Biochem test 3
Usually the quickest method of influencing an enzymatic activity is by: a. allosteric regulation. b. covalent modification. c. enzyme induction. d. activation of a zymogen. e. enzyme destruction.
a. Allosteric regulation
All are characteristics of ordered single-displacement reactions EXCEPT: a. Lineweaver-Burk plots with lines that intersect to the left of the 1/v axis. b. a chemically modified enzyme-intermediate. c. the lack of any exchange reaction activity. d. the lack of competitive substrate effects. e. single substrate initial binding activity.
b. a chemically modified enzyme-intermediate.
The International Units of an enzyme are based on the: a. ratio of enzyme to other proteins. b. micromoles of product formed per minute. c. moles of substrate reacted. d. micromoles of product produced at Vmax/2. e. none of the above.
b. micromoles of product formed per minute.
An enzyme's specificity can be due to: a. the ratio of catalyzed rate to the uncatalyzed rate of reaction. b. molecular recognition based on structural complementarity. c. amount of enzyme produced by the cell. d. amount of substrate available. e. metabolic activators.
b. molecular recognition based on structural complementarity.
The specific site on the enzyme where ____ binds and catalysis occurs is called the ____ site. a. coenzyme; substrate b. substrate; active c. coenzyme; regulatory d. regulatory; active e. none of the above
b. substrate; active
Which of the following statements regarding enzyme regulation is true? a. addition of an inhibitor to a V system results in kinetics similar to addition of a competitive inhibitor to a typical hyperbolic system b. allosteric effectors are always more powerful than covalent modification c. addition of an allosteric activator to a K system changes the plot of V vs. [S] from a sigmoidal curve to a more hyperbolic curve d. the T state of an enzyme generally has more activity than the R state e. none of the above are true
c. Addition of an allostericc activator to a K system changes the plot of V vs.[S] from a sigmoidal curve to a more hyperbolic curve.
How do catalysts work to accelerate a chemical reaction? a. They raise the average energy of the reactants. b. They provide a means of acceleration by being completely consumed in the reaction. c. They lower the energy of activation. d. They lower the overall free energy change of the reaction. e. They raise the overall free energy change of the reaction.
c. They lower the energy of activation.
Penicillin is an example of a mechanism-based enzyme inactivator and is a(n): a. competitive inhibitor. b. noncompetitive inhibitor. c. suicide substrate. d. uncompetitive inhibitor. e. none of the above.
c. suicide substrate.
All of the following are properties of a coenzyme EXCEPT: a. They are usually actively involved in the catalytic reaction of the enzyme. b. They tend to be stable to heat. c. They can serve as intermediate carriers of functional groups. d. They are protein components. e. They may contain vitamins as part of their structure.
d. They are protein components.
All are true for catalysts EXCEPT: a. They work by lowering the energy of activation. b. The average energy of the reaction is unchanged. c. They combine transiently with the reactants promoting a reactive transition state condition. d. They are regenerated after each reaction cycle. e. All are true.
e. all are true
All are true for kcat EXCEPT : a. referred to as the molecular activity of the enzyme. b. called the turnover number of the enzyme. c. measures the maximal catalytic activity or kinetic efficiency of an enzyme. d. defines the number of substrate molecules converted into product/enzyme molecule/unit of time when the enzyme is saturated with substrate. e. all are true.
e. all are true
If the rate constant for the enzyme catalyzed reaction is 2 × 10^5/sec and the rate constant for the uncatalyzed reaction is 2 × 10^−6/sec, the catalytic power of the enzyme is: A.10^11 B.2x10^-11 C. 10^-11 D.10^-1 E. 2x10^-1
A. 10^11
Catalytic antibodies, also called ____, are generated against an antigen that is: a. abzymes; an analog of the transition-state intermediate in the reaction. b. abzymes; the substrate of the reaction. c. zymogens; an analog of the product of the reaction. d. holoenzyme; an analog of the transition-state intermediate in the reaction. e. none of the above.
A. Abzymes; an anolog of the transition-state intermediate in the rection
Which of the following amino acids would NOT provide a nucleophilic center for covalent catalysis? a. alanine b. lysine c. glutamic acid d. serine e. cysteine
A. Alanine
The T form (tense or taut form) of deoxyhemoglobin differs from oxyhemoglobin (the R form or relaxed form) by all EXCEPT: a. covalent linkages between subunits. b. specific intrachain hydrogen bonds. c. between β-subunit salt links (ion-pair bonds). d. between α-subunits salt links (ion-pair bonds). e. intrachain salt bridges.
