Question 1

The oligomeric structure of all lactate dehydrogenase isozymes is a

Accepted Answer

The oligomeric structure of all lactate dehydrogenase isozymes is a tetramer, composed of four subunits.

Question 2

All of the following statements about the Michaelis constant are correct EXCEPT which one?

Accepted Answer

Km (Michaelis constant) does not approximate the equilibrium constant for the conversion of ES to E and P; it is a measure of the substrate concentration at which the reaction rate is half of its maximum value.

Question 3

Which is LEAST likely to be characteristic of a regulated enzyme in a metabolic pathway?

Accepted Answer

Enzymes that catalyze physiologically reversible reactions are less likely to be points of regulation in metabolic pathways. Regulation typically occurs at steps that are irreversible under physiological conditions, as these steps are more effective control points for the pathway's overall direction and rate.

Question 4

Stereospecificity may be explained by

Accepted Answer

Stereospecificity refers to the ability of an enzyme to selectively bind and catalyze a specific stereoisomer of a substrate. This specificity is often due to the requirement of three-point attachment, where the enzyme has specific binding sites for the functional groups on the substrate that are arranged in a particular spatial orientation. Therefore, choice C, three point attachment, is the best explanation for stereospecificity.

Question 5

For an enzyme-substrate pair, a graph of reaction rate versus substrate concentration gives an S-shaped curve. From these data which of the following is the most likely conclusion?

Accepted Answer

The answer of For an enzyme-substrate pair, a graph of...

Question 6

Considering the active site of serine protease enzymes, a fair generalization is that:

Accepted Answer

The active site of serine protease enzymes is a region on the protein that binds to the substrate and carries out the chemical reaction. This requires specific three-dimensional shape of the protein, which brings the amino acid residues involved in the active site into close proximity. While some amino acids may play a more direct role in catalysis than others, nearly all the residues in the active site play some role in substrate recognition or catalysis. The conformation of the active site region exists in the native state of the enzyme, but it may change slightly upon substrate binding. Therefore, the best generalization is that the amino acid residues which form the active site are in close proximity because of the specific three-dimensional shape the protein adopts in its native state.

Question 7

What happens when substrate concentration is much higher than Km?

Accepted Answer

When substrate concentration is much higher than Km, the rate of the reaction is limited only by the rate at which the enzyme can convert the substrate to product, which is determined by Kcat (turnover number) and the concentration of enzyme ([E]). At saturation, all the enzyme molecules are bound to the substrate and the reaction rate becomes constant and equal to Kcat [E].

Question 8

For an enzyme-substrate pair, a graph of reaction rate versus substrate concentration gives an S-shaped curve. From these data which of the following is the most likely conclusion?

Accepted Answer

The S-shaped curve indicates that the enzyme has more than one binding site for the substrate, which is characteristic of enzymes with cooperativity (positive allosteric regulation). This type of behavior cannot be explained by the Michaelis-Menten equation alone and suggests that there are additional binding sites that affect the catalytic activity of the enzyme. Additionally, the presence of product inhibition or multiple polypeptide chains would not necessarily result in an S-shaped curve.

Question 9

The movement of a tyrosyl residue in carboxypeptidase A when its substrate binds is an example of

Accepted Answer

The movement of the tyrosyl residue in carboxypeptidase A when its substrate binds is an example of induced fit. This is because the tyrosyl residue changes its conformation to fit the substrate, leading to the formation of the enzyme-substrate complex.

Question 10

Enzymes which are in an inactive form after the removal of a cofactor or prosthetic group are termed:

Accepted Answer

Enzymes which are in an inactive form after the removal of a cofactor or prosthetic group are called apoenzymes. Choice A (isozymes) refers to enzymes that have the same function but different structures. Choice B (holoenzymes) refers to enzymes with their cofactors or prosthetic groups attached. Choice C (zymogens) are also called proenzymes, which are inactive precursors of enzymes that require activation by cleavage of a specific peptide bond. Choice E (proenzymes) is the same as zymogens.

Question 11

Which of the following is NOT an assumption in the Michaelis Menten model for enzyme kinetic behavior?

Accepted Answer

The Michaelis Menten model assumes that a complex is formed between enzyme and substrate, the concentration of substrate is much larger than that of the enzyme, only initial velocities are measured, and a steady state concentration of enzyme-substrate complex is rapidly established and maintained during measurement. However, it does not assume that all substrates for the same enzyme will be used at the same rate.

Question 12

All reactions are technically reversible. Why are some considered de facto irreversible?

Accepted Answer

Some reactions are considered de facto irreversible because, at equilibrium, the concentration of the substrate is so low that it is effectively undetectable, making the reverse reaction practically impossible to observe.

Question 13

All of the following effects may occur in enzyme catalysis EXCEPT which one?

Accepted Answer

Enzymes can lower the activation energy required for a reaction to occur, making a forward reaction thermodynamically more favorable. All of the other options listed are potential effects of enzyme catalysis.

