH works in coordination with calcitonin to maintain this balance via its negative feedback loop.

Positive feedback, in contrast, involves the exponential increase of a reaction upon detection. Very few examples of positive feedback exist in the body, though oxytocin follows this model during childbirth.

Example Question #81 : Proteins

Sulfanilamide is an antibiotic that resembles the intermediate, 4-aminobenzoic acid (PABA), in the metabolic pathway to create folic acid. It binds to the active site of the enzyme that normally binds to PABA, and inhibits the binding of PABA temporarily. Since folic acid is necessary for bacterial growth, this antibiotic helps inhibit the spread of infection in humans.

Based on this information, what type of inhibitor is sulfanilamide?

Possible Answers:

Irreversible inhibitor

Noncompetitive inhibitor

Uncompetitive inhibitor

Competitive inhibitor

Correct answer:

Competitive inhibitor

Explanation:

Competitive inhibitors block substrates by binding noncovalently to the active site on enzymes. This prevents the substrate from entering the active site.

Noncompetitive inhibitors, in contrast, will act on regions outside of the active site to prevent binding. Uncompetitive inhibition is a specialized form of noncomptitive inhibition in which the inhibitor binds to the enzyme complex after the substrate has entered the active site. Irreversible inhibitors form covalent bonds, and fall outside the common inhibitor classifications.

Example Question #82 : Proteins

Which of the following strategies of enzymatic inhibition is used by noncompetitive inhibitors?

Possible Answers:

Bind to an allosteric site to cause a conformational shift in the enzyme

Target the enzyme for destruction using a protease

Induce another molecule to bind to the active site of the enzyme

Bind to the active site and prevent substrate from binding

Bind to substrate so that it cannot bind to the active site

Correct answer:

Bind to an allosteric site to cause a conformational shift in the enzyme

Explanation:

Noncompetitive inhibitors of enzymes function by binding to a site other than the active site on the enzyme, known as an allosteric site. This causes the enzyme to go through a conformational shift and inhibits binding of the substrate.

Competitive inhibitors bind to the enzyme active site to prevent substrate action. Uncompetitive inhibitors are a sub-category of noncompetitive inhibitors, and bind to an allosteric site only after the substrate has entered the active site, thus preventing the substrate from leaving.

Example Question #32 : Enzymes And Enzyme Inhibition

An inhibitor is added, disrupting the function of a particular enzyme. The experimenter adds more substrate, and enzyme function increases again. These results indicate the involvement of what type of inhibitor?

Possible Answers:

Competitive

Allosteric

There is not enough information to determine

Noncompetitive

Uncompetitive

Correct answer:

Competitive

Explanation:

Competitive inhibitors bind to the active site of the enzyme, blocking substrate entry. Increasing the concentration of the substrate helps overcome this type of inhibition by increasing the proportion of substrate to inhibitor in solution. By increasing substrate, the active site is more likely to bind substrate than inhibitor.

Noncompetitive inhibitors and uncompetitive inhibitors are types of allosteric inhibitors, meaning that they bind the enzyme in a site other than the active site. Increasing substrate concentration cannot overcome these types of inhibition because the inhibitor and enzyme are not in direct competition for binding sites.

Example Question #83 : Proteins

What type of inhibition affects both the Michaelis constant the maximum reaction rate of an enzyme?

Possible Answers:

All types of inhibition will affect the Michaelis constant or the maximum reaction rate, but never both

Non-competitive inhibition

All types of inhibition will affect both values

Uncompetitive inhibition

Competitive inhibition

Correct answer:

Uncompetitive inhibition

Explanation:

Enzymes are catalysts that speed up the rate at which a reaction occurs. Competitive inhibitors bind to the active site, affecting the Michaelis constant, but not the maximum reaction rate. Competitive inhibitors can be overcome by adding more substrate to displace the inhibitor. Non-competitive inhibitors bind to an allosteric site, affecting the maximum reaction rate, but not the Michaelis constant. No amount of additional substrate can overcome the inhibitor's effect.

In uncompetitive inhibition, the inhibitor binds to the enzyme-substrate complex. The substrate is required to bind the active site, affecting the Michaelis constant, and is then held in place by the inhibitor, affecting the maximum reaction rate. Uncompetitive inhibitors are, thus, as special class of inhibitor that will affect both values.

Example Question #81 : 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:

Uncompetitive inhibition

Noncompetitive inhibition

Competitive inhibition

Irreversible inhibition

Feedback 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 #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?

Possible Answers:

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

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 #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  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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