A. Covalent linkages between subunits
In the glutamate:aspartate aminotransferase catalyzed reaction mechanism, glutamate would react with ____ to form α-ketoglutarate. a. E-pyridoxal phosphate complex b. E-pyridoxamine phosphate complex c. aspartate d. oxaloacetate e. none of the above
A. E-paradoxal phosphate complex
Fetal hemoglobin (Hb F) has an intrinsically greater affinity for O2 than adult hemoglobin (Hb A) because: a. Hb F has a diminished capacity to bind BPG compared to Hb A. b. Hb A has a greater affinity for oxygen than does Hb F. c. BPG binds Hb F with greater affinity than it binds Hb A. d. The pH of fetal blood is less than the pH of maternal blood. e. All of the above are correct.
A. Hb F has a diminished capasity to bind BPG compared to Hb A
In uncompetitive inhibition: a. I combines only with ES. b. I combines only with E. c. I combines with E and ES. d. I combines with EP. e. none of the above.
A. I combines only with ES
Of the following, which promotes the release of oxygen by hemoglobin? a. increase in [H+ ] b. decrease in [CO2] c. decrease in [2,3-bisphosphoglycerate] d. Tyr HC2-Val FG5 hydrogen bond breaking e. none of the above
A. Increase in [H+]
All are true for inhibitor I if it is a competitive inhibitor EXCEPT: a. It binds a site other than the active site. b. It is structurally similar to the substrate. c. EI does not give rise to E + P. d. For a given [I], v decreases. e. At some point, S can displace all of I on E.
A. It binds at a site other than the active site.
Which statement is correct about the Michaelis-Menten constant, Km, for the kinetic mechanism below? K1. K2 E+S<—>ES<—>E+P K-1. K-2 a. It is numerically equal to the substrate concentration required to achieve one half the maximum velocity. b. Its defined as Km = k1/(k−1 + k2). c. It is approximately equal to the dissociation constant for the enzyme-substrate complex to E + P. d. The value of Km is constant for an enzyme regardless of the specific substrate molecule used to determine it. e. Its numeric value has the units of moles−1.
A. It is numerically equal to the substrate concentration required to achieve one half the maximum velocity.
Most covalent catalysis is carried out by enzymes using a: a. ping-pong kinetic mechanism. b. sequential bisubstrate kinetic mechanism. c. random bisubstrate kinetic mechanism. d. simple unimolecular kinetic mechanism. e. none of the above.
A. Ping-pong kinetic mechanism
Proinsulin is converted into insulin by: a. proteolytic excision of a specific peptide. b. allosteric binding of glucose. c. phosphorylation to the active form. d. removal of phosphate by converter enzymes. e. none of the above.
A. Proteolytic exccision of a specific peptide.
BPG shifts the oxygen saturation curve of Hb to the ____ because BPG binds to ____ making Hb an O2 delivery system eminently suited for ____. a. right; deoxyHb; humans and other primates b. right; deoxyHb; cattle, sheep and goats c. left; oxyHb; cattle, sheep and goats d. left; oxyHb; humans and other primates e. none of the above
A. Right; deoxyHb; humans and other primates
Which of the following statements regarding enzyme regulation is true? a. the conversion of trypsinogen to trypsin is an example of zymogen activation b. allosteric effectors are always more powerful than covalent modification c. addition of an inhibitor to a V system results in kinetics similar to addition of a competitive inhibitor to a typical hyperbolic system d. the T state of an enzyme generally has more activity than the R state e. none of the above
A. The conversion of trysinogen to trypsin is an example of zymogen activation
In the hydrolysis of p-nitrophenylacetate by chymotrypsin all of the following are correct EXCEPT: a. the first product is acetic acid. b. an acetyl-enzyme intermediate forms during the mechanism. c. attack of a water molecule on the acyl-enzyme intermediate yields the second product. d. the active site contains a serine residue, an unprotonated histidine, and an unprotonated aspartate residue at pH 7.0 prior to binding the substrate. e. none of the above.
A. The first product is acetic acid
All of the following are true statements about the transition state of a reaction EXCEPT: a. The transition state is not an appropriate indication of the rate of a reaction. b. The transition state is located at the height of a free energy diagram. c. The energy required to raise the average energy of one mole of reactant to the transition state is the free energy of activation. d. Reaching the transition state indicates that there is a high probability that the reaction will occur. e. The transition state energy level is the sum of the energy levels of the reactants and products.
