BIBC 102 Name______KEY for a DAY_______________________________Midterm 11/04/03page 1 Student ID _________________________________________________It’s Enzyme Time (12 points)You overhear a student say the following quote:“…it’s the kcat that’s important! If you know the kcat of an enzyme, you can predict what the maximumrate of the enzyme will be, and what the 50% saturation substrate concentration will be as well…”Let’s think about this quote:1)(1pt) Define the kcat for an enzyme:kcat = Vmax/ET, that is, the maximum velocity divided by the total enzymeconcentration. We also accepted something like “the number of product moleculesproduced per molecule of enzyme at maximum velocity”2 (2pt) Write the version of theMichaelis-Menton equation (kcat)[ET][S]that specifically includes Vo =the kcat (use space to right) [S] + Km3)(1pt) For an enzymes that follows theMichaelis-Menton relationship, what is thesubstrate concentration that will cause 50%of the maximal velocity of the enzyme? (usespace to right)4) (1pt) So, is the student’s quote correct, partially correct or totally wrong, and why (one sentence)?The student is partially correct: you can predict the maximum velocity but the 50%saturation concentration is independent of kcat, and is only predicted by the Km.5) (2pt) Consider an enzyme-catalyzed reaction. Suppose that the uncatalyzed reaction has an activationenergy that is higher by 8.7kJ per mole in the absence of enzyme. What letter best describes the effect onthe reaction caused by the presence of the enzyme?your choice: ___B___A) faster by a factor of (8,700/RT) B) faster by a factor of e(8,700/RT)C) faster by a factor of e-(8,700/RT) D) effect depends on the original Eact [S]50 = KmBIBC 102 Name______KEY for a DAY_______________________________Midterm 11/04/03page 2 Student ID _________________________________________________It’s Enzyme Time (cont')6) (4pt) Suppose the forwardreaction going from S to P has anactual ΔG’ of -33 kJ/mole. Drawan energy diagram using the graphto the right, showing the positionsof the S and P, and the effect ofthe enzyme on the free energydiagram. Indicate the distance onthe graph that corresponds toΔG’. Label the axes please.7) (1pt)What is the mathematical expression for the ratio of the forward and reverse rate constants for thespecific reaction we are discussing above? Choose the best answer from the letter options below:Letter answer: ____C_____A) kf/kr = e-(33,000/RT) B) kf/kr = (33, 000/RT) C) kf/kr =e(33, 000/RT)Sara Sara Sara Sara Sara Sara Sara Sara Sara Sara SaraD) kf/kr = (33,000/RT) E) kf/kr = e(ΔG‡/RT) F) kf/kr = e((ΔG‡ + 33,000)/RT)See problem set two, for where this comes from if you are confused. Note its not theabsolute rate or rates, but the ratio of the two.. so ΔG‡ plays no role.plus enz33 kJfreeenergyreactionSPBIBC 102 Name______KEY for a DAY_______________________________Midterm 11/04/03page 3 Student ID _________________________________________________Graphs and Corruption (10 points)In class we discussed a Lineweaver-Burke plot as a way to plot enzyme kinetic data. In the spaces that areprovided, describe this sort of plot8) (1pt) The points on the X (horzontal) axis are : 1/[S]9) (1pt) The points on the Y (vertical) axis are: 1/Vo10)(1pt)The X intercept is equal to: -1/Km11)(1pt)The Y intercept is equal to: 1/Vmax12) (1pt)Why do people use the Lineweaver Burke plot in analysis of enzyme kinetics (One sentence)When Michaelis-Menton enzymes are plotted in this manner, the resulting data is astraight line, making it easier to see when an enzyme behaves this way and easier todiscern what kind of inhibitor is involved in an experiment.Allosteric enzymes are often encountered in the study of metabolic regulation. Below is a simple rate plot,in which initial enzyme rate is plotted against substrate concentration S. You will draw a few curves.Please label each curve.13) (1pt)Draw a curve for acooperative enzyme. Include anapproximate Vmax for use below.14) (1pt)Draw a new curve forthis enzyme when a heterotropicallosteric activator is added, andlabel it A15) (2pt)Draw a third curve for Ewhen a covalent inhibitorirreversibly destroys half of theenzyme activity, label it C, andindicate on the graph the effecton Vmax16) (1pt) (I ask this every year, because I feel its my responsibility as a professor) What does this kineticbehavior indicate about the structure of this particular enzyme?The enzyme almost certainly has quaternary structure, that is, multiple subunits.[substrate]rateVmax1/2VmaxBIBC 102 Name______KEY for a DAY_______________________________Midterm 11/04/03page 4 Student ID _________________________________________________ Redox Redux (14 points)Here is a list of standard half-cell potentials for a number of reactions.1) aKG + CO2 +2H+ +2e- <---> isocitrate -0.33 V2) NAD+ + H+ + 2e- <---> NADH -0.32 V3) FAD + 2H+ + 2e- <---> FADH2 -0.22 V4) FMN + 2H+ + 2e- <---> FMNH2 -0.12 V5) CoQ + 2H+ + 2e- <---> CoQH2 +0.10 V6) Cyt c (Fe3+) + e- <---> Cyt c (Fe2+) +0.22 V7) Cyt a (Fe3+) + e- <---> Cyt a (Fe2+) +0.29 V17) (3pt)These are also known as the E’o for the reactions. What does the “prime o” after the E mean?all reactants and products 1M, 25 degrees C, ph 7, 55M water18) (2pt)Rewrite half-reaction #1 with structures+ C02 + H+ + 2e19) (1pt)What enzyme catalyzes the metabolicreaction that includes half reaction #1?: ___ ISOCITRATE DEHYDROGENASE__(Note: Remember, an enzyme will catatlyze the forward and the reverse reaction to thesame extent)20) (1pt)Of all the molecules on this table, which one is the best oxidizing agent ___ Cyt a___(It is the most prone to be reduced… so it is the best oxidizing agent)21) (4pt)Write a balanced complete reaction combining half-cell reactions #3 and #5 in the direction thatwould be spontaneous at standard conditions. Include the E’o value for the complete reaction(Just looking at the table, you can tell that the 5 will run spontaneously as a reductionwhereas 3 will run spontaneously as an oxidation. That is, 3 will run backward fromwritten and 5 will run as it is written. Thus)CoQ + FADH2 CoQH2 + FAD+ E’0 = 0.10 - (-.22) = 0.3222) (2pt) Set up the equation to compute ΔG’o for the spontaneous complete reaction you wrote in 21.You can leave constants as constants.ΔG’o = - n F
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