Name : ______________________ ID # : ______________________ Signature : _____________________ Chem & Biochem 153A-1 Summer 2004 Midterm II (100 pt, 50 min) Midterm II Histogram05101520250 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95Score sCountsAverage ~ 70STDEV ~ 15 High 98 Low 34 11) (20 pt) For the following, circle all that applies. (+2 pt for each correct answer; -2 pt for each incorrect answer. If you decide to change your answer, scratch out the circle and clearly indicate “not circled” next to it). Myoglobin : (Answer : b) a. Participates in oxygen transport. b. Is an example of a tertiary protein/structure. c. Is a fibrous protein. d. Predominantly contain β-sheets. e. Has cooperative binding to oxygen. Hemoglobin : (Answer : c, d, e) a. Binds oxygen (O2) with higher affinity in the presence higher [CO2]. b. Binds CO2 with higher affinity in the presence of higher [O2]. c. Transports CO2. d. Transports O2. e. Transports H+. f. Binds CO2 through electrostatic interaction. g. Is found in high concentrations in skeletal muscle cells. h. Contains 4 heme per subunit. Heme : (Answer : d, f, g) a. Binds oxygen with higher affinity than carbon monoxide. b. Has 6 coordination bonds to Fe2+. c. Has 5 coordination bonds to Fe3+. d. Has 4 coordination bond to Fe2+. e. Has 3 coordination bond to Fe3+. f. Is permanently bound to the α subunits of hemoglobin. g. Is bent/puckered in the T-state compare to the R-state. Others : (Answer : b, d, e) a. Sickle cell anemia is due to Val Æ Glu substitution. b. Sickle cell anemia is due to Glu Æ Val substitution. c. Sickle cell anemia is due to covalent cross-linking of hemoglobin into a fibrous network. d. 2,3-BPG (bis-phosphoglycerate) binds hemoglobin electrostatically. e. Oxygen partial pressure (pO2) is generally higher in the lungs than in the tissues. 22a) (10 pt) Draw the structure of 1-stearoyl (18:0)-2-palmitoyl (16:0)-3-phosphatidylethanolamine as you would expect to see it at pH 7. b) (5 pt) If we were to replace the stearic acid and palmitic acid with arachidic acids (20:0), and compare bilayer membranes made of these two different types of phospholipids, which membrane would you expect to be more fluid (compare part “a” to “b”)? Explain. Membrane “a” would be more fluid. Shorter hydrocarbon fatty acyl chains tend to “pack” less tightly, stabilized by a lesser degree of hydrophobic interaction, hence, less ordered and more fluid. c) (5 pt) If we were to replace the stearic acid and palmitic acid with linoleic acids (18:2∆9,12), which membrane would you expect to be more fluid (compare part “a” to “c”)? Explain. Membrane “c” would be more fluid. Higher degrees of unsaturation (compare to membrane “a” fatty acyl chains) tend to make the tails of phospholipids more bulky due to introduction of kinks in the fatty acyl chains. This results in less stable packing, hence, a more fluid membrane. d) (5 pt) If we added cholesterol (in reasonable quantity) to the membrane made of the phospholipids in part “a”, would you expect the membrane to be more or less fluid? Explain. Cholesterol has a very bulky structure relative to its hydrophobic region and tends to disrupt membrane structure when introduced. Hence, addition of cholesterol will tend to make membranes more fluid. 33) For the chymotrypsin illustration shown below (Nelson & Cox, 2000), answer the following questions. a) (10 pt) Clearly illustrate an instance of covalent catalysis from the mechanism of chymotrypsin catalyzed hydrolysis of the above peptide bond. Be sure to show relevant structure of product and reactants and electron pushing mechanism(s). (any reaction involving formation or breakage of a covalent bond between the enzyme and substrate is ok.) b) (6 pt) If the aromatic amino acid in the peptide segment above were to be replaced by arginine, would you predict cleavage of the peptide bond? Explain why or why not based on our discussion in class. There would probably be no cleavage. Chymotrypsin specifically recognizes and binds the sequence containing the aromatic AA through interaction of the aromatic AA side chain in a hydrophobic pocket. Arginine is charged/polar and hence would not have that specificity to bind in the hydrophobic pocket (charged/polar arginine would not interact well with hydrophobic moieties that lines the hydrophobic pocket. c) (6 pt) What is the role/function of glycine 193 relative to the mechanism of chymotrypsin function we discussed in class? Explain or illustrate briefly. Glycine 193 is in position to interact with (H-bonding) and stabilize the oxy-anion intermediate(s) formed during catalysis. (Can draw out structure and illustrate interaction as well). 44a) (10 pt) Given the Lineweaver-Burke plot of an enzyme under “uncompetitive” inhibition in the graph below. Based on your knowledge of simple enzyme kinetics and Lineweaver-Burke plots (note that the axis are not labeled), calculate the Km and Vmax (often referred to as the apparent or effective Km and Vmax) of this enzyme under inhibition as shown. Value at the X-intercept “A” is -100 mM-1. Value at the Y-intercept “B” is 0.02 min/µM. -1/Km = A = -100 mM-1 1/Vmax = B = 0.02 min/µM Km = 1/100 mM-1 = 0.01 mM Vmax = 1/0.02 min/µM = 50 µM/min b) (5 pt) On the graph, draw in the plot for the enzyme in the absence of inhibitor. Explain your reasoning. (dashed line above represents the plot for the enzyme without inhibition). Un-competitive inhibitors decrease both the Km and Vmax of the enzyme (or Km and Vmax of enzyme is higher than apparent Km and Vmax of uncompetitive inhibition). The dashed plot compare to the solid plot illustrates that. 565) Short answers. Draw out the relevant portions of the compounds in question to illustrate if need. “Primary” structure refers to the chemical composition and/or bonding within the linear form. “Secondary” structure refers to stable folding/conformation of these primary structures. a) (4 pt) What is the difference between the “primary” structure of α-amylose and amylopectin? Explain or illustrate. Amylopectins are essentially multiple “α-amylose-like” chains linked/branched through α1-6 glycosidic linkages. (can illustrate with structures). b) (4 pt) What is the difference
View Full Document
Unlocking...