A. The transition state is not an appropriate indication of the rate of a reaction
When every enzyme molecule in the reaction mixture has its substrate-binding site occupied by substrate, the kinetics become ____-order, and the velocity is ____. a. zero; Vmax b. first; Vmax c. second; Vmax/2 d. zero; Vmax/2 e. first; Vmax/2
A. Zero; Vmax
The good transition state analog is one which would serve also as an extremely effective: a. competitive inhibitor. b. noncompetitive inhibitor. c. allosteric inhibitor. d. mixed-noncompetitive inhibitor. e. irreversible inhibitor.
A. competitive inhibitor
All are catalytic mechanisms or factors that contribute to the performance of enzymes EXCEPT: a. entropy gain in ES formation. b. covalent catalysis. c. general acid or base catalysis. d. proximity and orientation. e. all are true.
A. entropy gain in ES formation
The initial bond formation in the covalent intermediate in the chymotrypsin catalyzed reaction is between: a. serine and the carbonyl carbon in the peptide backbone. b. serine and the nitrogen in the peptide backbone. c. histidine and the carbonyl carbon in the peptide backbone. d. histidine and the nitrogen in the peptide backbone. e. aspartate and the carbonyl carbon in the peptide backbone.
A. serine and the carbonyl carbon in the peptide backbone
What reaction would NOT proceed via bimolecular elementary steps? a. C + D → T + U b. A reaction with a rate constant in the units of s−1. c. 2A → D + E d. A reaction with a molecularity of 2. e. A reaction with a rate constant in the units of M−1s−1 .
B. A reaction with a rate constant in the units of s-1
In the Chymotrypsin reaction mechanism there is a low-barrier hydrogen bond(LBHB) formed between: A. Asp102 and Ser195 B. Asp102 and His57 C. His57 and ser195 D Ser195 and carbonyl oxygen in the peptide bond E. None of the above
B. Asp102 and His57
The correct sequence for the hormone-activated enzymatic cascade that leads to activation of glycogen phosphorylase is: A. Phosphorylation to activate phosphorylase kinase B. Activation of G-protein C. Activation of adenylyl cyclase to produce cAMP D. Phosphorylation of glycogen phosphorylase E. cAMP activation of protein kinase A (PKA) a. A, B, C, D, E b. B, C, E, A, D c. C, B, A, D, E d. B, D, E, A, C e. E, A, D, C, B
B. B, C, E, A, D
Which of the following statements regarding enzymes and transition states is true? a. stabilization of the transition state must be less than stabilization of ES for catalysis to occur b. binding of substrate to an enzyme often causes strain, thus promoting transition state formation c. the transition state conformation of an enzyme catalyzed reaction is identical to the conformation seen in the uncatalyzed transition state d. formation of the transition state always assures that the reaction will proceed to product e. none of the above are true
B. Binding of substrate to an enzyme often causes straain, thus promoting transition state formation.
Malonate inhibition of succinate dehydrogenase is an example of: a. noncompetitive inhibition. b. competitive inhibition. c. mixed noncompetitive inhibition. d. irreversible inhibition. e. uncompetitive inhibition.
B. Competative inhibition
The mechanism of chymotrypsin involves which of the following elements? a. deprotonation of an active site Asp residue by His to start the reaction b. formation of an acyl-enzyme intermediate that must be hydrolyzed to complete the reaction c. stabilization of the positively charged His by a Gln residue d. direct deprotonation of water by His to generate a hydroxide ion for initiation of the reaction e. both a and b occur
B. Fromation of an acyl-enzyme intermediaate that must bee hydrolyzed to complete the reaction
All of the following statements about noncompetitive inhibition are true EXCEPT: a. They interact with the enzyme as well as the enzyme-substrate complex. b. Increasing the concentration of [S] can overcome the inhibition. c. The Vmax value does not remain the same as for a reaction that is not inhibited. d. The inhibitor can cause a conformational change in the enzyme. e. The inhibitor binds to a different site than does the substrate.
B. Increasing the [S] can overcome the inhibition
Adenylyl cyclase catalyzes the synthesis of cAMP, and cAMP hydrolysis is catalyzed by a 5'-phosphodiesterase. If caffeine inhibits 5'-phosphodiesterases, drinking a caffeinated beverage would ____ cAMP levels and ____ glycogen phosphorylase activity. a. increase; decrease b. increase; increase c. decrease; decrease d. decrease; increase e. have no effect; have no effect
B. Increease; increase
Which of the following will cause an increase in the activity of glycogen phosphorylase? a. binding of ATP to the allosteric site b. phosphorylation of the enzyme c. high levels of glucose-6-phosphate d. conversion from the relaxed to tense state e. drinking coffee
B. Phosphorylation of the enzyme
The cause of cell sickling in sickle cell anemia is: a. interaction of oxy-Hb S with the cell membrane. b. precipitation of deoxy-Hb S into long, chain-like fibers. c. formation of oxy-Hb S complexes and subsequent cell disruption. d. precipitation of Hb S - Hb A hybrid molecules. e. none of the above.
