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Enzyme Reaction Rates Using ToothpickasePeggy Skinner AEPSkinner@aol.com Background information: \ This exercise was planned originally to work as a pre-lab activity for my advanced biology students and the AP Biology Lab on enzyme catalysis. Working with enzymes and V-max, initial rates of reaction and final rates of reaction were conceptually difficult because of so many abstractions. My students needed something visible to understand how enzymes work. They participate in a dialogue about enzymes, substrates, and the calculation of rates of reaction. Then, the students become the enzyme, toothpickase, which breaks toothpicks in timed intervals. Calculations are completed to determine how many toothpicks are broken per second to determine the rate of reaction of the enzyme toothpickase. The time for the completion of this activity is approximately 30 to 40 minutes. Teacher Preparation: Purchase flat toothpicks (each student or student group needs approximately 250 toothpicks. Toothpicks for each group will be divided into six piles of 40 toothpicks each and a few left over.) Two boxes of toothpicks is enough for six lab groups (or six students if completed individually). I always have the students count out the stacks of 40. Obtain a few "competitive inhibitors" which are the approximate shape of a toothpick but do not break easily (twisties from garbage sacks or dissecting pins). Discussion/Demonstration: The vocabulary and processes of an enzyme reaction are modeled with hands and toothpicks. The enzyme is a student's hands with a particular shape (it actually demonstrates quaternary structure with the two subunits coming together). The active site is the portion of the thumbs and index fingers which form a space within which the toothpick fits. The substrate is the toothpick. A toothpick can be broken into two products. The rate of the reaction can be measured by counting the amount of product produced or by counting the amount of substrate remaining. There are many factors that affect the rate of an enzyme reaction. Break a toothpick. Can a toothpick be broken faster? Can it be broken infinitely faster? Given a pile of toothpicks (substrate) and ideal conditions, it still takes some time for the enzyme to break the toothpick....that is the enzyme's V-max. If the room were filled with toothpicks, the rate would not increase, at least initially. Would it take longer to break the toothpick if it was across the room on the floor (lower substrate concentration)? If it was surrounded by look-alikes (competitive inhibitors)? What would happen to the amount of time to break toothpicks if two people were breaking at the same time? (Enzyme concentration). The stacks of toothpicks will then be broken for selected time intervals: 0 sec, 10 sec., 30 sec., 60 sec., 120 sec. 180 sec. The results recorded in a simple data table noting time and toothpicks metabolized. Of course, the students could also count how many toothpicks remained in their stack of 40. The AP biology lab on enzymes does "count" the substrate remaining with titration. A sample data table is on the next page. Time (in sec) Toothpicks Metabolized 0 10 30 60 120 180 The students will graph the number of toothpicks metabolized over time in seconds to establish the different rates of reactions in the different time intervals. We know that the rates change because the line does not have the same slope. The students also calculate the average rate of reaction at each of the time intervals using the formula for a slope: M2-M1 ---------- T2-T1 An average initial rate of reaction is .7 toothpicks/ sec. If taken to 180 seconds, the rate of reaction for the interval between 120 and 180 seconds is almost always 0 toothpicks/second. An interesting note is that invariably when the students are breaking toothpicks for 60 seconds, some student will anxiously say that they are out of toothpicks. Of course, that is the point because their rate will decrease as it becomes harder to find toothpicks to break. They have a sense of one of the factors that affect an enzyme. Possible extensions: Numerous and dependent upon the time allowed for the exercise. They can search for scattered toothpicks around the room. Students can put their hands in ice water for a minute before trying to break. Students can break in groups. They can have many competitive inhibitors in their stacks. There can be stacks with fewer toothpicks. They can model denatured enzymes by crossing their fingers. ..etc. contributed by Peggy O'Neill Skinner
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