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INTRODUCTORY BIOCHEMISTRY Bio. 28 First Midterm Examination February 24, 2005 Name Make sure that your name is on every page. This is the only way we have of matching you with your exam after grading it. Please work independently. Read each question carefully before answering. Unless otherwise indicated, there is only one correct answer for each multiple choice question. Points are indicated by the question within brackets []. Page 1 total __________/10 Page 2 total __________/8 Page 3 total __________/13 Page 4 total __________/13 Page 5 total __________/19 Page 6 total __________/6 Page 7 total __________/12 Page 8 total __________/9 Page 9 total __________/10 Exam total __________/100 Name 1 Consider the following three amino acids: (A) D-isoleucine (B) L-isoleucine (C) L-alloisoleucine 1. [1] Which one(s) will have identical melting points? ____________ 2. [1] Which one(s) will rotate the plane of polarized light to the same extent but in opposite directions? _________ 3. [1] Which one(s) would be expected to be found in proteins? __________ Consider the following three oligopeptides: (A) DDDAAGKAED (B) LIAAVALLA (C) WGGKFYARAGED 4. [1] Which one will absorb the most ultraviolet light? _________ 5. [1] Which one will be the most soluble in water? _________ 6. [1] Which one has the lowest pI? __________ 7. [2] Lysine has pKa1 = 2.18, pKa2 = 8.95, and pKaR = 10.53. What is the isoelectric pH of this amino acid? __________ 8. [2] A solution of an amino acid that has a light absorbance of 2.0 at 280 nm transmits _____% of the light at this wavelength. Lowering the concentration of this amino acid by 50% would change the transmission to ____%. Name 2 9. [2] In the Edman degradation of proteins, the amino acid that is removed from the protein at each cycle comes from the ___________ end? (indicate amino or carboxyl). 10. [2] Which one phrase best describes the hydrogen bonds in a β sheet? a) they occur mainly near the amino and carboxyl termini of the strands. b) they are perpendicular to the plane of the sheet. c) they occur mainly between atoms of the R groups. d) they occur only between some of the amino acids of each strand. e) they occur mainly between atoms of adjacent strands in a sheet. f) none of the above. 11. [2] The structure of an α helix is stabilized by (one or more may be correct): a) proline residues near the carboxy end b) glutamate residues near the carboxyl end c) aspartate residues near the amino end d) glycine residues near the amino end e) adjacent lysine and arginine residues f) adjacent lysine and aspartate residues 12. [2] A knowledge of all of the φ and ψ angles in a polypeptide allows determination of (more than one is possible): a) the primary structure of the polypeptide. b) the secondary structure of the polypeptide. c) the quaternary structure of the polypeptide. d) the degree of denaturation of the polypeptide. e) The position of the polypeptide on a Ramachandran chart Name 3 13. [4] Fill in the missing words: In the protein, __________________, additional strength is provided by the post-translational ____________________ of some amino acids to increase the number of __________________ bonds they can form. This modification becomes deficient when ____________________ is lacking in our diet. 14. [3] Which is/are true of the native conformation of a protein? a) In principle, it can be predicted from the primary structure. b) It is the lowest energy conformation. c) It is the most compact conformation. d) It is reached by sampling a random series of “trial and error” conformations. e) It is the most stable conformation under the conditions at which it is formed. f) It is the conformation that is most susceptible to denaturation. 15. [2] A mutant myoglobin has a Ka for O2 of 0.25 kPa–1. What is the value of θ when the partial pressure of O2 is 0.125 kPa? ______________ 16. [1] The proximal histidine residue in myoglobin acts to (choose one): a) prevent oxidation of the heme Fe2+ b) lower the relative affinity for CO c) assist in the binding of O2 d) prevent binding of N2 e) decrease θ 17. [3] For myoglobin, when θ = 0.25, what is the value of [PL]/[P]? ______. Name 4 Supply the missing word(s) in each of the following 4 questions. Choose from increase, decrease, or remain unchanged. 18. [2] Decreasing the concentration of HCO3– in a solution of hemoglobin (without changing the pH) causes the affinity for O2 to ___________. 19. [2] Decreasing the pH of a solution of hemoglobin causes the affinity for O2 to ___________. 20. [2] Decreasing the O2 tension (partial pressure) in a solution of hemoglobin causes the affinity for O2 to ___________. 21. [2] Adding BPG to a solution of adult hemoglobin causes the affinity for O2 to ___________. 22. [2] Enzymes catalyze the rapid formation of product by (choose one): a) shifting the position of the equilibrium between substrate and product in favor of product. b) lowering ΔG of the reaction. c) lowering ΔG´o of the reaction. d) raising the free energy of the substrate. e) decreasing the entropy of the product. f) raising ΔG‡ g) none of the above. 