B. Precipitation of deoxy-Hb S into long, chain-like fibers.
All of the following are examples of a zymogen and its activating protease EXCEPT: a. chymotrypsinogen and chymotrypsin. b. procarboxypeptidase and elastase. c. proelastase and elastase. d. pepsinogen and pepsin. e. trypsinogen and trypsin.
B. Procarboxypeptidase and elastase
All of the following are characteristics of hemoglobin's binding of oxygen EXCEPT: a. CO2 promotes dissociation of O2 from hemoglobin by lowering the pH. b. Protons promote binding of oxygen by Hb. c. 2,3-Bisphosphoglycerate (BPG) promotes release of O2 by Hb. d. CO2 can bind with Hb's free amino groups and stabilize deoxy-Hb. e. BPG and O2 are mutually exclusive allosteric effectors of Hb.
B. Protons promote binding of oxygen by Hb
Which of the following is true regarding effectors of oxygen-hemoglobin binding? a. actively metabolizing tissues produce acid which increases hemoglobin's affinity for oxygen b. the presence of CO2 enhances the release of oxygen from hemoglobin c. 2,3-bisphosphoglycerate must dissociate from hemoglobin before oxygen can be released d. the presence of protons will counter the effect of CO2 on the realease of oxygen by hemoglobin e. none of the above
B. The preesence of CO2 enhances the release of oxygen from hemoglobin
A plot of 1/V vs. 1/[S] for an enzyme catalyzed reaction gave a line with an equation of y = 0.5x + 0.2. The same enzyme with an inhibitor present gave a line with an equation of y = 1.1x + 0.2. Which of the following statements is true? a. the type of inhibition is competitive b. the type of inhibition is noncompetitive c. the type of inhibition is uncompetitive d. the Vmax with the inhibitor present has decreased e. the Km with the inhibitor present has decreased
B. The type of inhibition is noncompetative
All are true about isozymes of lactate dehydrogenase (LDH) that are present in a number of different tissues EXCEPT: a. They have differing Km values for lactate. b. The amount of subunit phosphorylation differs. c. They have differing Km values for pyruvate. d. The ratios of A and B subunits differ depending upon the tissue type e. They have different kinetic parameters.
B. Thee amount of subunit phosphorylation differs
Enzymes have active sites which have the greatest complementarity to the: a. substrate. b. transition state. c. product. d. both substrate and product. e. none of the above.
B. Transitio state
HIV-1 protease is different from most mammalian aspartic acid proteases in that it has: a. two subunits each with a two-aspartate active site. b. two subunits each contributing an aspartate to the active site. c. two active sites on one protein. d. two subunits, one with an active site, and the other with a regulatory activity. e. none of the above.
B. Two subunits eaach contributing an aspartate to the active site
The catalytically active complex of an apoenzyme and its prosthetic group is referred to a(n) ____. a. catalytic duo b. holoenzyme c. prosthetic enzyme d. dimeric enzyme e. none of the above
B. holoenzyme
For an enzyme-catalyzed reaction, the initial velocity was determined at two different concentrations of the substrate. Which of the following would be closest to the value of Vmax? [S] (mM). Vo(mM/min) 1.0. 2.0 4.0. 2.8 a. 4.7 mM/min b. 0.67 mM/min c. 3.19 mM/min d. 1.5 mM/min e. 0.32 mM/min
C. 3.19 mM/min
Effective HIV-1 protease inhibitors should have all of the following characteristics EXCEPT: a. effective delivery in sufficient quantities to the desired site(s) of action in the organism. b. relative specificity for the HIV-1 protease. c. a backbone −OH group that forms a weak association with the two active-site carboxyl groups of the protease. d. results in inhibiting the production of new virus particles in cells of infected patients. e. broad spectrum enough to be effective against mutant viral forms.
C. A backbone -OH group that forme a weak association with the two acttive-site carboxyl groups of protease.
All are distinctive features of enzymes EXCEPT: a. regulation. b. catalytic activity. c. ability to change ΔG. d. specificity. e. none is true.
