PairASeat # ________ Last initial ________ Chemistry and Biochemistry 153A Fall 2006 Midterm Examination Question # Value Score 1 21 2 22 3 31 4 27 5 20 6 31 TOTAL 152 ____________KEY________________ Print your full name (last name first) _____________________________________ Student ID Number _____________________________________ Print your TA's name ______________________________ Discussion section # [or time & day] INSTRUCTIONS YOU SHOULD HAVE ONLY YOUR WRITING MATERIALS, ERASER, APPROVED NON-PROGRAMMABLE, NON-GRAPHING CALCULATOR WITHOUT CASE OR COVER AND YOUR PICTURE I.D. ZIP EVERYTHING ELSE INSIDE YOUR BACKPACK AND STORE IT ALL THE WAY UNDER YOU CHAIR. NO OTHER ELECTRONIC DEVICES OR CELL PHONES ARE ALLOWED. NO BOOKS OR LOOSE PAPERS OF ANY KIND SHOULD BE IN THE VICINITY OF YOUR DESK. READ EACH QUESTION CAREFULLY! SHOW YOUR CALCULATIONS! WRITE YOUR LAST NAME IN THE SPACE PROVIDED ON EVERY PAGE OF THE EXAM. NO CREDIT WILL BE GIVEN FOR ANYTHING WRITTEN ON THE BACK OF A PAGE OR ANYWHERE OTHER THAN THE SPACE PROVIDED FOR EACH ANSWER. ANSWERS MUST BE BRIEF AND TO THE POINT. WHERE A WORD LIMIT IS SPECIFIED, NO CREDIT WILL BE GIVEN FOR THE PART OF THE ANSWER IN EXCESS OF THE LIMIT. Numbers in parentheses represent point values. I have read & understood the instructions on this page. I do not have any prohibited materials in my possession. I will not give, solicit or take any help on this exam. ______________________________________ SignatureSeat # ________ Last initial ________ 1. (21) a. 1) (1) How many classes of macromolecules are found in living systems? __4____ 2) (2) Macromolecules contribute greatly to the complexity of living cells as defined in the compendium. Very concisely state two reasons for this. [≤ 10 words each] a) They are extremely large. b) There are many different species of macromolecules in each cell. 3) (2) Although the statements in part (2) above are true, macromolecules also confer a degree of molecular simplicity on living systems, making the job of structural analysis by biochemists much easier than it would be otherwise. What structural characteristic, shared by all macromolecules, is largely responsible for this? [≤ 20 words] They are polymers of similar subunits. 4) (2) Considering the great diversity of life and the huge numbers of different organism on earth, what additional factor also contributes to a simplicity that makes biological studies at the molecular and chemical level easier? [≤ 20 words] Most organisms use the same or similar molecules and reaction pathways as nearly all other organisms to meet similar needs. 5) (2) Our cursory consideration of the many different types of functions to be carried out in a single organism has revealed to us that nature is very conservative in the use of resources by managing to adapt each class of macromolecules to a large number of diverse functions. The catch phrase we have used to describe this phenomenon is, “_the economy of life &/or simplicity of life”. b. (5) The expression for the equilibrium constant for the ionization of water is written as follows. [H+] [OH-][H2O]Keq = Starting with the following expression for the ionization of water, derive the expression above. (Show all steps) H2OH+ + OH–k1k-1 V = K1[H2O] – K-1[H+][OH-] V = 0 when K1[H2O] = K-1[H+][OH-] this is equilibrium Dividing both sides by K-1 and by [H2O] we have [H+][OH-][H2O] =k1k-1 Let Keq = k1/k-1 . Substituting into the above equation, we get the equilibrium expression given above. c. (7) 1) (5) On the grid below plot the titration of a weak acid, HA (pKa = 3 ), in dilute aqueous solution starting at pH 9 and adding HCl. H+ equiv.1pH0.51234567891011121314 2) (2) What is the ratio A- / HA of this weak acid at pH 8? 10-5__or 100000/1_______ [Show calculation below.]A-HAHAA-HAA- 1pH = pKa + log pH - pKa = log = 10(pH - pKa)= 10(8 - 3) = 10(5 ) o r 1000 00Name _______KEY_______________ 22. (22) a. (1) The pH of urine ranges from 5 to 8. This is a _1000_ fold range in the proton concentration. b. (2) Refer to this table of pKa’s for phosphoric acid. Species Ka pKa phosphoric acid H3PO4 1 X 10-2 2 phosphate, monobasic H2PO4- 2.0 X 10-7 6.7 phosphate, dibasic HPO42- 3.4 X 10-13 12.5 Based on the above pKa's, what predominant form(s) would be present at pH 4.35? ___2____ 1) H3PO4 is the predominant species 2) H2PO4- is the predominant species 3) HPO42- is the predominant species 4) H3PO4 is equal to H2PO4- 5) H2PO4- is equal to HPO42- c. (8) Two enzyme-catalyzed reactions were carried out separately using 0.2M TRIS chloride (pKa = 8) as a buffering agent. 1. glyceraldehyde-3-phosphate + inorganic phosphate + NAD+ → 1,3-bisphosphoglycerate + NADH + H+ catalyzed by glyceraldehyde-3-phosphate dehydrogenase at its pH optimum of 7.3. 2. CCH2OPO32-COOH+CCH3OCOO+ ATP+ ADPMg2+, K++ at the enzyme’s pH optimum of 7.3. a) (4) In which reaction above will the buffering by TRIS chloride be more effective at maintaining the optimum pH assuming that initial concentrations of reactants are the same for both reactions, and both reactions are allowed to proceed to the same extent? Write the number of your choice here. _2_ Explain? [≤ 35 words] At pH 7.3 more TRIS molecules are in the protonated form and able to deprotonate to replace protons consumed by reaction 2. There is less TRIS in the A- form to take up protons produced in reaction 1. b) (2) Suggest a means by which you could make the buffering by TRIS chloride more effective at the optimal pH in the reaction that you did not choose. [≤ 20 words] Increase the total concentration of TRIS in reaction 1. c) (2) Give the enzyme class for each of the above reactions 1. _oxidoreductase__ 2. __transferrase__ d. (7) Based on the microenvironmental effects we discussed in class and your general knowledge of chemistry suggest a reasonable pKa for Chloroacetic acid by comparison to acetic acid (pKa 4.8) and glycine (pKa 2.3). 2.98_ ( some credit will be given for any value between 2.8 and 4.2 supported by reasonable explanation.) Explain your choice. [≤ 40 words] Note you need not come up with the actual measured value just a reasonable estimate supported by a tenable explanation. No credit will be given for the numeric value
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