Question 1

In the hydrolysis of p-nitrophenylacetate by chymotrypsin all of the following are correct EXCEPT:

Accepted Answer

A

Question 2

In the catalytic triad common to many serine proteases, _____ increases the basicity of _____, thus allowing deprotonation of _____ to serve as a nucleophile.

Accepted Answer

Asp increases the basicity of His, which in turn allows the deprotonation of Ser, enabling it to act as a nucleophile in the catalytic mechanism of serine proteases.

Question 3

All are true for low-barrier hydrogen bonds EXCEPT:

Accepted Answer

All the statements provided are true characteristics of low-barrier hydrogen bonds. In these bonds, the hydrogen is indeed centered between the two heteroatoms, making the interactions more covalent in nature. The bond order does approach 0.5 for both O−H interactions, and the barrier for the hydrogen atom to exchange between oxygens is lower, which is a defining feature of low-barrier hydrogen bonds.

Question 4

All are true for the enzyme-transition state complex EXCEPT:

Accepted Answer

The answer of All are true for the enzyme-transition state...

Question 5

Most covalent catalysis is carried out by enzymes using a:

Accepted Answer

Covalent catalysis often involves a ping-pong kinetic mechanism where the enzyme forms a covalent intermediate with one substrate before releasing it and binding the second substrate.

Question 6

The initial bond formation in the covalent intermediate in the chymotrypsin catalyzed reaction is between:

Accepted Answer

In the chymotrypsin catalyzed reaction, the initial bond formation in the covalent intermediate is between the hydroxyl group of serine (Ser-195) and the carbonyl carbon of the peptide backbone of the substrate. This forms an acyl-enzyme intermediate.

Question 7

Because the pK_a is near 7, ____ side-chains are often involved in general acid-base catalysis.

Accepted Answer

The pK_a, also known as the pKa, is the pH at which half of the ionizable groups are protonated and half are deprotonated. It is close to 7, so it indicates that the amino acid side chain with a pKa close to 7 is likely to be involved in general acid-base catalysis. Histidine has a side chain pKa of approximately 6, making it a good candidate for this role. Therefore, choice E, histidine, is the best answer.

Question 8

Which of the following statements regarding enzymes and transition states is true?

Accepted Answer

Binding of substrate to an enzyme often causes strain, which helps to promote the formation of the transition state. This strain can come in the form of changes in bond angles or lengths, which can make it easier for the reaction to proceed. Choice A is incorrect because the stabilization of the transition state must actually be greater than the stabilization of ES for catalysis to occur. Choice C is incorrect because the transition state conformation is not necessarily identical in enzyme-catalyzed reactions compared to uncatalyzed reactions. Choice D is incorrect because the formation of the transition state does not guarantee that the reaction will proceed to product, although it does make it more likely.

Question 9

Metal ion catalysis include all EXCEPT:

Accepted Answer

All the options listed (A through D) are examples of roles that metal ions can play in catalysis, including increasing the acidity of a nucleophile, being required for the enzyme's native state, binding weakly during the catalytic cycle, and stabilizing increased electron density or negative charge during a reaction.

Question 10

The transition state has an estimated life-time of about:

Accepted Answer

The transition state in a chemical reaction is extremely short-lived, typically existing for about 10^−14 to 10^−13 seconds.

Question 11

The good transition state analog is one which would serve also as an extremely effective:

Accepted Answer

A good transition state analog would have a similar structure to the transition state of the enzyme-catalyzed reaction, and would therefore bind tightly to the active site of the enzyme. This tight binding would make it an effective competitive inhibitor, as it would prevent the formation of the transition state and therefore the reaction.

Question 12

The catalytic mechanism below is an example of:

Accepted Answer

The mechanism shown involves a specific base (histidine) catalyzing the deprotonation of a water molecule, which then acts as a nucleophile to attack the substrate. This is an example of general base catalysis, where the base (in this case, histidine) is deprotonated to become a better nucleophile and facilitate the reaction. Choice A is incorrect because there is no covalent bond formation with the catalytic residue. Choice B is incorrect because there is no electrophilic attack on the substrate. Choice C is incorrect because there is no specific acid/base pair involved in the catalysis. Choice E is incorrect because there is no evidence of an unusual hydrogen bonding network that leads to catalysis.

Question 13

The mechanism of chymotrypsin involves which of the following elements?

Accepted Answer

Chymotrypsin's mechanism involves the formation of an acyl-enzyme intermediate, which is a key step that must be hydrolyzed to complete the reaction.

Question 14

An enzyme-catalyzed reaction rate will be increased if the energy level of ES can be increased. Which of the following will not increase the energy level of ES?

Accepted Answer

Destabilization of ES (enzyme-substrate complex) by solvation typically stabilizes the complex rather than increasing its energy level. Solvation involves the interaction of solvent molecules with the ES complex, which can lead to a more thermodynamically favorable state, thus not increasing the energy level of ES.

Question 15

Because the enzymatic reaction rate is determined by the difference in energy between ES and ____, the tighter binding of the substrate, the ____ the rate of reaction.

Accepted Answer

The answer of Because the enzymatic reaction rate is determined...

