Enzyme Activity: The Ping-Pong Ball Torture Analogy Terry L. Helser
S.U.N.Y. College at Oneonta, NY 13820-4015 One biochemical concept that students often have difficulty visualizing and understanding is the effect of reactioiconditions on the initial rate of an enzymecatalyzed reaction. This seems to be true even when these lectures are supplemented with appropriate graphs and visual alds.' An analogy that I find particularly helpful is what I call -~ -- T h e Pine-Ponr!Hall Torture." It may also prove useful in describing other system showing satiratio; kinetics. I ask the students to imaeine that their professor is imprisoned by a terrorist group for causing stress and test anxietv in students (a readily believable fantasy for many of the&!). The cell is bare and the walls solid a i d perfectly elastic so that any halls thrown into the room continue to bounce around randomly and at their initial velocity. The ~rofessoris an enzyme with the palm of his hand being the Hctive site, where Ping-Pong balls (the substrate) are caught and crushed (the product). Here it is necessary to suspend credibility and envision the professor as Arnold Schwarzenegger or a NFL linebacker who can actually rnmh Pine-Pone balls with his bare hand! Since it does ..-.-------take some time to crush the ball and release it, the professor can crush only about one ball a second or 60 ballslmin with one hand (V,,)
- -
~
-
The Effect of Enzvme Concentration To convey this effect, 1 first draw the axes of a graph on
the board, and label them with V,(initial rate =y axis, and tEnzymel (enzyme concentration = x axis). I then ask the students, "If 60 crushed balls a minute is as fast as I can work with one hand when the terrorists first dump hundreds of balls into the room, (I put a point on the graph corresponding to a rate of 60 balldmin and 1unit of enzyme) how many can I crush if I use both hands?" Eventually someone ventures a guess that you muld crush 120 balldmin, which is then plotted relative to 2 units of enzyme. Adding another professor or stepping on balls with both feet (Ouch!)gives four active sites and double the rate again, which is plotted, and the straight line is then drawn to complete the direct relationship between initial reaction rate and enzyme concentration. The Effect of Substrate Concentration
The plot and label on the x axis are erased and the axis is relabelled "substrate concentration" ([S]). I then ask the students to imagine their professor (enzyme) with one hand out trying to crush Ping-Pong balls when only a dozen are bouncing around the room. I tell them to assume that I can catch one each minute. I then ask, "How would my initial rate increase if the terrorists dump in another dozen halls (two dozen total)?" Yon can eventually draw forth the answer that the rate should about double. ARer starting the plot to indicate this nearly direct relationship for low substrate concentrations, I ask what will happen when I'm crushing 50 ballslmin and the terrorists double
the number of balls in the mom. Since I can only crush 60balldmin at most, they soon realize that the rate can't double but can only approach the maximum, but never actually reach it. To explain this effect, I suggest that even when the terrorists are trying to suffocate me with PingPong balls, once in a while a ball is not going to be in the correct position in my palm for me to crush it, and I11 have to open my hand and try again. Most can now easily grasp (pun intended) that the plot will curve to approach V-, a result called "saturation." The Effects of Inhibltors/Activators
To envision the effect of a competitive inhibitor, the student can simply consider that the terrorists, angered at me crushing their Ping-Pong balls, have added several white, solid rubber balls ("Superballs") to the cell. Since the professor can't tell the difference until he tries to crush one, the solid balls will slow down his effectiveness, at least when Ping-Pong halls and Superballs are in relatively equal proportions. If the terrorists dump in more PingPong balls, however, eventually the few Superballs rarely, if ever, have a chance to get to the professor's palm. Again the rate will approach V, but only at a higher concentration than without the Superballs. If the terrorists want to prevent the professor fmm destroying their Ping-Pong balls, they might tape his thumb or fingers to the back of his hand, preventing him from closing it. Since the tape is removable without permanently damaging his hand, this is analogous to noncompetitive inhibitors. Taping one hand and not the other effectively halves the [enzyme]and the rate, including V,,. The Effect of Temperature
You can extend the "torture" analogy to explain a temperature "optimum" curve as well. When the cell is cold, both the number and force of collisions between the balls and the professor's hand are decreased, so he is not going to crush balls as rapidly as at higher temperatures. Warming his cell improves both effects, until the professor starts to sweat. This makes catching and crushing the balls increasingly more difficult until the professor shrivels and dies of dehydration (he "denatures").At that point it isjust the speed of the balls colliding that cause some to crush. Summary
"The Ping Pong Ball Torture" analogy has proven to be a useful tool in teaching the effects caused by changing the conditions in an enzyme-catalyzed reaction. And if you are talented enough to juggle some Ping-Pong balls while discussing this analogy, the effect should he improved "dramatically." 'Helser, T. L. J. Chem. Educ. 1991,68,286287.
Volume 69 Number 2 February 1992
137
