CHM 341 C: Biochemistry I. Test 2: October 24, 2014

Transcription

1 CHM 341 C: Biochemistry I Test 2: ctober 24, 2014 This test consists of 14 questions worth points. Make sure that you read the entire question and answer each question clearly and completely. To receive credit for a problem, you must SHW YUR WRK! If you expect full credit, you better include units! When you have completed the exam, please consider the following: I affirm that I have neither committed nor witnessed a violation of academic integrity in the completion of this exam. Signed Group/AA pka value C-terminus 3.5 Aspartate 3.9 Glutamate 4.1 Histidine 6.0 Cysteine 8.4 N-terminus 9.0 Tyrosine 10.5 Lysine 10.5 Arginine 12.5

2 Multiple Choice. Choose the answer that best fits the question. There is only one correct answer for each question. For potentially half credit, you may circle 2 answer choices. (4 pts) 1. Which amino acid is least likely to participate in acid-base catalysis? a. lysine b. methionine c. aspartate d. histidine e. tyrosine 2. Which of following is an anomeric pair? a. D-glucose and D-fructose b. D-glucose and L-fructose c. D-glucose and L-glucose d. α-d-glucose and β-d-glucose e. α-d-glucose and β-l-glucose 3. Which one of the following statements is true of enzyme catalysts? a. They bind to substrates but are never covalently attached to substrate or product. b. They increase the equilibrium constant for a reaction, thus favoring product formation. c. They increase the stability of the product of a desired reaction by allowing ionizations, resonance, and isomerizations not normally available to substrates. d. They lower the activation energy for the conversion of substrate to product. e. To be effective, they must be present at the same concentration as their substrates. 4. In uncompetitive inhibition, an inhibitor: a. binds at several different sites on an enzyme. b. binds covalently to the enzyme. c. binds only to the ES complex. d. binds reversibly at the active site. e. does not affect the characteristic Vmax of the enzyme. 5. A transition-state analog: a. is less stable when binding to an enzyme than the normal substrate. b. resembles the active site of general acid-base enzymes. c. resembles the transition-state structure of the normal enzyme-substrate complex. d. stabilizes the transition state for the normal enzyme-substrate complex. e. typically reacts more rapidly with an enzyme than the normal substrate.

3 6. True/False. If a statement is false, correct it to make it true. (12 pts) a. kcat/km represents the overall efficiency of an enzyme. b. The cyclization of glucose results in the formation of an acetal. c. Mixed inhibitors decrease the Vmax but may increase or decrease the Km. d. Allosteric regulation occurs on multisubunit enzymes, resulting only in inhibition of the enzyme. e. The higher the value of KI, the tighter the inhibitor binds to the enzyme. f. In a Lineweaver-Burk plot, the slope of the line is equal to kcat/km. 7. The turnover number for an enzyme is known to be 5000 min -1. The Lineweaver-Burk plot is shown below for a set of experiments with this enzyme. Calculate the Km and the total amount of enzyme present in these experiments. (6 pts) y = 0.004x /vo (min/μm) /[S] (mm) Km =. (b) Total enzyme =. 8. Consider the chemical equation below used to describe the enzyme-catalyzed conversion of substrate to product: k 1 E S ES k 2 E P k -1 a. Write a rate equation for the formation of ES. (3 pts) b. Write a rate equation for the breakdown of ES. (3 pts) c. Explain what is meant by the steady-state assumption. You may find it helpful to use the equations from part a and b in your explanation. (3 pts)

4 9. Consider the structures of the D aldopentoses: D-ribose D-arabinose D-xylose D-lyxose a. Draw the enantiomer of D-arabinose. (3 pts) b. Identify the relationship between the enantiomer of D-arabinose and the C2 epimer of L-ribose. (3 pts) 10. List and describe two limitations of Michaelis-Menten kinetics. (4 pts) 11. In serine protease, all of the following steps occur. Indicate the order which they happen by numbering them. (16 pts) a. His 57 donates H to N of sissile peptide bond, tetrahedral intermediate decomposes. b. The portion (the C-terminal end) of the original substrate with the new amino terminus diffuses away. c. Resulting H attacks carbonyl of remaining substrate. d. His 57 donates H to Ser 195, leading to collapse of tetrahedral intermediate. e. Water donates H to His 57. f. The portion (N-terminal end) of original substrate with new carboxylate terminus diffuses away. g. His 57 catalyzes removal of H from Ser 195 hydroxyl. h. Ser 195 s nucleophilic attacks carbonyl C of substrate.

5 12. You measure the initial rate of an enzyme reaction as a function of substrate concentration in the presence and absence of an inhibitor. The following data are obtained: (10 pts) [S] (μm) V0 (μm/sec) Inhibitor Inhibitor a. What is the Vmax in the absence of inhibitor? b. What is the Km in the absence of inhibitor? Make it clear how you determined this. c. When [S] = μm, what will V0 be in the absence of inhibitor? SHW WRK! d. Calculate KI when [I] = 2.0 μm. SHW WRK! e. What kind of inhibitor is it likely to be? 13. Match the enzyme class with the appropriate example. Some categories may be used more than once or not at all. (6 pts) elastase a. oxidoreductase alcohol dehydrogenase b. transferase hemoglobin c. hydrolase peptidyl transferase d. lyase HIV protease e. isomerase triose phosphate isomerase f. ligase g. not an enzyme

6 14. Renin belongs to the aspartyl protease family, like HIV protease. It catalyzes the hydrolysis of a peptide bond in the protein angiotensinogen, converting it to angiotensin I, a precursor of a hormone involved in regulating high blood pressure. Two Asp residues make up the catalytic dyad shown below: Asp 32 Asp 215 H a. Using your knowledge of the HIV protease mechanism, draw a mechanism for the hydrolysis of a peptide bond by Renin. (6 pts) b. What type of catalytic mechanism(s) is/are used by aspartyl proteases? (2 pts) c. What is the major mechanistic difference between aspartyl proteases and serine proteases? (3 pts)