Mechanism - Molecular and Cell Biology Department at the ：机制的分子和细胞在生物学系.docVIP

University of Cape Town Department of Molecular Cell Biology Course Code: MCB2000F Course Name: Intro to Biochemistry Paper: Theory Date: 13 June 2007 @ 12h30 No.of pages: 17 Marks: 137 Time: 3 Hours Venue: NSLT Attachments: 2 pages ANSWER ALL THE QUESTIONS In the interest of economy please write on both sides of the exam scriptPlease answer each section in a different answer book. Note that multiple choice questions are subjected to negative marking Section 1 Dr Jerry Rodrigues 33 Marks Section A (Amino acids and proteins) A highly idealized titration of 1 mol of fully protonated cysteine with sodium hydroxide is shown in the following figure: (a) Write the three ionization reactions, with structures, starting with protonated cysteine. (4) (b) What regions in the graph correspond to each ionization step ie indicate the pKas (choose from I to VII)? (3) (c) At what pH value is the average nett charge on the cysteine equal to -1 (read off the graph or choose from I to VII)? (1) (d) At what pH value is the average nett charge on the cysteine equal to -1/2 (read off the graph or choose from I to VII)? (1) (e) At what pH value is the average nett charge on the cysteine equal to -2 (read off the graph or choose from I to VII)? (1) (f) What point corresponds to the isoelectric point of cysteine (choose from I to VII)? (1) (g) At which point is the pH equal to the pKa value for the dissociation of the carboxyl group? (1) [12] 2. Consider the following compound: (a) What kind of peptide does this structure represent? (For example, dipeptide) (1) (b) How many peptide bonds are present? (1) (c) Give the name for the N-terminal amino acid. (1) (d) Give the name for the C-terminal amino acid. (1) (e) Using three-letter abbreviations, write the sequence from N- to C-terminal. (2) [6] 3. The figure below shows the 13C chemical shifts occurring in titrations of the amino aci