EXAM 1, BICH 410( MWF 1:50 PM), Friday, Sept. 22, 1995
Write your name on each page. Write concise answers to demonstrate effectively your mastery of the subject material. Show your work in order to receive partial credit where applicable.
1. (15 pts) If 50 ml of 0.01 M HCl is added to 100 ml of 0.06 M phosphate buffer (pH 7.16), what is the resultant pH? For the reaction H2PO4- to HPO4-2 + H+, the pKa = 6.86. Show your work and your reasoning explicitly.
2. (10 pts) Consider the following tripeptides:
- A. leucine-valine-phenylalanine
- B. glycine-proline-arginine
- C. aspartic acid-histidine-glutamic acid
- D. aspartic acid-tryptophan-tyrosine
- E. tyrosine-lysine-methionine
Which one of these tripeptides:
- ______ (a) contains the largest number of nonpolar side chains?
- ______ (b) will have the greatest light absorbance at 280 nm?
- ______ (c) is most negatively charged at pH 7?
- ______ (d) contains sulfur?
- ______ (e) will yield DNP-tyrosine when reacted with 1-fluoro-2,4-dinitrobenzene and hydrolyzed in acid?
3. (20 pts) Fill in the blanks with an appropriate answer or circle the correct answer. In some cases, several answers are possible, but write just one.
- a. Amino acid with a guanidino group _____________________
- b. What is the pH of 0.1 M HCl? _____________________
- c. A reagent that will reduce disulfide bonds in proteins __________________________
- d. Amino acid with two asymmetric carbon atoms _______________________
- e. Amino acid with an isoelectric pH above 9 ___________________________
- f. Buffer system that operates in blood plasma ____________________________
- g. The force of an ionic interaction between two opposite charges is greatest in:
- i) water ii) benzene iii) ethanol
- h. What is the pH of 0.01 M NaOH? _____________________
- i) Electrophoresis technique that uses ampholytes ____________________________
- j) Approximate molecular weight of a protein containing 200 amino acids ________________
4. (15 pts) Use the table below that describes several properties of some hypothetical enzymes in order to answer the following questions. Before purification, these enzymes are combined in a single tube.
|enzyme||pI||subunit composition and molecular weight||mass (mg)||activity units|
|CUase||8.0|| one @ 25,000||100||300 field goals|
|ATMase||8.0|| one @ 50,000||100||800 touchdowns|
|TUase||4.5|| two@ 25,000, each||150||200 safeties|
|Ricease||5.0|| three @ 10,000, each||50||50 touchdowns|
(a) What chromatography method could be used to separate ATMase from TUase? Why?
(b) What chromatography method could be used to separate ATMase from CUase? Why?
(c) After steps (a) and (b) would ATMase be separated from the other enzymes?
(d) What is the specific activity of purified ATMase if 50% of the starting activity was recovered?
(e) After purification, by what factor would the ATMase be purified when compared to the starting mixture?
5. (10 pts) In their optimal orientations, draw two types of hydrogen bonds (as dotted lines) that could form between the peptide backbone and water molecules.
6. (10 pts) Using a flow chart, briefly describe the sequence of steps involved in the chemical synthesis of a dipeptide by the Merrifield method.
7. (a) (12 pts) Draw the structure of the dipeptide aspartylproline as it would exist at pH 7. You do not have to depict the proper stereochemistry in this structure.
(b) (3 pts) The pKas for the ionizable groups in aspartylproline are 1.99, 3.65 and 9.60 (using values from the individual amino acids). Assign each pKa to the correct ionizable group (either in writing below or on your structure drawn above).
(c) (5 pts) What is the pI (isoelectric point) of aspartylproline using the pKas given in part (b)? Show your reasoning for full credit.