Chapter 11: Enzyme Catalysis

Transcription

1 Chapter 11: Enzyme Catalysis Matching A) high B) deprotonated C) protonated D) least resistance E) motion F) rate-determining G) leaving group H) short peptides I) amino acid J) low K) coenzymes L) concerted acid-base M) orient N) oxidoreductase 1. For efficient nucleophilic catalysis, a group such as the sulfhydryl on a cysteine residue must be able to form a good, in addition to being a good nucleophile. 2. If an enzyme-catalyzed reaction has a low rate at low ph and high rate at higher ph, this implies that a group on either the enzyme or the substrate must be for an efficient reaction. 3. On a transition state diagram for a multistep reaction, the step with the greatest G is the step. 4. On a transition state diagram for a one-step, very spontaneous reaction (large negative G), a large peak (high G ) would imply a rate for the reaction. 5. Metal ion cofactors can the substrate at the active site, providing catalytic enhancement. 6. Large rate enhancement in enzyme catalysis may occur when the binding of substrate

2 reduces the of the reacting groups. 7. Metal ion cofactors with more than one oxidation state may assist in catalysis by the class of enzymes. 8. Since proteins are limited in their abilities to catalyze oxidation-reduction reactions, enzymes often employ to assist with catalysis. 9. Some serine proteases are believed to have developed by convergent evolution, because the sequences of some serine proteases show no resemblance to those of others. 10. Clustering several amino acid residues with favorable pk values at an active site can promote a(n) catalytic mechanism. 11. Which of these amino acid groups would not make a good nucleophilic catalyst? A) amino B) sulfhydryl C) imidazole D) methyl E) hydroxyl 12. The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst because: A) in the physiological ph range both H + and OH are present at high concentrations. B) in the physiological ph range, the nitrogen in the ring can be easily protonated/deprotonated. C) one guanidino group is protonated, and the other is deprotonated at physiological ph. D) the imidazole group is a strong reducing agent at physiological ph. E) the sulfur atoms in the ring can either gain or lose a proton at physiological ph. 13. Chymotrypsin, a serine protease, preferentially cleaves a peptide bond adjoining a bulky non-polar side chain. This is because chymotrypsin's specificity pocket: A) contains a sulfhydryl group that forms a disulfide bond with the substrate. B) is lined with small hydrophobic side chains, leaving considerable room in the nonpolar pocket. C) contains a negative charge. D) is mostly filled with large side chains.

3 E) contains a positive charge. 14. Which one of the following is correct? A) All enzymes are highly specific for the reactions they catalyze. B) Prosthetic groups are loosely associated with the polypeptide chain of an enzyme. C) Activation of zymogens, such as proelastase, requires an oxidation-reduction reaction at a particular amino acid side chain. D) If an enzyme-catalyzed reaction requires a group with a low pk to be deprotonated and a group with a higher pk to be protonated, the ph vs. velocity curve will have a peak in the middle. E) The proximity effect, a result of bringing substrates close to their catalytic groups in the active site, can result in a rate enhancement on the order of Which of the following amino acid residues would not provide a side chain for acid-base catalysis? A) leucine B) serine C) lysine D) aspartic acid E) histidine 17. is a serine protease that has a specificity pocket that binds small hydrophobic side chains. A) chymotrypsin B) trypsin C) lysozyme D) trypsinogen E) elastase 18. Simultaneous stimulation of a reaction by general acid and general base catalysis is defined as: A) covalent catalysis. B) electrostatic catalysis. C) proximity catalysis. D) concerted acid/base catalysis. E) transition-state catalysis. 19. The formation of a Schiff base in the enzymatically catalyzed decarboxylation of acetoacetate is an example of: A) covalent catalysis. B) electrostatic catalysis. C) proximity catalysis. D) concerted acid/base catalysis. E) transition-state catalysis. 20. The catalytic triad in chymotrypsin consists of:

4 A) substrate, transition state, and product B) enzyme, substrate, and metal-ion cofactor C) three amino acids: SER, HIS, and ASP D) three amino acids: CYS, HIS, and ASP E) three amino acids: SER, HIS, and GLU Chap What is the velocity of a first-order reaction when the reactant concentration is 6 x 10-2 M and the rate constant is 8 x 10 3 sec -1? A) 1.33 x 10 5 M -1 sec -1 B) 1.33 x 10 5 M sec C) 7.5 x 10-2 M sec D) 4.8 x 10 2 M sec -1 E) not enough data are given to make this calculation 2. For a reaction A + B C, if the concentration of B is much larger than [A] so that [B] remains constant during the reaction while [A] is varied, the kinetics will be: A) sigmoidal B) pseudo-first-order C) unimolecular D) zero-order E) enzymatic 3. K M is: A) a measure of the catalytic efficiency of the enzyme. B) the rate at which the enzyme dissociates from the substrate. C) the rate constant for the reaction ES E + P. D) the [S] that half-saturates the enzyme. E) the rate at which the enzyme binds the substrate. 4. The K M can be considered to be the same as the dissociation constant K S for E + S binding if: A) this statement cannot be completed because K M can never approximate K S. B) ES E + P is fast compared to ES E + S. C) the turnover number is very large. D) k 2 << k -1. E) k cat /K M is near the diffusion-controlled limit. 5. An enzyme is considered to have evolved to its most efficient form if A) k cat is a large number B) k cat /K M is near the diffusion-controlled limit C) K M is a large number

