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Unformatted text preview: Page 1 Name: MCB102 Practice Midterm 1 Fall 2007 This practice test is similar in form but perhaps easier than the actual Midterm 1 Exam. NAME with LAST NAME FIRST STUDENT ID Section Day Time TA Maximum Points Your Points Question 1 10 Question 2 25 Question 3 20 Question 4 25 Question 5 10 Question 6 10 REFERENCE INFORMATION: Henderson-Hasselbalch pH = pK a + log ([proton acceptor]/[protein donor]) Histidine side chain pK a = 6.0 Page 2 Name: Question 1: only bold words are needed for full credit A. What percentage of histidine molecules have no net charge at pH 7.0? NOTE: reference information is provided on the cover page! Answer: About 90% would have no net charge. (At pH 7.0, because pK a of the side chain is 6.0, log ([proton acceptor]/[proton donor]) must be 1 and so proton acceptor concentration is 10 times that of the proton donor. For histidine, the proton acceptor is the uncharged side chain.) B. What net charge do the charged histidine molecules carry at pH 7.0? Answer: One positive charge . (About 10% of molecules would have a positive charge due to protonation of the side chain.) Question 2 A. The artificial sweetener NutraSweet ® is a dipeptide of aspartate and phenylalanine (with the free carboxyl of phenylalanine esterified to methyl alcohol). Draw the structure of aspartame (without the carboxyl terminal modification) at pH 7.0. Make sure to indicate all atoms, bonds and charges. Answer: you should have drawn the complete amino acid structure of aspartate (with a free amino group bearing a positive charge) and phenylalanine (linked to aspartate through a peptide bond, with a negative charge on the free carbonyl group). The asparate side chain has a negative charge at pH 7.0. B. Draw the polypeptide backbone (label side groups = R) and hydrogen bonding of two 5 amino acid beta strands paired together in a parallel beta sheet. Please draw only the 5 amino acid segments, without any connector between them. Blue = nitrogen Red = oxygen Black = carbon White = hydrogen Purple = R (you do not need to draw this pointing into or out of the paper) You do not need to draw the arrows....
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This note was uploaded on 04/02/2008 for the course MCB 102 taught by Professor Staff during the Spring '08 term at Berkeley.
- Spring '08