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WRITE YOUR NAME LEGIBLY ON EVERY PAGE . PAGES WILL BE SEPARATED FOR GRADING ! CHECK TO BE SURE YOU HAVE 7 PAGES , NAME (print): INCLUDING COVER PAGE. Section & T.A. I swear/affirm that I have neither given nor received any assistance with this exam. Signature: Date: BIOCHEMISTRY 462a FIRST HOUR EXAMINATION Form A -- ANSWERS September 17, 2007 A SCIENTIFIC CALCULATOR IS NEEDED FOR THIS EXAM. SHOW YOUR WORK FOR ALL CALCULATIONS , AND BE SURE TO STATE UNITS OF ANY NUMERICAL ANSWERS. If the reasoning, calculations, or answer are shown anywhere other than in the space provided, make a note in the space provided and put answer on back of SAME PAGE for same grader, since pages are separated for grading . USEFUL CONSTANTS: R (gas constant) = 8.315 J •mol –1 •Kelvin –1 = 8.315 x 10 –3 kJ •mol –1 •Kelvin –1 (25 °C) T = 298 K; human physiological temperature (37 °C) T = 310 K. Ionizable group in peptides and proteins Approximate ("generic") pK a in α -amino 8.0 α -carboxyl 3.0 ε -amino 10.0 guanidino 12.0 thiol 8.5 imidazole 6.5 aromatic hydroxyl 10.0 side chain carboxyl 4.0 p. 2 (31 points) p. 3 (21 points) p. 4 (14 points) p. 5 (14 points) p. 6 (10 points) p. 7 (10 points) TOTAL: (100 points)
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Biochemistry 462a, Exam #1 Form A ANSWERS NAME (every page!!) September 17, 2007 page 2 1. (15 pts) Consider the peptide whose sequence is A K L Y E N F M . A. (10 pts) At pH 5 , what would be the approximate net charge on the peptide ? Use the “generic” pK a values on the cover sheet of the exam; show your reasoning in Part A of table below; start by listing all ionizable groups and their approximate pK a values. See lefthand column below. PART A PART B ionizable group pKa approx. charge at pH 5? approx. charge at pH 1 α -carboxyl ~3 –1 0 R-carboxyl (E) ~4 –1 0 <–– pI α -amino ~8 +1 +1 aromatic OH (Y) ~10 0 0 ε -amino (K) ~10 +1 +1 approx. net charge: 0 (pH 5) +2 (pH 1) B. (5 pts) Calculate the isoelectric point ( pI ) for this peptide, using the “generic” pK a values on the cover sheet of the exam. Start by filling out the Part B column above, listing the charges of all ionizable groups at pH 1. Net charge at pH 1 is +2 , so 2 equivalents of OH would be needed to get from pH 1 to net charge of zero (pI), so pI is halfway between pKa of side chain carboxyl group of Glu and α - amino group. pI = (4 + 8) / 2 = 6 2. (16 pts) The flat projection structure of a 3-amino acid residue segment of a protein is drawn below. A. (3 pts) What is the sequence shown (1-letter or 3-letter codes)? Lys (K) – Ser (S) – Asn (N) B. (3 pts) Circle 6 atoms that are coplanar (all 6 atoms in the same plane). Circle only ONE group of 6 atoms; you have several correct choices. Do NOT include any side chain atoms. C. (3 pts) With a
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