ACDSeat # ________ Last initial ________ Chemistry and Biochemistry 153A Winter 2002 Midterm Examination Question # Value Score 1 23 2 23 3 20 4 10 5 11 6 25 7 10 8 30 TOTAL 152 __________KEY_______________ Print your full name (last name first) ______________________________ Student ID Number ______________________________ Print your TA's name ______________________________ Discussion section # [or time & day] I have read the instructions below. ______________________________ Signature INSTRUCTIONS 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 the point values.Name ______KEY__________________ 2 1. (23) a. (3) Circle an expression that represents an acid/base reaction that could cause a resistance to change in pH upon addition of hydroxide to a dilute aqueous solution containing a weak acid, HA. H2OH+OH-H++H2O + H2OH3O+ + OH-OH- + HAH2O + A-H2O + HAH3A+ + OH-H3O+ + HAH2A + b. (6) Fill in the blanks giving the concentrations of each of the following species in a dilute aqueous solution at pH 8. [H2O] = __55.5_____ M [H+] = __10-8___ M [OH-] = ____10-6____ M c. (6) OH- equiv.1pH0.51234567891011121314abcd 1) The above plot shows the titration of 2mmoles of a weak acid, HA, in dilute aqueous solution. Write a balanced equation showing the predominating reaction occurring between points "a" and "b" on the plot as OH- is added. H2OH+OH-+H3O+ + OH-or2H2O 2) What is the pKa for deprotonation of the weak acid, HA, represented in the above plot? __6_______ 3) What is the ratio A- over HA at point c on the plot? _1000/1 or _10-3___ 4) What is the pH of the solution represented by the plot when 1.5 mmoles of OH- have been added? __7_ d. (8) Name the four classes of biological macromolecules. ___________proteins_____________________ ____________polysaccharides____________ ___________lipids_______________________ ____________nucleic acids_______________ These four classes of macromolecules play a large number of biological roles in living organisms. This suggests a high level of structural diversity leading to great complexity in the structures of these macromolecules. What two major factors permit the necessary structural diversity while maintaining a degree of simplicity in structure? 1) __Macromolecules are polymers of known simple subunits___________________________________ _____________________________________________________________________________________________ 2) ___Many_macromolecules are virtually ubiquitous (i.e., similar or identical macromolecules in many different and diverse organisms____________________________________________________Name ______KEY__________________ 3 2. (23 ) a. (1) Which of the 20 protein amino acid residues is least likely to be found in an α-helix? ___proline____________ b. (1) Glutamic acid is the most strongly hydrophilic protein amino acid according to the hydropathic index given in the compendium. TRUE FALSE c. (1) Rotation about the peptide (amide) bond is restricted due to steric hindrance. TRUE FALSE d. (2) If a weak acid has a pKa of 6 and the ratio of deprotonated to protonated molecules is 1 to 10,000, what is the pH of the solution? (Write your answer on the line provided.) ___2_____________ e. (1) The term tertiary structure refers to the overall folding pattern (3-dimensional conformation) of a polypeptide. TRUE FALSE f. (10) Draw a titration curve (pH vs. H+ equiv.) for methionyl-aspartyl-lysine using standard pKa’s given in the compendium. H+ equivalents0.5 1.0 1.5 2.0 2.5 3.02468101214pH3.5 4.0 g. (2) Calculate the isoelectric point for this tripeptide. pI = (4 + 8) /2 = 12/2 = 6 h. (5) Briefly explain why the average pKa for the alpha amino group of the N-terminus of a polypeptide is lower than the average pKa for the alpha amino group of a free amino acid. [50 words or les In the free amino acid, the inductive effect of the α-carboxyl group tends to lower the pKa of the α-amino group, and the electrostatic effect tends to raise it. Since the electrostatic effect is missing when the carboxyl group of the amino terminal residue participates in a peptide bond, the pKa of the N-terminus is lower.Name ______KEY__________________ 4 3. (20) The figure below represents the tertiary structure of a segment of a cytoplasmic protein at pH 7. The complete amino acid sequence is indicated by three letter symbols at regular intervals along the backbone represented by the solid line. The dotted lines represent interactions stabilizing the native conformation of this protein. Note that ten of these dotted lines are labeled. leuth rphealametaspalaleualaglyproglyglycyscysalaarglysaspasp thrasnvalphepheglugluaspcysmethisglyleuglnalasercyscysphevalglymettrphis1235 679108val4N-terminusC-terminusproileval leuph ecysserthrglyalamettrpglyalathrtrpleuasntyrargileglyglugluser a.(10) Below is a list of descriptions of interactions. Place each number from the figure above on the line following the most precise description of the interaction that is depicted next to that number in the figure. Use all the numbers. There can be more than one number per line, and at least one number should go on each line. Ion pair (electrostatic attraction) _____6_______ Hydrogen bond between side chain atoms ___4_______ Electrostatic repulsion ____7_______ Ion-polar bond ____2______ Hydrogen bond between backbone atoms __3, 5_____ Hydrophobic interaction __1, 10______ Covalent bond ____8_____ Steric hindrance ___9_______ b. (6) Match each of the following terms on the left with the phrase that best describes it with reference to proteins. Letter Term Description D backbone atoms A. amide G peptide plate B. depends entirely on ionic interactions A peptide bond C. largely entropy driven F secondary structure D. everything except the side chains E native conformation E. tertiary structure in single polypeptide proteins C hydrophobic effect F.
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