Question 16

Enzymes have active sites which have the greatest complementarity to the:

Accepted Answer

Enzymes catalyze reactions by lowering the activation energy required for the reaction to occur. The active site of an enzyme is specifically shaped to bind to the transition state of the reaction, which is the unstable intermediate state between the substrate and product. By binding to the transition state, the enzyme stabilizes it and lowers the energy required for the reaction to proceed. Therefore, the active site has the greatest complementarity to the transition state, not the substrate or product.

Question 17

Transition-state analogs are:

Accepted Answer

Transition-state analogs are designed to mimic the transition state of a substrate in an enzyme-catalyzed reaction. They bind more tightly to the enzyme than the substrate itself, competing for the active site. Because they are stable molecules that resemble the transition state, they can be used to study enzyme mechanisms and design inhibitors.

Question 18

All are catalytic mechanisms or factors that contribute to the performance of enzymes EXCEPT:

Accepted Answer

Entropy gain in ES formation is not a catalytic mechanism but rather a thermodynamic factor that contributes to the stability of the enzyme-substrate complex. The other choices listed (covalent catalysis, general acid or base catalysis, and proximity and orientation) are all catalytic mechanisms that enhance the rate of enzymatic reactions.

Question 19

Since enzymes are not rigid molecules but rather flexible proteins, which of the following may be attributed to this fact?

Accepted Answer

All of the above choices are correct. The flexibility of enzymes allows them to adapt to the shape of the substrate, orient catalytic residues for effective bond breakage, place strain on the substrate during binding, and achieve near-attack conformations during transition state formation.

Question 20

Which of the following amino acids would NOT provide a nucleophilic center for covalent catalysis?

Accepted Answer

Alanine does not have a side chain that can act as a nucleophile. Its side chain is a simple methyl group, which lacks the functional groups necessary for nucleophilic activity seen in the side chains of lysine, glutamic acid, serine, and cysteine.

Quiz 14: Mechanisms of Enzyme Action

In the hydrolysis of p-nitrophenylacetate by chymotrypsin all of the following are correct EXCEPT:

In the catalytic triad common to many serine proteases, ___ increases the basicity of _, thus allowing deprotonation of ___ to serve as a nucleophile.

All are true for low-barrier hydrogen bonds EXCEPT:

All are true for the enzyme-transition state complex EXCEPT:

Most covalent catalysis is carried out by enzymes using a:

The initial bond formation in the covalent intermediate in the chymotrypsin catalyzed reaction is between:

Because the pK_a is near 7, ____ side-chains are often involved in general acid-base catalysis.

Which of the following statements regarding enzymes and transition states is true?

Metal ion catalysis include all EXCEPT:

The transition state has an estimated life-time of about:

The good transition state analog is one which would serve also as an extremely effective:

The catalytic mechanism below is an example of:

The mechanism of chymotrypsin involves which of the following elements?

An enzyme-catalyzed reaction rate will be increased if the energy level of ES can be increased. Which of the following will not increase the energy level of ES?

Because the enzymatic reaction rate is determined by the difference in energy between ES and , the tighter binding of the substrate, the the rate of reaction.

Enzymes have active sites which have the greatest complementarity to the:

Transition-state analogs are:

All are catalytic mechanisms or factors that contribute to the performance of enzymes EXCEPT:

Since enzymes are not rigid molecules but rather flexible proteins, which of the following may be attributed to this fact?

Which of the following amino acids would NOT provide a nucleophilic center for covalent catalysis?

Quiz 14: Mechanisms of Enzyme Action

In the hydrolysis of p-nitrophenylacetate by chymotrypsin all of the following are correct EXCEPT:

In the catalytic triad common to many serine proteases, _____ increases the basicity of _____, thus allowing deprotonation of _____ to serve as a nucleophile.

All are true for low-barrier hydrogen bonds EXCEPT:

All are true for the enzyme-transition state complex EXCEPT:

Most covalent catalysis is carried out by enzymes using a:

The initial bond formation in the covalent intermediate in the chymotrypsin catalyzed reaction is between:

Because the pKa is near 7, ____ side-chains are often involved in general acid-base catalysis.

Which of the following statements regarding enzymes and transition states is true?

Metal ion catalysis include all EXCEPT:

The transition state has an estimated life-time of about:

The good transition state analog is one which would serve also as an extremely effective:

The catalytic mechanism below is an example of:

The mechanism of chymotrypsin involves which of the following elements?

An enzyme-catalyzed reaction rate will be increased if the energy level of ES can be increased. Which of the following will not increase the energy level of ES?

Because the enzymatic reaction rate is determined by the difference in energy between ES and ____, the tighter binding of the substrate, the ____ the rate of reaction.

Enzymes have active sites which have the greatest complementarity to the:

Transition-state analogs are:

All are catalytic mechanisms or factors that contribute to the performance of enzymes EXCEPT:

Since enzymes are not rigid molecules but rather flexible proteins, which of the following may be attributed to this fact?

Which of the following amino acids would NOT provide a nucleophilic center for covalent catalysis?

In the catalytic triad common to many serine proteases, ___ increases the basicity of _, thus allowing deprotonation of ___ to serve as a nucleophile.

Because the pK_a is near 7, ____ side-chains are often involved in general acid-base catalysis.

Because the enzymatic reaction rate is determined by the difference in energy between ES and , the tighter binding of the substrate, the the rate of reaction.