BCCB2000 Lecture 8 Questions Flashcards
(25 cards)
Enzymes act as catalysts because: A. they are specific for only one substrate B. they bind the transition state of the reaction catalysed C. their active sites are complementary to the substrate of the reaction D. they are not altered by the reaction catalysed E. they do not change the position of a thermodynamic quilibrium
B. they bind the transition state of the reaction catalysed
When no other non-protein factor is needed for an enzyme to work then the enzyme is known as: A. A Holoenzyme B. An Apoenzyme C. Conjugated D. Unconjugated
D. Unconjugated
Allosteric enzymes: A. depend mainly upon covalent modification of the enzyme B. bind ligands reversibly C. have a Km that is 1/3 Vmax D. usually show strict Michaelis-Menten kinetics. E. have a Vmax that depends only on substrate concentration
B. bind ligands reversibly
Consider the following information: Acid-base catalysis is the general term given to an enzyme-catalysed reaction that uses ionized groups in the enzyme active site to act as proton donors or acceptors. The ionizable groups that participate in this reaction must be in the correct protonation state for catalysis to occur. Some pKa’s for amino acid side chains are as follows: E = 4.3 D = 3.9 C = 8.3 Y = 10.1 K = 10.8 R = 12.5 H = 6.0 An enzymatic reaction works best at pH 6 to 8. This is compatible with the assumption that the reaction mechanism requires two ionizable amino acid side chains in the active site of the enzyme possibly: A. protonated glutamate and deprotonated aspartate B. protonated histidine and deprotonated lysine C. protonated glutamate and deprotonated histidine D. protonated arginine and deprotonated lysine E. protonated cysteine and deprotonated histidine
E. protonated cysteine and deprotonated histidine
Which of the following potentially provide an answer to the question ‘Why are enzymes so big?’ when compared with small non-enzyme catalysts? A. The large size of the enzyme may protect it the active site from damage. B. The large size of the enzyme is needed to increase the activation energy of a reaction C. The large size of the enzyme may be involved in its regulation D. The large size of the enzyme forms a scaffold to optimally interact with the substrate.
A. The large size of the enzyme may protect it the active site from damage. C. The large size of the enzyme may be involved in its regulation D. The large size of the enzyme forms a scaffold to optimally interact with the substrate.
Catalase is an enzyme that catalyses the following reaction: 2H2O2 —> 2H2O + O2 Choose the most likely and appropriate answer from the following list of statements. A. Catalase provides the energy to make the reaction proceed faster B. Catalase can change the reaction mechanism C. Catalyse alters the equilibrium in favour of the products. D. Catalase reduces the free energy change (ΔG) for the activation of the reaction E. Catalase reduces the free energy change (ΔG) for the reaction F. Catalase is used up in the reaction
B. Catalase can change the reaction mechanism D. Catalase reduces the free energy change (ΔG) for the activation of the reaction
Enymes increase the rates of both forward and reverse reactions but do not affect the equilibrium of chemical reactions. True or False?
True
Enzymes are highly specific and catalyse only one reaction with only one particular substrate. True or False?
False
Enzymes differ from other catalysts in that enzymes: A. lower the activation energy of the reaction catalyzed. B. fail to influence the equilibrium point of the reaction. C. form an activated complex with the reactants. D. usually display specificity toward a single reactant. E. are not consumed in the reaction.
D. usually display specificity toward a single reactant.
Enzymes are not as efficient as most catalysts used in organic chemistry, since they must function at body temperature. True or False?
