LSM1401 Semester 2 2007‐08
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Tutorial 1
(10, 11, and 12 March 2008)
Questions
1. Refer to the following structure for aspirin (acetylsalicylic acid).
(a) Draw the structure for the predominant form of aspirin as it would exist in (i) blood plasma,
(ii) in gastric juice (pH ≈ 1), and (iii) in the small intestine (pH ≈ 6).
(b) To be absorbed into the bloodstream, it must pass through the membrane lining the stomach and the small intestine. Electrically neutral molecules can pass through a membrane more easily than charged molecules. Would you expect more aspirin to be absorbed in the stomach or in the small intestine? Explain your answer.
2. An experiment measuring the initial velocity of an enzyme as a function of substrate concentration was conducted, first in the absence of inhibitors, and then in the presence of inhibitor Y. The following data were obtained.
[Substrate] Velocity (µmol min‐1) (mM) No Inhibitor Inhibitor Y
0.010 22.3 4.5
0.020 29.8 5.9
0.050 37.0 7.3
0.075 39.5 7.9
0.100 40.6 8.1
(a) Use the Lineweaver‐Burk plot to determine the values of Km and Vmax of the enzyme in the absence of inhibitors. Express your Km and Vmax values in µM and µmol min‐1, respectively.
(b) On the same graph determine the apparent Km and Vmax values (in µM and µmol min‐1) of the enzyme in the presence of inhibitor Y.
(c) What type of reversible inhibitor is Y? Explain your answer.
LSM1401 Semester 2 2007‐08
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3. The pKa values of the α‐carboxyl, side chain, and α‐amino groups of aspartic acid are 1.88, 3.65, and 9.60, respectively.
(a) Draw the predominant form of aspartic acid at pH 7.
(b) What percentage of the side chain group is deprotonated at pH 4?
(c) A researcher was able to modify the aspartic acid by replacing the hydrogen that is attached to the α‐carbon with another amino group.
(i) Draw the structure of the new amino acid when its net charge is zero.
(ii) Calculate the isoelectric point for this new amino acid. Assume that the pKa for the new amino group is 10.5.
4. A novel enzyme, tyrosine aminomutase, which catalyses the conversion of L‐α‐tyrosine to β‐ tyrosine, was isolated from Streptomyces globisporus. To investigate the kinetics of the novel enzyme, the initial rate of reaction was measured with variable initial concentrations of L‐α‐ tyrosine, in the absence and presence of inhibitor H. The following data was obtained:
[L‐α‐Tyrosine] Initial Rate (µM/sec)
(mM) Without Inhibitor H With Inhibitor H
0.0125 0.0295 –
0.0250 0.0465 –
0.0500 0.0670 0.0145
0.0750 – 0.0210
0.1000 0.0835 0.0260
0.2000 – 0.0435
0.2500 0.1045 –
1.0000 0.1105 0.0790
(a) Which Enzyme Commission class should this enzyme be classified under? Provide both the class number and name.
(b) Using the Lineweaver‐Burk double‐reciprocal plot, determine the Km (in µM) and Vmax (in µM/sec) values of the novel tyrosine aminomutase.
(c) On the same graph, determine the apparent Km and Vmax values (in µM and µM/sec, respectively) of the enzyme in the presence of inhibitor H.
(d) What type of inhibitor is H?
(e) Predict and briefly describe the structure of inhibitor H. No drawings required.
5. You are examining three types of cells under the electron microscope. You know that one is a bacterium, one is a plant cell, and the third is an animal cell. From their structures, how can you tell which is which?
LSM1401 Semester 2 2007‐08
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6. The pKa values of the α‐carboxyl, α‐amino, and R groups for lysine are 2.16, 9.06, and 10.54, respectively.
(a) Examine the following structure of lysine. Can this structure exist at any pH in aqueous solution? Explain.
(b) Calculate the isoelectric point of lysine.
7. Many types of cells in the human body secrete products such as enzymes.
(a) What method of transportation might such cells use to secrete the large amounts of enzymes they produce?
(b) Why do such cells use the method to transport the enzymes?
8. A beaker of pure distilled water sitting out on your lab bench is slightly acidic. Explain.
9. Antibiotics are medicines that target bacterial infections. How can an antibiotic kill all the bacterial cells and not harm our cells? What part of the bacterial cell must antibiotics be targeting and why?
10. When a protein is unfolded (denatured), it becomes less water soluble and often precipitates from solution. Explain.
