Test

Among enzymes, kinases catalyze phosphorylation, while phosphatases catalyze removal of phosphate(s). A cellʹs use of these enzymes can therefore function as an on-off switch for various processes. Which of the following is probably involved? A) the change in a proteinʹs charge leading to a conformational change B) the change in a proteinʹs charge leading to cleavage C) a change in the optimal pH at which a reaction will occur D) a change in the optimal temperature at which a reaction will occur E) the excision of one or more peptides

A

Competitive inhibitors block the entry of substrate into the active site of an enzyme. On which of the following properties of an active site does this primarily depend? A) the ability of an enzyme to form a template for holding and joining molecules B) the enzymeʹs ability to stretch reactants and move them toward a transition state C) the enzyme providing an appropriate microenvironment conducive to a reactionʹs occurrence D) the enzyme forming covalent bonds with the reactants E) the enzyme becoming too saturated because of the concentration of substrate

A

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate. The reaction is inhibited by malonic acid, which resembles succinate but cannot be acted upon by succinate dehydrogenase. Increasing the ratio of succinate to malonic acid reduces the inhibitory effect of malonic acid. 47) What is the purpose of using malonic acid in this experiment? A) It is a competitive inhibitor. B) It blocks the binding of fumarate. C) It is a noncompetitive inhibitor. D) It is able to bind to succinate. E) It replaces the usual enzyme.

A

Which of the following statements is (are) true about enzyme-catalyzed reactions? A) The reaction is faster than the same reaction in the absence of the enzyme. B) The free energy change of the reaction is opposite from the reaction in the absence of the enzyme. C) The reaction always goes in the direction toward chemical equilibrium. D) A and B only E) A, B, and C

A

A series of enzymes catalyze the reaction X → Y → Z → A. Product A binds to the enzyme that converts X to Y at a position remote from its active site. This binding decreases the activity of the enzyme. 63) Substance A functions as A) a coenzyme. B) an allosteric inhibitor. C) the substrate. D) an intermediate. E) a competitive inhibitor.

B

Besides turning enzymes on or off, what other means does a cell use to control enzymatic activity? A) cessation of all enzyme formation B) compartmentalization of enzymes into defined organelles C) exporting enzymes out of the cell D) connecting enzymes into large aggregates E) hydrophobic interactions

B

During a laboratory experiment, you discover that an enzyme-catalyzed reaction has a △G of -20 kcal/mol. If you double the amount of enzyme in the reaction, what will be the △G for the new reaction? A) -40 kcal/mol B) -20 kcal/mol C) 0 kcal/mol D) +20 kcal/mol E) +40 kcal/mol

B

How does a noncompetitive inhibitor decrease the rate of an enzyme reaction? A) by binding at the active site of the enzyme B) by changing the shape of a reactant C) by changing the free energy change of the reaction D) by acting as a coenzyme for the reaction E) by decreasing the activation energy of the reaction

B

In order to attach a particular amino acid to the tRNA molecule that will transport it, an enzyme, an aminoacyl-tRNA synthetase, is required, along with ATP. Initially, the enzyme has an active site for ATP and another for the amino acid, but it is not able to attach the tRNA. What must occur in order for the final attachment to occur? A) The ATP must first have to attach to the tRNA. B) The binding of the first two molecules must cause a 3-dimensional change that opens another active site on the enzyme. C) The hydrolysis of the ATP must be needed to allow the amino acid to bind to the synthetase. D) The tRNA molecule must have to alter its shape in order to be able to fit into the active site with the other two molecules. E) The 3ʹ end of the tRNA must have to be cleaved before it can have an attached amino acid.

B

Reactants capable of interacting to form products in a chemical reaction must first overcome a thermodynamic barrier known as the reactionʹs A) entropy. B) activation energy. C) endothermic level. D) heat content. E) free-energy content.

B

The active site of an enzyme is the region that A) binds allosteric regulators of the enzyme. B) is involved in the catalytic reaction of the enzyme. C) binds the products of the catalytic reaction. D) is inhibited by the presence of a coenzyme or a cofactor.

B

The mechanism in which the end product of a metabolic pathway inhibits an earlier step in the pathway is known as A) metabolic inhibition. B) feedback inhibition. C) allosteric inhibition. D) noncooperative inhibition. E) reversible inhibition.

B

Which of the following is likely to lead to an increase in the concentration of ATP in a cell? A) an increase in a cellʹs anabolic activity B) an increase in a cellʹs catabolic activity C) an increased influx of cofactor molecules D) an increased amino acid concentration E) the cellʹs increased transport of materials to the environment

B

Which of the following statements regarding enzymes is true? A) Enzymes decrease the free energy change of a reaction. B) Enzymes increase the rate of a reaction. C) Enzymes change the direction of chemical reactions. D) Enzymes are permanently altered by the reactions they catalyze. E) Enzymes prevent changes in substrate concentrations.

B

A series of enzymes catalyze the reaction X → Y → Z → A. Product A binds to the enzyme that converts X to Y at a position remote from its active site. This binding decreases the activity of the enzyme. 62) What is substance X? A) a coenzyme B) an allosteric inhibitor C) a substrate D) an intermediate E) the product

C

A solution of starch at room temperature does not readily decompose to form a solution of simple sugars because A) the starch solution has less free energy than the sugar solution. B) the hydrolysis of starch to sugar is endergonic. C) the activation energy barrier for this reaction cannot be surmounted. D) starch cannot be hydrolyzed in the presence of so much water. E) starch hydrolysis is nonspontaneous.

