Biochemistry I Exam 3 Fall 2002 Name ________________
1. ( 15 points) Enzymes greatly enhance the rates of reactions by lowering the activation energy of the reaction.a. If an enzyme lowered the activation energy by 60 kJ, what would the increase in the rate of the reaction be? The uncatalyzed reaction rate is 1.5 µmole/sec. Consider the concentration of the reactants to be constant. The temperature is 25C.
b. How would the Keq for a reaction change if an enzyme inhibitor was added?
2. (10 points) In the popluar TV show, "ER", they are always wanting to know the pH and blood gases (eg: CO2) for the victim. Several episodes have had pregnant women as cases. What effect does each of the following have on the oxygen affinity of hemoglobin F (fetal hemoglobin) in vitro? Please fully describe your reasoning.
a. Decrease in pH from pH 7.2 to 6.8.
b. Increase in CO2 from 10 to 40 torrs.
c. Decrease in the concentration of BPG (bisphosphoglycerate).
3. (10 points) Many enzymes are inhibited irreversibly by heavy metal ions. Mercury, silver and copper ions can react with sulfhydryls to form mercaptides. Silver nitrate reacts with the cysteines in a protein and this can be used to determine the minimum molecular weight of the protein. A 5 mL solution of enzyme that contained 3 mg of protein per mL was titrated with a 1 x 10-4 M solution of silver nitrate. It took 6.36 mL of this silver nitrate solution (AgNO3) to inactivate the enzyme. The temperature is 25C.
a. Calculate the minimum molecular weight of the protein.
b. Why is this the minimum molecular weight? What would make this value higher?
4. (20 points) A new drug inhibits COX-2 but not COX-1. COX-1 produces prostaglandins that increase the secretion of mucin that protects the stomach. A drug that inhibits COX-1 will increase stomach irritation due to a decrease in these prostaglandins. COX-2 produces prostaglandins that are involved in pain and inflammation. How would you determine if the new drug is a competitive, non competitive, mixed inhibitor. Describe the experiments and the results (using graphs) that you would expect for each type of inhibition. The temperature is 25C.5. (15 points) The pKa of an acid depends upon its environment. Explain the following changes in pKa values. (There is more than one possible explanation for each, just give one clearly explained reason. X and Y are unknown proteins.) The temperature is 25C.
a. The pKa of histidine in protein X is 8.5.
b. The pKa of one lysine in acetoacetate decarboxylase is 6.8.
c. The pKa of aspartate in protein Y is 6.6. Chymotrypsin (Mr 21,600) degrades peptides by cleaving the amide bond on the carboxyl side of aromatic amino acids. Under saturation conditions, the substrate glycyltyrosinylglycine, is cleaved at a rate of 6.2 moles/min, kcat is 100 sec-1, and Km is 108 mM. The rate of the uncatalyzed reaction is 120 nmoles/min. The temperature is 25C.
a. What is the enzyme concentration?
b. What is the difference in activation energy for the catalyzed vs uncatalyzed reactions?
7. At what substrate concentration will an enzyme with a kcat of 50 sec-1 and a Km of 3 x 10-3 show 1/4 of its maximum rate?
8. Studying disease states helps us to understand the crucial components of a biochemical system. The following diseases were presented in class: Hereditary Fetal Hemoglobinemia, Thallasemia and Methemoglobinemia. Choose one of these diseases and write a brief summary of what you learned from each presentation and how this information is related to understanding how oxygen normally is transported in the blood. Be specific and use examples.
9. Studying disease states helps us to understand the crucial components of a biochemical system. The following diseases were presented in class: Systemic Lupus Erythmatosis (SLE) and Multiple Schlerosis. Choose one of these diseases and write a brief summary of what you learned from each presentation and how this information is related to understanding how the immune system normally functions. Be specific and use examples.