Question 14

Which of the following statements relative to the maximum velocity (Vmax) of an enzyme catalyzed reaction (assayed under conditions of excess substrate) is correct?

Accepted Answer

Vmax is reached when all the enzyme molecules are bound to substrate molecules, meaning the concentration of the enzyme-substrate complex is equal to the original concentration of the enzyme. This condition reflects that all active sites of the enzyme are occupied, and the reaction rate cannot increase further by adding more substrate.

Question 15

The turnover number of an enzyme catalyzed reaction is:

Accepted Answer

The turnover number, also known as kcat, is the maximum number of substrate molecules that an enzyme can convert to product per unit time when the enzyme is saturated with substrate. Hence, it is a measure of the efficiency of the enzyme in catalyzing the reaction. The turnover number is specific to an enzyme-catalyzed reaction and is not related to the equilibrium constant, nor does it change with the concentration of the enzyme. It can, however, be affected by factors such as pH, temperature, and enzyme inhibitors.

Question 16

The fact that an enzyme-catalyzed reaction is first order with respect to substrate at low substrate concentration but becomes zero order with respect to substrate at high substrate concentration is explained by

Accepted Answer

This phenomenon is explained by the concept of saturation. At low substrate concentrations, the rate of the enzyme-catalyzed reaction increases linearly with substrate concentration, indicating first-order kinetics. However, at high substrate concentrations, the enzyme becomes saturated with substrate, meaning all active sites are occupied, and the reaction rate reaches a maximum and becomes independent of substrate concentration, indicating zero-order kinetics.

Question 17

Induced fit

Accepted Answer

Induced fit is a conformational change that occurs when the substrate binds to the active site of an enzyme, causing it to change its shape slightly and facilitating the reaction. This mechanism ensures that substrates are held in the correct orientation for the reaction to occur, and can also help to lower the activation energy required for the reaction.

Question 18

In the spectrophotometric assay for lactate dehydrogenase (LDH) activity which of the following reaction components is monitored at a wavelength of nanometers?

Accepted Answer

The spectrophotometric assay for LDH activity measures the rate of NADH production at a wavelength of 340 nm as LDH catalyzes the conversion of lactate and NAD+ to pyruvate and NADH.

Question 19

In the presence of effector A an enzyme displays sigmoidal behavior when initial velocity is plotted against substrate concentration. With increasing concentrations of A the curve is displaced to the left (i.e., towards the ordinate). Effector A is thus

Accepted Answer

The sigmoidal behavior indicates cooperative binding of the substrate to the enzyme. The displacement of the curve to the left indicates an increase in affinity of the enzyme for the substrate. This kind of behavior is seen in positive allosteric effectors, which bind to the enzyme and enhance substrate binding.

Question 20

Standard free energy of a reaction can be used for all of the following EXCEPT

Accepted Answer

The standard free energy of a reaction can only predict the direction and extent of a reaction under standard conditions. The rate of a reaction is not directly related to the free energy change of the reaction.

Quiz 10: Enzymes: Classification, Kinetics, and Control

The oligomeric structure of all lactate dehydrogenase isozymes is a

All of the following statements about the Michaelis constant are correct EXCEPT which one?

Which is LEAST likely to be characteristic of a regulated enzyme in a metabolic pathway?

Stereospecificity may be explained by

For an enzyme-substrate pair, a graph of reaction rate versus substrate concentration gives an S-shaped curve. From these data which of the following is the most likely conclusion?

Considering the active site of serine protease enzymes, a fair generalization is that:

What happens when substrate concentration is much higher than Km?

For an enzyme-substrate pair, a graph of reaction rate versus substrate concentration gives an S-shaped curve. From these data which of the following is the most likely conclusion?

The movement of a tyrosyl residue in carboxypeptidase A when its substrate binds is an example of

Enzymes which are in an inactive form after the removal of a cofactor or prosthetic group are termed:

Which of the following is NOT an assumption in the Michaelis Menten model for enzyme kinetic behavior?

All reactions are technically reversible. Why are some considered de facto irreversible?

All of the following effects may occur in enzyme catalysis EXCEPT which one?

Which of the following statements relative to the maximum velocity (Vmax) of an enzyme catalyzed reaction (assayed under conditions of excess substrate) is correct?

The turnover number of an enzyme catalyzed reaction is:

The fact that an enzyme-catalyzed reaction is first order with respect to substrate at low substrate concentration but becomes zero order with respect to substrate at high substrate concentration is explained by

Induced fit

In the spectrophotometric assay for lactate dehydrogenase (LDH) activity which of the following reaction components is monitored at a wavelength of nanometers?

In the presence of effector A an enzyme displays sigmoidal behavior when initial velocity is plotted against substrate concentration. With increasing concentrations of A the curve is displaced to the left (i.e., towards the ordinate) . Effector A is thus

Standard free energy of a reaction can be used for all of the following EXCEPT