C

An important group of peripheral membrane proteins are enzymes, such as the phospholipases that attack the head groups of phospholipids leading to the degradation of damaged membranes. What properties must these enzymes exhibit? A) resistance to degradation B) independence from cofactor interaction C) water solubility D) lipid solubility E) membrane spanning domains

C

Increasing the substrate concentration in an enzymatic reaction could overcome which of the following? A) denaturization of the enzyme B) allosteric inhibition C) competitive inhibition D) saturation of the enzyme activity E) insufficient cofactors

C

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate. The reaction is inhibited by malonic acid, which resembles succinate but cannot be acted upon by succinate dehydrogenase. Increasing the ratio of succinate to malonic acid reduces the inhibitory effect of malonic acid. 46) Based on this information, which of the following is correct? A) Succinate dehydrogenase is the enzyme, and fumarate is the substrate. B) Succinate dehydrogenase is the enzyme, and malonic acid is the substrate. C) Succinate is the substrate, and fumarate is the product. D) Fumarate is the product, and malonic acid is a noncompetitive inhibitor. E) Malonic acid is the product, and fumarate is a competitive inhibitor.

C

Sucrose is a disaccharide, composed of the monosaccharides glucose and fructose. The hydrolysis of sucrose by the enzyme sucrase results in A) bringing glucose and fructose together to form sucrose. B) the release of water from sucrose as the bond between glucose and fructose is broken. C) breaking the bond between glucose and fructose and forming new bonds from the atoms of water. D) production of water from the sugar as bonds are broken between the glucose monomers. E) utilization of water as a covalent bond is formed between glucose and fructose to form sucrase.

C

When you have a severe fever, what may be a grave consequence if this is not controlled? A) destruction of your enzymesʹ primary structure B) removal of amine groups from your proteins C) change in the folding of enzymes D) removal of the amino acids in active sites E) binding of enzymes to inappropriate substrates

C

Which of the following is an example of cooperativity? A) the binding of an end product of a metabolic pathway to the first enzyme that acts in the pathway B) protein function at one site affected by binding at another of its active sites C) a molecule binding at one unit of a tetramer allowing faster binding at each of the other three D) the effect of increasing temperature on the rate of an enzymatic reaction E) binding of an ATP molecule along with one of the substrate molecules in an active site

C

Which of the following statements describes enzyme cooperativity? A) A multi-enzyme complex contains all the enzymes of a metabolic pathway. B) A product of a pathway serves as a competitive inhibitor of an early enzyme in the pathway. C) A substrate molecule bound to an active site affects the active site of several subunits. D) Several substrate molecules can be catalyzed by the same enzyme. E) A substrate binds to an active site and inhibits cooperation between enzymes in a pathway.

C

According to the induced fit hypothesis of enzyme catalysis, which of the following is correct? A) The binding of the substrate depends on the shape of the active site. B) Some enzymes change their structure when activators bind to the enzyme. C) A competitive inhibitor can outcompete the substrate for the active site. D) The binding of the substrate changes the shape of the enzymeʹs active site. E) The active site creates a microenvironment ideal for the reaction.

D

Assume that the reaction has a △G of -5.6 kcal/mol. Which of the following would be true? A) The reaction could be coupled to power an endergonic reaction with a △G of +6.2 kcal/mol. B) The reaction could be coupled to power an exergonic reaction with a △G of +8.8 kcal/mol. C) The reaction would result in a decrease in entropy (S) and an increase in the total energy content (H) of the system. D) The reaction would result in an increase in entropy (S) and a decrease in the total energy content (H) of the system. E) The reaction would result in products (C + D) with a greater free-energy content than in the initial reactants (A + B).

D

How can one increase the rate of a chemical reaction? A) Increase the activation energy needed. B) Cool the reactants. C) Decrease the concentration of the reactants. D) Add a catalyst. E) Increase the entropy of the reactants.

D

Some enzymatic regulation is allosteric. In such cases, which of the following would usually be found? A) cooperativity B) feedback inhibition C) both activating and inhibitory activity D) an enzyme with more than one subunit E) the need for cofactors

D

Which of the following is true of enzymes? A) Enzymes may require a nonprotein cofactor or ion for catalysis to take speed up more appreciably than if the enzymes act alone. B) Enzyme function is increased if the three-dimensional structure or conformation of an enzyme is altered. C) Enzyme function is independent of physical and chemical environmental factors such as pH and temperature. D) Enzymes increase the rate of chemical reaction by lowering activation energy barriers.

D

Zinc, an essential trace element for most organisms, is present in the active site of the enzyme carboxypeptidase. The zinc most likely functions as a(n) A) competitive inhibitor of the enzyme. B) noncompetitive inhibitor of the enzyme. C) allosteric activator of the enzyme. D) cofactor necessary for enzyme activity. E) coenzyme derived from a vitamin.

D

In coupled reactions, in which direction would the endergonic reaction be driven relative to the clockwise direction of the ATP reaction above and shown in the figure? A) from left to right at the top of the figure B) under the symbol for energy doing cellular work in the figure C) from right to left at the bottom of the figure D) it would be shown separately after the figure E) it would be shown in a clockwise direction at the top of the figure

E