Question 1

In the acid-catalyzed hydrolysis of an amide, why is the initial proton transfer to the carbonyl oxygen considered a fast step?

Accepted Answer

Because proton transfers are always fast due to the high mobility of protons in aqueous solution.

Question 2

What is the main consequence of protonating the carbonyl oxygen in an amide during acid-catalyzed hydrolysis?

Accepted Answer

It makes the carbonyl carbon more electrophilic, increasing its susceptibility to nucleophilic attack by water.

Question 3

Why is water a slow nucleophile in the acid-catalyzed hydrolysis of amides?

Accepted Answer

Because water is a weak nucleophile and the reaction requires overcoming a high activation energy.

Question 4

Which of the following best describes the role of general acid-base catalysis in enzyme-catalyzed reactions?

Accepted Answer

It involves the transfer of protons to stabilize charged intermediates during the reaction.

Question 5

In the catalytic mechanism of chymotrypsin, which amino acid acts as a nucleophile to attack the carbonyl carbon of the substrate?

Accepted Answer

Serine (Ser-195)

Question 6

Given the following equation for enzyme kinetics, $$k_{cat}/K_M = rac{V_{max}/[E]}{K_M}$$, what does a high value of $$k_{cat}/K_M$$ indicate about the enzyme?

Accepted Answer

The enzyme is highly efficient and approaches catalytic perfection.

Question 7

Superoxide dismutase (SOD) catalyzes the conversion of superoxide to hydrogen peroxide. Why is it important for this reaction to be extremely fast?

Accepted Answer

Superoxide is a strong oxidizing agent that can cause damage to cells if not quickly removed.

Question 8

Which of the following is a reason why the rate of the SOD-catalyzed reaction exceeds the diffusion-controlled limit?

Accepted Answer

The active site contains a positively charged copper ion that attracts the negatively charged superoxide, increasing the rate of substrate binding.

Question 9

Which of the following is NOT a way to regulate the activity of an enzyme-catalyzed reaction?

Accepted Answer

Changing the pH of the cytoplasm.

Question 10

Allosteric regulation of enzymes typically involves:

Accepted Answer

Non-covalent binding of a small molecule at a site distinct from the active site, causing a conformational change.

Question 11

Which statement best describes the difference between an allosteric regulator and an uncompetitive inhibitor?

Accepted Answer

Uncompetitive inhibitors bind only to the substrate-bound enzyme within the active site, while allosteric regulators bind outside the active site.

Question 12

ATCase binds cooperatively to its substrates. What would the graph of $$V_0 vs$$. $$[Asp]$$ look like in the presence of saturating amounts of the second substrate?

Accepted Answer

A sigmoidal curve, indicating cooperative binding.

Question 13

CTP is an allosteric inhibitor of ATCase and ATP is an allosteric activator. What is the biological logic of this regulation?

Accepted Answer

CTP signals that the cell does not need more pyrimidines, so ATCase slows down; ATP signals the need for more pyrimidines for RNA synthesis.

Question 14

Which of the following is a correct statement about zymogen activation?

Accepted Answer

Zymogens are inactive enzymes that are activated by peptide bond cleavage.

Question 15

Which catalytic mode involves the formation of a covalent bond between the enzyme and the substrate?

Accepted Answer

Covalent catalysis

Question 16

In the mechanism of triose phosphate isomerase (TPI), what is the main reason for its high catalytic efficiency?

Accepted Answer

The reaction consists of rapid proton transfers, which are very fast.

Question 17

What determines substrate specificity in the chymotrypsin-catalyzed reaction?

Accepted Answer

The presence of a binding pocket in which the side chain of the substrate fits.

Question 18

Which of the following statements about enzyme inhibition is correct?

Accepted Answer

Inhibitors can bind either inside or outside the active site, affecting enzyme activity.

Question 19

Which step in acid-catalyzed hydrolysis of an amide is typically the rate-limiting step?

Accepted Answer

Nucleophilic attack by water on the protonated carbonyl carbon.

Mechanisms and Regulation of Enzyme-Catalyzed Reactions

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