23. [3] Two samples of the same enzyme were purified by different students in an undergraduate laboratory. The students determined that the two samples had identical Vmax values, but very different values of Km. Which one of the following explanations for these facts is most likely? a) One sample inadvertently contained a noncompetitive inhibitor. b) One sample inadvertently contained a mixed inhibitor. c) The samples had different affinities for the substrate. d) The students had used very different substrate concentrations. e) The students had used very different enzyme concentrations. Name 5 24. [4] A series of initial velocity measurements for an enzyme revealed that the substrate concentration required for Vo to be one-fourth of Vmax was 3.0 mM. What is the Km for this enzyme? ______ mM 25. [4] For an enzyme that has a Km = 5 mM, what is the substrate concentration required for Vo to be one third of Vmax? _________ mM 26. [4] If the enzyme described above has a Vmax of 3 µmol/min at an enzyme concentration of 4 µg/ml, what would be the enzyme concentration required for a Vmax of 9 µmol/min? ___________ µg/ml 27. [3] In the following oligopeptide, indicate by drawing vertical line(s) where chymotrypsin would hydrolyze peptide bond(s): D-A-V-T-W-C-L-A-F-T-K-S-G-S-Y-H-I-L-R 28. [2] Which of the following is/are true for a competitive inhibitor? (There may be more than one.) a) it lowers kcat b) it lowers the apparent Km c) it raises the substrate concentration required for the rate to equal Vmax d) it raises the substrate concentration required for the rate to be 1/2 of Vmax e) it lowers the substrate concentration required for the rate to be 1/2 of Vmax 29. [2] Chymotrypsin is an example of an enzyme that catalyzes the following type of reaction: a) single substrate reaction b) random bi-substrate reaction with a ternary complex c) ordered bi-substrate reaction with a ternary complex d) ordered bi-substrate reaction without a ternary complex Name 6 30. [2] The substrate specificity of chymotrypsin is determined largely by a) the oxyanion hole b) the catalytic triad c) the hydrophobic pocket d) the heme cofactor 31. [2] Chymotrypsin has a) no covalently bound reaction intermediate b) a reaction intermediate that is covalently bound to the active site Ser c) a reaction intermediate that is covalently bound to the oxyanion hole d) a reaction intermediate that is covalently bound in the hydrophobic pocket e) a reaction intermediate that is covalently bound to the active site His 32. [6] For an enzyme to be allosterically regulated, it must (choose one or more): a) have a variable Vmax. b) have multiple subunits. c) have multiple substrate binding sites. d) have multiple regulatory domains. e) have two or more binding sites for substrate(s) and/or regulatory effector(s) Name 7 33. [2] If the ΔG of the reaction A → B is –30 kJ/mol, which of the following statements is/are correct? (There may be more than one.) a) The reaction would proceed spontaneously from left to right at standard conditions. b) The equilibrium constant could be calculated if the concentrations of A and B and the temperature were known. c) The reaction would proceed spontaneously from left to right at the given conditions. d) The value of ΔG´o is negative. e) The equilibrium constant favors the formation of B over the formation of A. 34. [6] For each class of enzyme inhibitor listed below, indicate the effect on the Vmax and apparent Km of the enzyme. Choose from raise, lower, or no effect. a) Competitive: effect on apparent Km _____________; effect on Vmax ___________ b) Noncompetitive: effect on apparent Km__________; effect on Vmax ___________ c) Uncompetitive: effect on apparent Km ___________; effect on Vmax ___________ 35. [2] A Lineweaver-Burk douple reciprocal plot in which all of the data points fall very close to the straight line implies (choose one): a) The enzyme does not exhibit cooperative substrate binding. b) There is no competitive inhibitor present. c) The enzyme has a high catalytic efficiency. d) The enzyme has a constant Vmax. 36. [2] Which of the following feature(s) do a micelle and a lipid bilayer have in common? (There may be more than one.) a) Both are made up of amphipathic molecules. b) Both are very large, sheetlike structures. c) Both are stabilized by electrostatic interactions. d) Both consist of molecules having two long-chain hydrocarbons. e) Both require proteins for stable assembly. Name 8 37. [3] Indicate the effect on the membrane fluid transition temperature of the following changes to the fatty acids in the membrane lipids (choose raise, lower, or no change): a) Increase the average chain length of fatty acids ______________ b) Increase the average number of double bonds in the fatty acids _________ c) Change some cis double bonds to trans double bonds _______________ 38. [3] Fructose 6-phosphate is converted to glucose 1-phosphate in two successive reactions, with glucose 6-P as an intermediate. Given: Glucose 1-P → glucose 6-P ΔG´o = –7.3 kJ/mol Fructose 6-P → glucose 6-P ΔG´o = –1.7 kJ/mol What is ΔG´o for the overall reaction fructose 6-P → glucose 1-P? a) 12.4 kJ/mol b) –12.4 kJ/mol c) 5.6 kJ/mol d) –5.6 kJ/mol e) 9.0 kJ/mol f) –9.0 kJ/mol 39. [3] If the reaction glucose 1-P → glucose 6-P has ΔG´o = –7.3 kJ/mol, the ΔG for the reaction when the concentration of glucose 6-P is ten times that of glucose 1-P will be a) less negative than –7.3 kJ/mol b) more negative than –7.3 kJ/mol c) –7.3 kJ/mol Name 9 40. [2] Which pair or pairs of the above molecules are epimeric? _____________ 41. [2] Which pair or pairs of the above molecules are enantiomeric? ________ 42. [2] Which of the above molecules contain (a) pyranoside(s)? –––––––– 43. [2] Which of the above molecules is/are (a) reducing sugar(s)? –––––––– 44. [2] Which of the above molecules contain(s) at least one α anomer? ––––––––