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UCLA CHEM 153A - sample 153A exam #1

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1 Your Name: _________________________ Chemistry and Biochemistry 153A Summer 2013 First Midterm 50 total points 1. Part VI. (25 points) Protein structure. Consider whether the following statements are reasonable (R) or unreasonable (U) in terms of what you know of the stuctures and properties of proteins. Indicate either R or U for each statement and write a brief justification for your answer for those statements you judge as unreasonable. No more than three sentences should be required. Credit will not be given without written justification. (A) Only the hydrogen bonds that are surface-exposed are important in stabilizing the three dimensional structure of a globular protein. (B) Polylysine (a polymer of lysine monomer units linked by alpha-peptide bonds) is likely to fold into an alpha helical structure at pH 7.0 (C) A coiled-coil protein possesses alpha helices that contain only hydrophobic amino acid residues. (D). Interactions between hemoglobi subunits are stabilized in high salt concentrations. (E)2 Your Name: _________________________ 2. Consider the peptide: trp-pro-arg-phe-ala-asp (a) (12 points) Draw the chemical structure of this peptide at pH 7.0. (b) (6 points) Using the following approximate pKa values, sketch the titration curve of this peptide. Indicate regions of buffering on the curve. pKa terminal COOH 2.0 terminal -NH3 9.0 R-COOH 4.0 R-NH3 10.03 Your Name 3. (12 points) A novel protein of unknown structure has been purified. Using chromatography, the native molecular weight is determined to be 240,000. Chromatography in the presence of 6M guanidine results in a protein with an apparent molecular weight of 60,000. Chromatography in the presence of both 6M guanidine and 10 mM b-mercaptoethanol results in two polypeptides of molecular weights equaling 34,000 and 26,000. What does this experiment tell you about the subunit composition of this protein? It may be helpful if you sketch a cartoon of the protein.4 Your Name 4. The graph below shows that polyglutamate, a polypeptide composed entirely of L-glutamate residues, has an a-helical conformation at pH 3. When the pH is shifted to approximately pH 7 the polyglutamate adopts a random conformation. (a) (7 points) Provide an explanation for this effect. Why is the transition between these two forms so sharp? Be sure to note the forces involved in stabilizing each structure. (continued on next page) pH1 2 3 4 5 6 7 8 9 10 % alphahelixrandomalpha helix5 Your Name (b) (5 points) What would happen if polylysine were tested over this range of pH values (from pH 1 to pH 10)? Show the results on a graph. 5. (10 points) Two polypeptides, A and B, have very similar tertiary structures, but A normally exists as a monomer and B normally exists as a dimer. What differences would you expect in the composition and distribution of amino acids in A versus


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