C. Ability to change ΔG
If the substrate for an enzyme catalyzed reaction contained a negative charge, which of the following amino acids would most likely be present in the active site to provide electrostatic destabilization of the ES complex? a. Val b. Asp c. Arg d. Ser e. Gln
C. Arg
Which of the following statements regarding enzymes is true? a. stabilization of the transition state must be less than stabilization of ES for catalysis to occur b. an enzyme mechanism is vastly different from the uncatalyzed reaction c. binding of substrate to an enzyme often causes strain, thus promoting transition state formation d. a random single displacement mechanisms requires that substrates bind to the enzyme in a specific order e. none of the above
C. Binding of substrate to an enzyme often causes strain, thus promoting transition state formation
The function of glycogen phosphorylase is: a. the conversion of glucose-1-phosphate to glucose-6-phosphate. b. to break down ATP. c. to catalyze the phosphorolysis of glucose-1-phosphate from glycogen molecules. d. to inhibit the production of glucose-1-phosphate. e. to stimulate the build up of glycogen.
C. Catalyze the phosphorolysis of glucose-1-phosphate from glycogen molecules
The addition of an effector to an enzyme that acts as a K system of the MWC model causes the half-maximum velocity (K0.5) to ____ and Vmax to ____. a. remain constant; remain constant b. remain constant; change c. change; remain constant d. change; change e. none of the above
C. Change; remaain constant
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. a. S; higher b. P; lower c. EX ++; lower d. EX ++; higher e. S; lower
C. EX++; lower
Which statement below about contrasting Hb and Mb is FALSE? a. Hb shows sigmoidal, whereas Mb shows hyperbolic oxygen saturation curves. b. Hb shows cooperativity, whereas Mb does not. c. Hb binds O2 more tightly than Mb. d. Oxygen binds to a ferrous ion in both proteins. e. Hb-oxygen binding is dependent on physiological changes in pH, whereas Mb-oxygen binding is not
C. Hb binds O2 more tightly than Mb
We are protected to some extent against low level CO poisoning because: a. His F8 on Hb distorts the heme iron atom out of the heme plane and attracts O2, but not CO. b. Affinity of Hb for O2 is much greater than for CO. c. His E7 blocks perpendicular binding of CO to heme iron allowing for better binding competition by O2. d. The competition of O2 for heme is much greater than CO for heme. e. None of the above.
C. His E7 blocks perpendicular binding of CO to heme iron allowing for better binding competition by O2
Regulation of metabolism by activation of the gene encoding a particular enzyme is called ____. a. covalent modification b. repression c. induction d. allosteric activation e. none of the above
C. Induction
The Gα subunit of GTP-binding protein (G protein) has all of the following characteristics EXCEPT: a. activates adenylyl cyclase. b. hormone-receptor complex promotes Gα release from Gβγ. c. has a very active GTPase activity. d. binds GDP in the Gαβγ complex. e. GTP hydrolysis leads to Gα-adenylyl cyclase dissociation.
C. It has very acive GTPase activity
Enzymes work by: a. providing an alternate reaction pathway b. increasing the spontaneity (ΔG) of the reaction c. lowering the activation energy of the reaction d. altering the concentration of the reactants to achieve a favorable ΔG e. proceeding in only one direction, from reactants to products
C. Lowering the activation energy of the reaction
Which of the following is true regarding the Briggs and Haldane steady state assumption? a. It is defined by the equation E+S<—> ES<—> E+P b. It states the rate of enzyme-substrate complex formation differs from the rate of enzyme-substrate disappearance. c. The concentration of the enzyme-substrate complex reaches a constant value even in a dynamic system. d. The enzyme-substrate complex will always dissociate to form E + P. e. The total amount of enzyme is variable, depending on the amount of substrate available.
C. The concentration of the enzyme-substrate complex reaches a constant value even in a dynamic system.
The free energy of activation, ΔG , is defined as: a. The average free energy of the product formed. b. The rate of a chemical reaction in relationship to the concentration of reactant molecules. c. The energy required to raise the average energy of one mole of reactant to the transition state energy. d. The amount of energy released by a spontaneous reaction. e. The lowest point on a free energy diagram.
