COMMUNICATION pubs.acs.org/jchemeduc
More Nuts and Bolts of Michaelis Menten Enzyme Kinetics Joseph H. Lechner* Department of Chemistry, Mount Vernon Nazarene University, Mount Vernon, Ohio 43050, United States ABSTRACT: Several additions to a classroom activity are proposed in which an “enzyme” (the student) converts “substrates” (nut bolt assemblies) into “products” (separated nuts and bolts) by unscrewing them. KEYWORDS: Upper-Division Undergraduate, Biochemistry, Analogies/Transfer, Collaborative/Cooperative Learning, HandsOn Learning/Manipulatives, Inquiry-Based/Discovery Learning, Catalysis, Enzymes, Kinetics, Student-Centered Learning atthew Junker1 described a classroom activity that illustrates several aspects of Michaelis Menten enzyme kinetics. In his analogy, an “enzyme” (the student) picks up a “substrate” (nut bolt pair) and converts it to “products” (separated nut and bolt) via a “reaction” (unscrewing them). The enzyme can be hindered in this process by a “competitive inhibitor” (cemented nut bolt pair that cannot be unscrewed) or by a “noncompetitive inhibitor” (another student who restrains the first individual’s hands). I suggest several additions that extend the usefulness of Junker’s analogy. (i) Include some unpaired nuts or bolts among the objects to be encountered by the enzyme. When the student picks one of these up, he or she discards it after realizing that it is not a substrate; however, some time is wasted by this interaction. Unpaired nuts or bolts simulate product inhibition. Product inhibition could also be demonstrated by providing nut bolt pairs in a container and instructing the student to return separated nuts and bolts to the same container after the reaction. Thus, as the reaction proceeds, the enzyme is increasingly likely to pick up a product rather than a reactant. (ii) Instruct the student to pick up substrates without directly looking into the container. This ensures that his or her behavior is random; that is, the enzyme cannot consciously avoid inhibitors or products. (iii) After a student has done this exercise using his or her hands in the preferred way, require the same student to repeat the exercise using hands in the opposite way, demonstrating the importance of chirality.
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the products into a separate container. This trial was regarded as the control against which all other runs were compared. Trial 2: Each enzyme picked up substrates from the jar and unthreaded them, but returned the products to the original reactant container, simulating product inhibition. Trial 3: To each jar, the instructor added three nut bolt pairs that were identical to the substrates except that the nuts had been cemented in place with cyanoacrylate glue. Each enzyme picked up nut bolt pairs from the jar and unthreaded them if possible or discarded them. This simulated competitive inhibition as suggested by Junker.1 As in trial 1, students were permitted to drop products and rejected inhibitors into a separate container so that these were not encountered again during this run. Cemented bolts had a small identifying mark on the head to facilitate their removal after this trial. Trial 4: This was identical to trial 1, except that a third team member sat behind each enzyme and restrained that individual’s hands, simulating noncompetitive inhibition as suggested by Junker.1 Trial 5: In previous trials, the third member of each team had been asked to observe which hands the enzyme used when performing his or her task. As it happened, all participants held the bolt with their left-hand and turned the nut with their righthand. Enzymes were now instructed to reverse hands; that is, to hold the bolt with their right-hand and turn the nut with their left-hand. This demonstrates the stereochemical selectivity of enzymes.
’ RESULTS On their first attempt, students could unthread 3 4 nut bolt pairs during a 30 s interval, averaging 9.2 s per bolt (Table 1). With practice, their performance improved to 5 6 nuts bolts per 30 s interval, or an average of 5.7 s per bolt (Table 1, trial 1). This underlines the importance of allowing practice runs to familiarize students with the activity before taking quantitative measurements. Rate of unthreading became somewhat slower when students were instructed to return products to the substrate container (Table 1, trial 2). The unthreading rate was significantly slower when students were required to perform the task using their opposite hands (Table 1, trial 5). In agreement with Junker,1 the rate of unthreading was significantly reduced by the
’ EXPERIMENT Junker’s classroom activity and my proposed additions were tested by four teams of three students. In each team, one person acted as the enzyme; one served as scorekeeper; the third person acted as an inhibitor when required. This activity was presented as a contest where teams competed with each other to “catalyze” the most reactions during the allotted time. Each team was given 12, fully threaded nut bolt pairs (1 in. long 1/4 in. diameter hex bolts with matching hex nuts) in a plastic jar (250 mL capacity). The instructor announced the beginning and end of each 30 s time interval. A preliminary practice run was performed but not scored. After each run, students rethreaded nuts all the way onto the bolts and returned them to the jar. Trial 1: Each enzyme (student) picked up substrates (nut bolt pairs) from the jar, unthreaded them, and dropped Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Published: April 15, 2011 845
dx.doi.org/10.1021/ed100761g | J. Chem. Educ. 2011, 88, 845–846
Journal of Chemical Education
COMMUNICATION
Table 1. Classroom Results for Nut Bolt Enzyme Simulation Trial 2 Trial 3 Trial 4 Trial 5 Trial 1 (product (competitive (noncompetitive (opposite (control) inhibition) inhibition) inhibition) chirality)
Teama
Practice Run
1 2 3 4
4 3 3 3
5 5 5 6
5 3 4 4
3 4 3 3
4 4 4 5
4 3 2 4
3.25
5.25
4
3.25
4.25
3.25
Mean a
Data represent the number of nut bolt pairs completely unthreaded by one student during a 30-s interval.
presence of permanently glued nut bolt pairs (competitive inhibitors), and somewhat reduced when another person restrained the enzyme’s hands (noncompetitive inhibition) (Table 1, trials 3 and 4).
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The author thanks Luke D. Vogan for assistance. ’ REFERENCES (1) Junker, M. J. Chem. Educ. 2010, 87 (3), 294–295.
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dx.doi.org/10.1021/ed100761g |J. Chem. Educ. 2011, 88, 845–846
