14. A researcher has developed an assay to measure the activity of an important

Question

14. A researcher has developed an assay to measure the activity of an important liver enzyme present in liver cells, which are being cultured in a lab. She adds the substrate for the enzymatic reaction to the dish of cells, and then measures the appearance of the reaction products. The results are graphed as the amount of product on the y-axis versus time on the x-axis. The researcher noted four sections of the graph. For a short period of time, no products appeared (section A). In section B, the reaction rate was quite rapid (the slope of the line was steep). After some time, the reaction slowed down considerably (section C), although products continued to appear (the line was not flat). Still later, the reaction resumed its original rapid rate (section D). Draw the graph, and propose a model to explain the molecular events underlying this interesting reaction profile.

Explanation / Answer

So, the graph drawing is trivial, and so I've attached a version of what it might look like. To figure out whats going on and why will take some more thought. In the first section, of nothing happening, there must be something occuring to stop activity, or their is a lag time. Think of where the reaction is occuring. In a cell, and the substrate is just being poured onto the cell. Transport takes time. You have a lag during which the substrate must penetrate the cell and get to the enzyme, section A. In section B, an excess of substrate has been added to an enzyme system and so the reaction takes off, this is not surprising, it is expected and a nice result. Section C, the slowing down this is the interesting bit. Now, remember this is almost an in vivo, rather than an in vitro experiment. The cells might be in dish, but they are alive and functional, so a lot more than just one reaction is going on. You've forced the cell to make a lot of product, and then the afterwards the product is used, or it isnt, allowing build up. Now, all processes in cells are balanced dynamic equilibria. If you upset that balance (by pouring substrate on the cells and making lots of needed product) it'll do as you want at first but then begin to rebalance. In this case, I would say you have a negative feedback loop. This means that if you have enough of the product, it will actually begin to inhibit the enzymes reactivity. In this case whatever is beign made is necessary, and is made fast and then might be used for something where it remains its the form being tested for, however, when enough is there it begins to slow down production. However, if you slow down production, and the stuff is used all over the place, eventually, when its been slowed down enough, but more is still needed, the loop may begin again, spurrring the creation of more at a faster rate to make up for the use. The amount being emasured may be in the whole cell, rather than locally around the enzyme, so as enough is drawn form the enzyme it will pick up again, creating a never ending cycle. Since you have no information abotu what happens next, this is certainly possible. My apologies for the mildly rambling answer. I hope it makes sense, let me know if it doesn't and ill give it another go. Chris

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