C. The energy required to raise the average energy of one mole of reactant to the transition state energy.
An enzymatic reaction has a Vmax of 30 μM/min and a Km of 50 μM. If the concentration of the substrate is 25 μM, which of the following is true? a. the velocity will be 7.5 μM/min b. the velocity will be 20 μM/min c. the velocity will be 10 μM/min d. the velocity will be 25 μM/min e. the velocity will be 15 μM/min
C. The velocity will be 10 μM/min
All are characteristic of allosteric enzymes EXCEPT: a. Effectors may show stimulatory or inhibitory activity. b. They have multiple subunits. c. They obey Michaelis-Menten kinetics. d. The regulatory effect is by altering conformation and interaction of subunits. e. Binding one subunit impacts binding of substrate to other subunits.
C. They obey Michaelis-Menten kinetics
In transforming the Michaelis-Menten equation into a straight line equation, y = mx + b, the Lineweaver-Burk double reciprocal plot, which of the following is NOT a true representation? a. slope = Km/Vmax b. y-intercept is 1/Vmax c. x-intercept is 1/Km d. y = 1/V e. x = 1/[S]
C. X-intercept is 1/Km
The transition state has an estimated life-time of about: a. microseconds (10^−6s). b. nanoseconds (10^−9s). c. (10^−2s). d. (10^−14 to 10^−13s). e. milliseconds (10^−3 s)
D. (10^-14 to 10^-13 s)
For an enzyme-catalyzed reaction, the initial velocity was determined at two different concentrations of the substrate. Which of the following would be closest to the value of Km? [S] (mM). Vo(mM/min) 1.0. 2.0 4.0. 2.8 a. 0.17 mM b. 5.7 mM c. 2.7 mM d. 0.60 mM e. 1.7 mM
D. 0.60 mM
If an enzyme has a Vmax of 15 mM/min, what is the velocity if the substrate is present at 1/5 of the Km? a. 12 mM/min b. 6 mM/min c. 3 mM/min d. 2.5 mM/min e. cannot be determined from given information
D. 2.5 mM/min
In the catalytic triad common to many serine proteases, _____ increases the basicity of _____, thus allowing deprotonation of _____ to serve as a nucleophile. a. Ser-His-Asp b. His-Ser-Asp c. Ser-His-His d. Asp-His-Ser e. Cys-His-Ser
D. Asp-His-Ser
Aspartate proteases display a variety of substrate specificities, but normally they are most active in cleavage of peptide bonds: a. on the carboxyl side of the basic amino acids. b. on the carboxyl side of aromatic amino acids. c. on the carboxyl side of small, neutral residues. d. between two hydrophobic amino acid residues. E. None of the above
D. Between two hydrophobic amino acid residues
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? a. destabilization of ES by strain caused by non-covalent interactions between E and S. b. loss of entropy due to binding of E and S. c. destabilization of ES by distortion. d. destabilization of ES by solvation. e. destabilization of ES by electrostatic effects.
D. Destablization of ES by Solvation
Carbon dioxide affects O2 binding to Hb by: a. Hb competing with carbonic anhydride for CO2. b. directly binding to heme-Fe in the oxygen binding site. c. forming iron carbonate with the heme-iron. ++ d. forming H + HCO3− where the H is an antagonist to oxygen binding to Hb. e. forming HCO3− that combines with H to increase CO2 binding.
D. Forming H + HCO3- where the H is an antagonist to oxygen binding to Hb
If the transition state of an enzyme catalyzed reaction contained a positive charge, which of the following amino acids would most likely be present in the active site to aid in transition state formation? a. Lys b. Val c. Asn d. Glu e. Cys
D. Glu
Glycogen phosphorylase displays allosteric activation and inhibition by multiple modes. Which of the following is a correct relation? a. phosphate: positive heterotropic effector b. AMP: negative heterotropic effector c. ATP: positive heterotropic effector d. glucose-6-phosphate: negative heterotropic effector e. phosphorylation: covalent inhibitor
D. Glucose-6-phosphate; negative heterotropic effector
Glycogen phosphorylase displays allosteric activation and inhibition by multiple modes. Which of the following is a correct relation? a. phosphate: positive heterotropic effector b. AMP: negative heterotropic effector c. ATP: positive heterotropic effector d. glucose-6-phosphate: negative heterotropic effector e. phosphorylation: covalent inhibitor
D. Glucose-6-phosphatee; negative heterotropic effector
All of the following statements about competitive inhibition are correct EXCEPT: a. Competitive inhibitors are often chemical analogs of the substrate. b. For a group-specific enzyme, one substrate would be a competitive inhibitor of reactions of the other possible substrate. c. Sometimes a product of an enzyme-catalyzed reaction is a competitive inhibitor of its own production. d. In the presence of a competitive inhibitor, the apparent Km would be altered and Vmax would be decreased. e. Competitive inhibitors usually interact with the enzyme at the binding site for a substrate.
