Enzymes A Workshop for Secondary School Students C. Larry Bering Clarion University, Clarion, PA 16214
Recently a series of weekend science workshops was developed a t Clarion University for students in the Upward Bound Program (grades $12). Aworkshop on enzymes developed by the author is described below. The workshop included several projects that involve students directly, and parts of it can be incorporated into a traditional chemistry, biology, or physical science course at the secondary level. The Lesson Plan
The workshop began with the definition of a catalyst, and two students were recruited to perform an exercise that could he used to illustrate how catalysts work. The students eaeh added about 2.5 g of sodium chloride to a large beaker with about 500 mL of water. The rest of the class timed this "reaction" (the dissolution of sodium chloride). One student simply added the salt, while the other student was given a stirring rod, and was told to stir the solution. Although not a catalyst, the stirring rod does speed up the dissolution and can he recovered a t the end, thus making it a useful analogy to catalysts. After a brief discussion of catalytic converters in cars, the discussion moved to enzymes as catalysts in living organisms. One example discussed was the process in which glucose is converted to carbon dioxide and water plus a large amount of free energy. It was noted that a teaspoon of glucose in air eventually would be converted to the same products, but with no catalysts or enzymes present, it probably would take an immeasurably long time. The importance of enzymes for living organisms was noted when we discussed the need for the energy from this process to move quickly out of the way of a speeding car. Enzymes as consumer products was the next topic considered. Many such products are available a t the local grocery or pharmacy, and can be passed around during this discussion. Laundry detergents wntain enzymes, particularly starch-degrading amylases and protein-degrading proteases. Clomx IIcontains several light blue beads of detergent, but one also may see small brown beads that are macmscopic aggregates of enzymes. Meat tenderizers also contain a protease, papain, as the active ingredient. Although students may not be familiar with collagen (the principle target of tenderizers), they would know that meat is protein, and they can then predict how papain might work. SoR contact lenses are cleaned by enzymes. A glance at the label of a lens cleaner indicates that the active agent is subtilisin, also a protease. Another group of pmducts that has been advertised widely includes the enzyme lactase, or Rgalactosidase. Many adults lack this digestive system enzyme and cannot break milk sugar (lactose) into its constituent sugars galactose and glucose. This enzyme deficiency leads to lactose intolerance in which sufferers cannot digest lactose in dairy products. Enzymes also have been used in a variety of industrial applications. A handout was given that lists several consumer goods such as cheese, leather, paper and textiles, vitamins and pharmaceuticals, whose production relies on enzymes.
Finally enzymes have become important in clinical diagnoses. One application is the use of immobilized glucose oxidase and horseradish peroxidase coupled to a dye whose color is directly proportional to glucnse concentration. Pmduas such as Clini-Stlxor Tes-Tape are used by l a b e t ics to monitor urine glucose levels. lb illustrate these pn~ducts, students took several strips and dipped them into any of four solutions-a blank with no glucose, two solutions mimicking mild and severe glucose levels, and one solution of fructose. The fructose solution that failed to react. led to a discussion of specificity. The tremendous rate acceleration afforded bv enzvmes was demonstr;ited using two 500-mL gruduated cylinders The demmposition of hvdroecn and 3 3 hydrogen - peroxide. . peroxide to water and oxygen was discusssed, and it was pointed out that the reaction could be observed noting the escape of oxygen bubbles. In one graduated cylinder, about 300 mL of 3% H202were added, and students were asked to count the oxygen bubbles that appeared over a given time. In this uncatalyzed reaction, no bubbles were ohserved. Next. a small amount of the enzvme catalase that catalyzes ~ 2 0 2decomposition was addedto the other graduated cvlinder, and about 300 mL of 3% H707 - - added. This catalyzed reaction was vigorous, and it was impossible for students tocount theoxveen bubbles. The visual asoerts of this demonstration can beenhanced by addition offbod coloring to the peroxide solution. Catalase is ubiquitous in aerobic organisms, and students can observe catalase activity on freshly cut potato slices. Students were given a potato slice and a vial of 3% Hz02.After adding a few drops of hydrogen peroxide, foaming appeared on the surface of the slice. While students performed this task, several slices of potato were added to a boiliie water bath for about 1min. These heated slices were then passed around, and students repeated the exercise. In the heated slices. no activitv was observed. and it was concluded that catalase, like most enzymes, was destmved hv heat. The final part of the workshop dealt with specificity of enzvmes. Achiral tetrahedral model and its mirror imaee ., weri made, and when asked, students stated that the two were identical. Then a model of an enzyme active site was drawn on the board withthree points ofattachment foronr of the models. Althouxh the first model exactly matches U D point for point, the mirror image does not fit a t all three points. The mirror image relationship between the models was then revealed and a discussion of specfiity of suhstrates ensued. This discussion included the role of D-sugars and L-amino acids, as well as the important differences between enantiomeric drugs such as L-DOPA or dextromethorphan. The components of this workshop fit nicely into a 90-min. time slot and can be split into two shorter sessions. The hands-on and applied nature of the workshop seemed to hold the interest of students. It is hoped that participation of the students might spark a long-term interest in chemistry or hiotechnology. ~
Volume 71 Number 3 March 1994
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