5 D) k cat /K M is a very small number E) K M is a small number 6. Find the initial velocity for an enzymatic reaction when V max = 6.5 x 10 5 mol sec 1, [S] = 3.0 x 10 3 M, and K M = 4.5 x 10 3 M. A) not enough information is given to make this calculation B) 2.6 x 10 5 mol sec 1 C) 1.4 x 10 2 mol sec 1 D) 8.7 x 10 3 mol sec 1 E) 3.9 x 10 5 mol sec 1 7. Parallel lines on a Lineweaver-Burk plot are diagnostic of: A) competitive inhibition. B) non-competitive inhibition. C) allosteric activation. D) allosteric inhibition. E) none of the above. The following questions refer to the overall transformation: 8. The overall transformation A) is composed of two elementary reactions. B) can be zeroth order in [S] if [S]>>[E] C) may be described by the Michaelis-Menten equation if certain assumptions are made D) all of the above E) none of the above 9. For the reaction, the steady state assumption assumes that A) [S] = [P] B) [ES] is constant C) [P]>>[E] D) [P] is constant E) k 1 >>k The Michaelis constant K M is defined as A) (k 1 + k 2 )/k 1 B) (k 1 + k 1 )/k 2 C) ([P] + [E])/[ES] D) [ES] E) none of the above 11. The most efficient enzymes have k cat /K M values that approach

6 A) k 2 B) k 1 C) k 1 D) k 1 + k 2 E) M s A compound that reduces the concentration of enzyme available for substrate binding is called: A) a transition-state analog B) a non-competitive inhibitor C) an allosteric effector D) an enzyme inactivator E) a competitive inhibitor 13. Compounds that function as mixed inhibitors A) interfere with substrate binding to the enzyme B) bind to the enzyme reversibly C) can bind to the enzyme/substrate complex D) all of the above E) none of the above 14. Enzyme activity in cells is controlled by processes including: A) temporary covalent modifications. B) modulation of expression levels. C) feedback inhibition. D) binding to allosteric effectors. E) all of the above. 15. Allosteric activators bind to enzymes A) in a way to inhibit substrate binding. B) and stabilize the T-state, which has a low substrate affinity. C) and stabilize the R-state, which has an enhanced substrate affinity. D) covalently. E) none of the above. 16. Protein kinases are involved in A) the digestion of drugs to potentially toxic byproducts B) the degradation of enzymes to the component amino acids C) the phosphorylation of a wide variety of proteins D) the metabolism of drugs to water-soluble, excretable compounds E) all of the above Chapter 13: Biochemical Signaling

7 1. Insulin binds to a receptor that A) is coupled to a G protein. B) possesses tyrosine kinase activity. C) possesses serine/threonine phosphatase activity. D) migrates to the nucleus to affect transcription. E) activates MAP kinase. 2. The ability of receptors to adapt to long-term stimulation by responding to changes in stimulation rather than the level of stimulation is called A) dual-specificity B) signal transduction C) autoinhibition D) desensitization E) an emergent property 3. The interaction of various signaling pathways is called A) dual-specificity B) signal transduction C) autoinhibition D) desensitization E) cross-talk 4. Phospholipase C cleaves PIP 2, generating A) IP 3 and DAG (inositol-1,4,5-trisphosphate and 1,2-diacylglycerol) B) camp and PP i C) camp and DAG D) IP 3 and PP i E) IP 3 and P i 5. G proteins A) consist of three subunits, G, G, and G B) hydrolyze GTP to GMP + P i + P i C) completely span the cell membrane D) A and B E) A and C 6. The insulin signaling pathway includes the following steps: A) ligand-induced dimerization of the receptor and phosphorylation of IRS-1 and/or IRS-2 proteins B) autophosphorylation and phosphorylation of IRS-1 and/or IRS-2 proteins

8 C) IRS-1 induced dimerization of the receptor, autophosphorylation, and phosphorylation of SH2 domains D) autophosphorylation and dephosphorylation of IRS-1 and/or SH2 domains E) none of the above 7. What is (are) the advantages of protein phosphorylation as a signaling event? A) Phosphorylation is rapid and specific. B) Phosphorylation is enzymatically reversible. C) Phosphorylation can be amplified by cascade systems. D) A and B E) A, B, and C 8. Which of the following is a secondary messenger? A) insulin B) epinephrine C) cortisol D) cyclic AMP E) rhodopsin 9. Ras is an example of A) a monomeric G protein B) a heterotrimeric G protein C) a receptor protein D) a cytokine E) an interferon 10. Which of the following is (are) associated with camp binding to camp-dependent protein kinase A? A) camp binds to the regulatory subunits B) regulatory subunits dissociate from catalytic subunits C) catalytic subunits phosphorylate multiple targets with specifc serines and threonines D) A and B E) A, B, and C 11. Which of the following statements concerning the action of G proteins on adenylate cyclase is true? A) G proteins always serve to activate adenylate cyclase. B) G proteins always serve to inhibit adenylate cyclase. C) Some G proteins activate adenylate cyclase, while others inhibit it. D) G proteins do not act on adenylate cyclase at all.