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UA BIOC 460 - THIRD HOUR EXAMINATION

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WRITE YOUR NAME AND I.D. NUMBER LEGIBLY ON EVERY PAGE – PAGES MAY BESEPARATED FOR GRADING!CHECK TO BE SURE YOU HAVE 6 PAGES, NAME (print): INCLUDING COVER PAGE. 5-digit course ID # I swear/affirm that I have neither given nor received any assistance with this exam.Signature: Date: BIOCHEMISTRY 460THIRD HOUR EXAMINATIONFORM A (YELLOW)ANSWER KEYApril 11, 2005A CALCULATOR MAY BE USED ON THIS EXAM. IF YOUR CALCULATOR ISPROGRAMMABLE, ALL MEMORY BANKS MUST BE CLEARED BEFORE YOU ENTERTHE EXAM, AND WE MAY CHECK & CLEAR CALCULATORS DURING THE EXAM.SHOW YOUR WORK FOR ALL CALCULATIONS, AND BE SURE TO STATE UNITS OFANY NUMERICAL ANSWERS. If the reasoning, calculations, or answer are shown anywhereother than in the space provided, make a note in the space provided and put answer on back ofSAME PAGE so the grader for that page will have it.USEFUL CONSTANTS:R (gas constant) = 8.315 x 10–3 kJ•mol–1•Kelvin–1F (Faraday constant) = 96.48 kJ•Volt–1•mol–1If temperature = 25 °C, absolute temperature T = 298 ºKEnergy Charge (EC):€ [ATP] + 0.5[ADP][ATP] + [ADP] + [AMP]Change in Free Energy:€ ΔG'= ΔGº '+RT ln[ product]actual[substrate]actualAt equilibrium (when ΔG’ = 0):€ ΔGº '= −RT ln[ product]equilibrium[substrate]equilibriumChange in Reduction Potential:€ ΔE º '= E º ' e − acceptor( )− E º ' e − donor( )Relating ΔGº' to ΔEº':€ ΔGº '= −nFΔE º 'Biochemistry 460, Exam #3 Form A ANSWERS NAME April 11, 2005 5-digit course ID# page 21. (6 pts) Sildenafil (Viagra) is a member of a class of drugs that treat erectile dysfunction by increasingblood flow to target tissues through a cyclic GMP-mediated mechanism.a. (3 pts) What is the mechanism by which sildenafil increases cyclic GMP levels?Sildenafil inhibits the activity of cyclic GMP phosphodiesterase.b. (3 pts) What upstream signal is required for sildenafil to be effective?Sildenafil requires upstream nitric oxide (NO) signaling which stimulates cyclic GMPproduction (by activating guanylyl cyclase).2. (6 pts) Extracellular signals can trigger phosphorylation events that "recruit" intracellular signalingproteins to the cytosolic side of the plasma membrane. Epidermal growth factor (EGF) andphosphoinositide-3-kinase (PI-3 kinase) signaling pathways both utilize this protein dockingmechanism to transmit intracellular signals.a. (3 pts) What amino acid residue is phosphorylated on the cytoplasmic tail of the EGF receptor thatserves as the binding site for SH2 domain-containing proteins?The amino acid tyrosine (Tyr, Y)b. (3 pts) PI-3 kinase phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2), a membraneglycolipid, to generate a binding site for pleckstrin homology (PH) domain-containing proteins.What is the name of the phosphorylated product of the PI-3 kinase reaction that serves as thebinding site for PH domains? (The abbreviated name is acceptable.)Phosphatidylinositol 3,4,5-trisphosphate (PIP3)3. (5 pts) Order the steps of a glucagon-stimulated Gα protein coupled receptor signaling pathwaybeginning with the number 1 for the first step and ending with number 5 as the last step (1 pt. for eachcorrect answer only, no partial credit).3 or 4 Cyclic AMP activates protein kinase A (PKA) by binding to PKA regulatory subunits.__5__ Cyclic AMP phosphodiesterase reduces the level of cyclic AMP and PKA is inhibited.__2__ Gα-GTP stimulates activity of adenylate cyclase leading to rapid build-up of cyclic AMP.4 or 3 Adenylate cyclase activity returns to basal levels after Gα-GTP is converted to Gα-GDP.__1__ Glucagon receptor binding initiates the exchange of GDP for GTP in Gα subunits./17p. 2 (17 points) p. 3 (21 points) p. 4 (20 points) p. 5 (22 points) p. 6 (20 points) TOTAL: (100 points)Biochemistry 460, Exam #3 Form A ANSWERS NAME April 11, 2005 5-digit course ID# page 34. (6 pts) Name the two peptide hormones that are most responsible for regulating blood glucose levelsin humans and state their functional role in liver cell signaling before and after breakfast.Before breakfast, glucose levels are low and glucagon signaling stimulates glucose export.After breakfast, glucose levels are high and insulin signaling stimulates glucose import.5. (5 pts) Fill in the blanks with the correct number of moles, or abbreviated name of the substrate orproduct, in the following net reaction for glycolysis.Glucose + 2 _____ + 2 ______ + 2 _____  2 ________ + 2 NADH + 2 H+ + __ATP + 2 H2OGlucose + 2 NAD+ + 2 ADP + 2 Pi  2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O6. (10 pts) The phosphoglucose isomerase reaction interconverts glucose-6-phosphate and fructose-6-phosphate.a. (2 pts) Write the reaction catalyzed by phosphoglucose isomerase (just words, no structures) in thedirection in which it would occur in glycolysis.Glucose-6-P  fructose-6-P ΔGº’ = +1.7 kJ/molb. (5 pts) Using +1.7 kJ/mol for standard free energy change (ΔGº’) of this reaction in the glycolyticdirection, calculate the actual change in free energy (ΔG’) in the cell at 310 K for thephosphoglucose isomerase reaction in the context of the glycolytic pathway, using the intracellularsteady-state concentrations of 8.3 x 10–5 M for glucose-6-phosphate and 1.4 x 10–5M for fructose-6-phosphate. Show your work.Glucose-6-P  fructose-6-P ΔGº’ = +1.7 kJ/mol[glucose-6-P]actual = 8.3 x 10–5 M[fructose-6-P]actual = 1.4 x 10–5 MΔG’ = ΔGº’ + RT • ln [product]actual [substrate]actualΔG’ = +1.7 kJ/mol + RT • ln [fructose-6-P]actual [glucose-6-P]actualΔG’ = +1.7 kJ/mol + (8.315 x 10–3 kJ/mol • K) (310 ºK) • ln(1.4 x 10–5 M) (8.3 x 10–5 M)ΔG’ = +1.7 kJ/mol + (– 4.6 kJ/mol)ΔG’ = – 2.9 kJ/molc. (3 pts) Is the reaction as you wrote it in part A (the direction of glycolysis) favorable orunfavorable under the specified intracellular concentration conditions, i.e., would it proceedspontaneously in the direction of glycolysis or in the reverse direction (gluconeogenesis)? Brieflyexplain why you gave that answer.ΔG’ < 0 (or ΔG’ is a negative number) for the direction of the reaction in glycolysis(favorable in direction of glycolysis, glucose-6-P --> fructose-6-P)./21Biochemistry 460, Exam #3 Form A


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UA BIOC 460 - THIRD HOUR EXAMINATION

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