Additional Homework Problems for Exam 3
1. A metalloenzyme has been found to contain 0.25% zinc by weight.
- a. What is the minimum molecular weight of the enzyme.
- b. It has been found that the total molecular weight of this enzyme is 52,296 g/mole. How many zinc atoms are present in the enzyme?
2. On average, human hair grows 18 cm per year. The fundamental structure of hair is keratin, a rope like protein that consists primarily of alpha helices. The alpha helix contains 3.6 amino acids and has a rise of 0.56 nm per turn. In other words each .56 nm of hair contains 3.6 amino acids. The rate determining step for hair growth is the formation of peptide bonds in the keratin chains. Calculate the rate of peptide bond formation in the keratin chains that would correspond to the observed yearly hair growth.
3. Carbonic anhydrase catalyzes the hydration of carbon dioxide to form the bicarbonate ion. It has a molecular weight of 30,000 g/mole.
- a. If 15 ug of pure carbonic anhydrase catalyzes the hydration of 0.42 grams of carbon dioxide at 37oC in one minute, what is the turnover number of carbonic anhydrase (in units of min-1).
- b. From your answer in a, calculate the activation energy of the enzyme catalyzed reaction (in kJ/mole).
4. Catalase protects the body from oxidation by catalyzing the breakdown of H202, a potentially potent oxidant, to form carbon dioxide and water.
- a. If 10 ml solution of catalase destroys H202 at a rate of 1.24 moles/min at 37oC, and the kcat for catalase is 2.4 x 109(min-1), what is the enzyme concentration?
- b. The Km value for catalase is 1.1M, what is concentration of substrate (H202) where the velocity is 1/3 of Vmax?
- c. What is the activation energy for the reaction catalyzed by catalase?
5. An oligomeric protein has a molecular weight of 64,500. It is a heme protein, with 0.34% iron. (The atomic weight of iron is 55.47.) When 390 mg of the protein is reacted with 1-fluoro-2,4-dinitrobenzene and the peptide bonds completely hydrolyzed with 6N HCl at 100 C overnight, 2.86 mg of the 2,4 dinitro derivative of threonine was produced and 3.26 mg of the 2,4 dinitro derivative of methionine was also produced.
- a. How many iron atoms are present in each oligomeric protein molecule?
- b. How many subunits are present in the oligomeric protein?
6. Benzo(a)pyrenes are present as the incomplete combustion products of fossil fuel buring. They are converted to reactive epoxide molecules by the enzyme, p450. These reactive molecules react with guanine nucleotides to cause DNA damage. An enzyme is needed to repair this damage. Consider the following data:
1. The enzyme hydrolyzes the 5' to 3' phophodiester bond of DNA that contains a (5') Guanine that has been modified with a large hydrophobic group such as the benzo(a)pyrene metabolites
2. The optimal pH is 5
3. Using x-ray crystallography, the following amino acids were found to surround a nonhydrolyzable substrate: R, Y, G, L, D, E, H
- a. Suggest a possible nonhydrolyzable structure for use in structure determinations.
- b. Draw the amino acids around the substrate to show how the protein stablilizes the transition state of the substrate.
- c. Suggest a possible mechanism for this enzyme.
Note: this is a practice enzyme, it does not exist except in this cyberspace.
Please send questions and comments to: Koni Stone (koni@chem.csustan.edu)
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