D. In the presence of a competitive inhibitor, the apparent Km would be altered and Vmax would be decreased
The polypeptide of myoglobin serves all of the following functions EXCEPT: a. protects the heme iron from oxidation. b. cradles the heme group. c. provides a pocket for O2 to fit. d. keeps the heme iron in the ferric form. e. none of the above.
D. Keeps the heme iron n the ferric form
Which of the following statements is true regarding enzyme pathways? a. the most effective way to control a pathway is to regulate every enzyme in the pathway b. an enzyme pathway always proceeds in only one direction, never in reverse c. a regulatory enzyme is regulated only by molecules within the given pathway d. metabolic pathways are necessary since enzymes usually catalyze only one specific reaction e. none of the above are true
D. Metabolic pathways are necessary since enzymes usually catalyze onlt one specific reaction.
All are properties of regulatory enzymes EXCEPT: a. pathway end-products may act as allosteric inhibitors. b. v vs [S] plots are sigmoid- or S-shaped. c. substrate binding is cooperative. d. monomeric enzymes with a single regulated active site. e. may be stimulated by allosteric activators.
D. Monomeric enzymes with single regulated active sites
The presence of a negative allosteric effector on an allosteric protein would: a. cause a shift to the left in the sigmoidal curve. b. increase the number of R conformations. c. decrease the cooperativity of the substrate. d. raise the apparent value of the equilibrium constant, L. e. increase the likelihood of the binding of S.
D. Raise the apparent value of the equalibrium constaant, L
Which of the following statements regarding inhibitors is correct? a. a transition state analog binds covalently to the enzyme b. a non-competitive inhibitor causes an increase in the Vmax c. the presence of a competitive inhibitor can be overcome by addition of more inhibitor d. a competitive inhibitor results in a higher apparent Km value e. none of the above are correct
D. a competitive inhibitor results in a higher apparent Km value
Sildenafil citrate (Viagra) was developed as an inhibitor of: a. acid phosphatases. b. cAMP phosphodiesterases. c. glycogen phosphorylase. d. cGMP phosphodiesterases. e. testosterone reductase.
D. cGMP phosphodiesterase
The mechanism of aspartate protease catalysis is proposed to be: a. covalent nucleophilic catalysis. b. covalent electrophilic catalysis. c. specific base catalysis. d. general base-general acid catalysis facilitated by a low barrier hydrogen bond. e. all of the above.
D. general base-general acid catalysis facilitated by a low barrier hydrogen bond
All are characteristics of the enzyme hexokinase EXCEPT: a. It is not highly specific and will catalyze phosphorylation of a number of hexoses at the six position. b. Hexoses bind the active site and induce a solvent inaccessible fit to the hexose. c. Glycerol may fit into the active site, but it does not bind to induce a conformational change necessary for catalysis. d. When a hexose binds the active site, the active site is modified to promote changes in the substrate to a transitional-state intermediate. e. All are true.
E. All are true
All are true for the enzyme-transition state complex EXCEPT: a. It is designated as EX . b. The enzyme stabilizes the transition-state complex more than it stabilizes the substrate complex. c. The enzyme is "designed" to bind the transition-state structure more tightly than the substrate or product. d. The energy barrier between ES and EX is less than the energy barrier between S and X . e. All are true.
E. All are true
Characteristics of glycogen phosphorylase include all EXCEPT: a. covalently linked pyridoxal phosphate. b. an active site at the center of each of two identical subunits. c. regulatory phosphorylation site on a Ser residue. d. allosteric effector sites for ATP and glucose-6-phosphate. e. all are true.
E. All are true
Metal ion catalysis include all EXCEPT: a. increased acidity of a nucleophile with an ionizable proton. b. metal ion requirement to maintain the stable, native state of the enzyme. c. metal binding weakly, perhaps only during the catalytic cycle. d. electrophilic catalysis, stabilizing the increased electron density or negative charge that can develop during a reaction. e. all are true.
E. All are true
All are characteristics of ribozymes EXCEPT: a. They are critical for removal of introns from mRNA b. They are substrate specific. c. They enhance the reaction rate. d. On the ribosome, they catalyze the peptidyl transferase reaction e. All are true.
E. All is true
Transition-state analogs are: a. approximations of the transition state that bind more tightly than the substrate. b. compounds that compete for the active site, but are not necessarily very similar to the substrate. c. stable molecules that can not be expected to resemble the true transition state too closely. d. stable chemically and structurally similar molecules to the transition state. e. all of the above.
