Welcome to week 17 of Science Friday! This edition has been posted on Tuesday because I will be out of town for Fourth of July weekend. Last week, we discussed the basics of the biological cell. This week, we will talk about the ever-important role of enzymes in life systems. Before we get into what enzymes are, let’s first discuss the role they fulfill. The fundamental unit of life is the cell; within these small organisms, all of the chemical reactions that sustain life occur: food is metabolized to harness energy; amino acids are linked to form proteins; and DNA is replicated for reproduction. Without these processes, life as we know it would not exist. Here’s the problem. If we were to analyze these chemical reactions in abiotic (nonliving) environments, we would find that the rate at which they occur is far too slow to generate useful products. In other words, the reactions would still occur, but they would occur much more slowly—to the point where they become entirely useless for organisms. Cells need to synthesize proteins and energy-rich molecules rapidly and efficiently. So how is this problem solved? The answer is enzymes. Put briefly, enzymes are special molecules that [i]catalyze[/i] chemical reactions in cells—that is, they speed up the reactions significantly. How exactly is this done? In order for molecules to chemically react to make a product, they must collide with sufficient kinetic energy to break existing bonds and allow new ones to form. This amount of energy is unique to every reaction and is known as the [i]activation energy[/i]. Even chemical reactions that give off energy once the products have formed have positive activation energy. In these cases, you can think of the activation energy as a small hump a ball must surmount before rolling down a steep hill. In order for the ball to roll down the hill, it must have enough kinetic energy to climb the hump. Enzymes function by lowering this activation energy or “hump.” The actual means to lower the activation energy varies per enzyme, but there are a few common methods most enzymes utilize. One method is to strain the existing bonds to make them easier to break. If they are easier to break, then the overall rate of the chemical reaction will increase. One way to imagine this if we need to construct a building where an old building currently stands. To do so, we must first dismantle the existing structure and then construct the new edifice. If, however, the old building’s structural integrity is compromised intentionally to make disassembly easier, the overall speed of disassembly and reconstruction will be faster. Another way enzymes lower the activation energy is by orienting the reactant (input) molecules so that they react. In order for a chemical reaction to occur, molecules must possess the activation energy as aforementioned, but they also must collide in the correct orientation. In an environment where trillions of molecules are constantly bumping into one another, it is guaranteed that only a fraction will collide with the orientation that will facilitate a reaction. Enzymes effectively act as a scaffold; they hold the molecules in place in the correct orientation to ensure reaction. These two different mechanisms of enzyme action are simply the only ones I have mentioned here. There are many others. Memorizing them is not as important as recognizing how all the mechanisms ultimately lower the activation energy of reactions to speed them up. After all this explanation, you may wonder why life systems use enzymes specifically to speed up chemical reactions. After all, why not raise the temperature so that the molecules collide with more energy and avoid the complicated arena of organic catalysts? The problem is that raising the temperature will speed up [i]all[/i] of the reactions within a cell, many of which are extremely dangerous. This method of reaction catalysis is extremely imprecise. With specific enzymes, only reactions that are essential to the cell’s life are catalyzed. And there is another massive benefit of enzymes: the ability to regulate metabolic pathways. As I mentioned in the beginning of this post, most of the chemical reactions that occur within a cell would happen much too slowly in an abiotic (or enzyme-less) environment. The cell exploits this fact to only execute certain reactions at certain times. To understand this better, let’s consider the following example. Let’s say the cell has raw materials A and B, which it can either turn in to product C [i]OR[/i] product D. Products C and D have common raw materials, so the cell must partition A and B according to its physiological needs. We have two chemical reactions then. A + B → C A + B → D Let’s further assume both reactions are catalyzed by different enzymes. Now let’s say there is an abundant amount of product C in the cell’s environment, meaning it need not commit energy to the production of C. In order to ensure raw materials A and B do not make C but continue to make D, the cell can [i]inhibit[/i] the enzyme that catalyzes the first reaction, effectively ensuring it does not occur. Meanwhile, the second reaction proceeds since a different enzyme catalyzes it. In fact, in a process called [i]feedback inhibition[/i] the inhibitor (molecule that stops the normal function of an enzyme by attaching to it) of an enzyme is [i]the product of the chemical pathway that is inhibited[/i]. So, in our example above, the abundant presence of product C would physically inhibit the enzyme that catalyzes the first reaction. There is also positive feedback where the presence of a product actually stimulates enzymatic activity, amongst other regulation mechanisms. Here in lies the beauty of enzymes: they offer [i]reaction specific[/i] catalysts that can be [i]precisely regulated[/i]. Simply raising the temperature is neither reaction specific nor precise. I’ll end this week’s post by briefly mentioning that many of the adverse effects we suffer from exposure to poisons is the direct result of [i]irreversible[/i] inhibition of critical enzymes. For example, cyanide is so deadly because it irreversibly inhibits an enzyme in the electron transport chain of the mitochondrion, which is part of a chemical pathway that transforms the food we eat into energy. I hope you enjoyed this week’s Science Friday. If there is only one thing you take away from this week’s post, it is that enzymes speed up important chemical reactions in cells by lowering activation energy and provide a precise way to regulate cellular activity. The rest of the details—specific mechanisms of action and fancy names of specific enzymes—are important but not fundamental to basic understanding. It is truly incredibly to me how nature shows such ingenious methods to regulate the biological phenomena responsible for our existence. Stay tuned for more science! In the meantime, check out my previous works by clicking the #sciencefriday tag on this thread. Please also comment in this thread. I enjoy engaging in discussion with my readers and answering questions. See you next week!