False
Which of the following statements does not apply to the effect of temperature on an enzyme-catalysed reaction? A. At low temperatures the rate doubles (approximately) for each 10°C rise B. The temperature at which the maximum occurs on the activity versus temperature graph is called the optimum temperature C. Denatured enzymes always regain their lost activity on cooling D. At high temperatures the enzyme may be denatured E. Enzymes are inactivated both by being held for long periods of time at moderate temperatures and by being held for short periods of time at high temperatures
C. Denatured enzymes always regain their lost activity on cooling
Chemical reactions in the absence of a catalyst may be distinguished from enzyme-catalysed reactions because: A. chemical reactions are saturable with substrate B. the free energy of activation is lower for an enzyme-catalysed reaction C. chemical reactions proceed at lower temperatures D. enzyme-catalysed reactions are not first-order with respect to substrate concentration below the Km value E. enzyme-catalysed reactions produce a variety of products from a single substrate
B. the free energy of activation is lower for an enzyme-catalysed reaction
A histidine was determined to be the critical residue involved in an enzyme-catalyzed reaction. If the pKa of the histidine is known to be 6.5 in the active site and the pH of maximum catalytic activity is 7.2, what is likely the primary role of histidine in the catalytic reaction? A. acts as a proton donor B. forms a covalent bond with the substrate C. stabilizes a charged intermediate D. reduces the entropy of the substrate
A. acts as a proton donor
Which of the following statements is true of enzyme catalysts? A. They increase the stability of the product of a desired reaction by allowing ionizations, resonance, and isomerizations not normally available to substrates. B. They lower the free energy of the reaction C. They increase the equilibrium constant for a reaction, thus favoring product formation. D. They lower the free energy of the transition state E. They specifically bind to substrates, but are never covalently attached to substrate or product.
D. They lower the free energy of the transition state
Oxidases, peroxidases, oxygenases or reductases are all: A. lyases. B. synthases. C. synthetases. D. oxidoreductases. E. hydrolases.
D. oxidoreductases.
Why is it important that the enzymes in lysosomes are more active at acidic pH than at neutral pH? A. The pH dependence allows for minimum efficiency for the digestion of food B. It prevents their diffusion out of the lysosomes. C. It maximizes the interaction with their substrates which are always bases. D. It prevents them from accidentally degrading the macromolecules in the cytosol. E. It allows for regulation of their uptake by the mitochondria.
D. It prevents them from accidentally degrading the macromolecules in the cytosol.
An enzyme that catalyzes conversions of L-sugars to D-sugars is called a/an ________.
Isomerase
When a non-protein factor is needed for an enzyme to work then the enzyme is known as: A. Non-allosteric B. Conjugated C. Allosteric D. Unconjugated
B. Conjugated
An enzyme can only use one substrate at a time. True or False?
False
Allosteric modulators seldom resemble the substrate or product of the enzyme. What does this observation show? A. Modulators likely bind at a site other than the active site. B. Modulators always act as activators. C. Modulators bind non-covalently to the enzyme. D. The enzyme catalyzes more than one reaction
A. Modulators likely bind at a site other than the active site.
When a non-protein factor is needed for an enzyme to work, and the factor is bound, the enzyme is known as: A. A holoenzyme B. An allosteric enzyme C. An apoenzyme D. A unconjugated enzyme
A. A holoenzyme
Which of the following are typical characteristics of enzymes: A. Enzymes are usually highly regulated B. Enzymes act on only one substrate molecule C. Enzymes increase the rate of reactions much more than non-enzyme catalysts for the same type of reaction. D. Enzymes are highly specific E. Enzymes alter the equilibrium of a reaction
A. Enzymes are usually highly regulated C. Enzymes increase the rate of reactions much more than non-enzyme catalysts for the same type of reaction. D. Enzymes are highly specific
The enzyme fumarase catalyzes the reversible hydration of fumaric acid to L-malate; however, the enzyme will not catalyze the hydration of maleic acid, the cis isomer of fumaric acid. This is an example of: A. chiral activity. B. stereospecificity. C. racemization. D. stereoisomerization. E. biological activity.
B. stereospecificity.
The release of free energy as a result of the interaction between enzyme and substrate is sometimes called the binding energy. Which of the following is true of the binding energy derived from enzyme-substrate interactions? A. Most of it is used for enzyme stability B. Most of it is derived from covalent bonds between enzyme and substrate. C. It is sometimes used to hold two substrates in the optimal orientation for reaction. D. It is the difference in initial and final free energy between reactants and products. E. It is used to provide more kinetic energy to the reactants
C. It is sometimes used to hold two substrates in the optimal orientation for reaction.