E. All of the above
Which of the following are relevant to the reaction catalyzed by chorismate mutase? a. the reaction involves a concerted intramolecular rearrangement of chorismate to prephenate during thesynthesis of phenylalanine. b. the enzyme catalyzed and uncatalyzed reactions follow almost identical routes c. the enzymatic reaction is thought to involve transition state stabilization by 12 electrostatic and hydrogen bond interactions d. the geometry of the enzyme active site is such that the difference in energy between ES and the near attack conformation is less than 1 kJ/mol e. all of the above are true
E. All of the above
Since enzymes are not rigid molecules but rather flexible proteins, which of the following may be attributed to this fact? a. active sites are able to conform to the shape of the substrate b. catalytic residues are oriented in such as way to aid in bond breakage and bond formation c. strain is placed upon the substrate while it binds to the enzyme d. near-attack conformations are achieved during formation of the transition state during an enzyme catalyzed reaction e. all of the above are correct
E. All of the above are correct
An enzyme that displays negative cooperativity has ____ activity when the substrate is present at concentrations ____ than the value of K0.5 when compared with enzymes that display no cooperativity. a. increased; greater b. increased; less c. decreased; greater d. decreased; less e. both b and c are correct
E. Both b and c are correct
Which of the following statements is correct regarding isozymes? a. they catalyze the same reaction but have vastly different structures b. they are always monomeric proteins c. their differences are based upon the type of tissue in which they are present d. they often respond to different inhibitors and activators e. both c and d are correct
E. Both c and d are correct
When O2 binds to heme in hemoglobin, the ____ ion is drawn into the plane of the ____ causing a conformational change that is transmitted to adjacent subunits enhancing the ____ for additional O2 binding. a. Mg 2+; globin; planarity b. Fe 3+; heme; folding c. Mg 2+; globin; attraction d. Fe 3+; porphyrin; affinity e. Fe 2+; porphyrin; affinity
E. Fe2+; porphyrin; affinity
Because the pKa is near 7, ____ side-chains are often involved in general acid-base catalysis. a. cysteine b. aspartate c. glutamate d. lysine e. histidine
E. Histidine
Which of the following statements is NOT characteristic of kcat/Km? a. It corresponds to a second-order rate constant. b. It provides an excellent parameter for comparison of the catalytic efficiency of enzymes. c. It reflects the property of the enzyme when substrate concentration is at saturation d.Theupperlimitforthekcat/Kmvalueisfixedbythediffusion-controlledlimitforreactions,which is 10^9 M− 1 s−1. e. It is also referred to as the turnover number.
E. It is also refered to as the turnover number
All of the following statements are true about the relationships between [S], Km and Vmax EXCEPT: a. As the [S] is increased, v approaches the limiting value, Vmax. b. Km = Vmax/2. c. The rate of the reaction, v, follows a first order rate equation v = K'[A] and K' = Vmax/Km. d. The rate of product formed, v, is at Vmax when [S] >> Km. e. Km and Vmax assist in finding the rate of the enzyme catalyzed reaction only if the reaction is irreversible.
E. Km and vmax assist in finding the rate of the enzyme catalyzed reaction only if thee reaction is irreversable
Penicillin and other β-lactam antibiotics form a covalent bond to the enzyme glycoprotein transpeptidase. What type of bond is formed? a. ester b. thioester c. amide d. phosphate e. none of the above
E. None of the above (the answer is covalent)
All are true for cAMP-dependent protein kinase EXCEPT: a. also known as PKA. b. phosphorylase kinase is a substrate. c. consists of a pair of catalytic subunits. d. two regulatory subunits block catalytic activity without cAMP binding. e. phosphorylates glycogen phosphorylase.
E. Phosphorylates glycogen phosphorylase
All of the following are correct statements about enzyme regulation EXCEPT: a. Enzymes can be inhibited by the products they produce. b. Enzymes can be inactivated by the addition of a functional group. c. Coenzyme and substrate availability can regulate enzyme reaction rate. d. The reaction rate slows as equilibrium is approached. e. The activity of an enzyme is covalently affected by allosteric regulators.
E. The activity of aan enzye is covalently affected by allosteric regulators
All are true for low-barrier hydrogen bonds EXCEPT: a. The hydrogen is centered between the two heteroatoms. b. The interactions are more covalent. c. The bond order approaches 0.5 for both O−H interactions. d. The barrier that the hydrogen atom must surmount to exchange oxygens becomes lower. e. All are true.
E.all are true
