|MadSci Network: Biochemistry|
Dear Hannah: I understand how you feel. When I teach this lesson, my students often have trouble with this topic. However, it can be overcome. Enzymes are organic (i.e. carbon-containing) molecules, that are large and complex. They are usually proteins (i.e. they are made up of long chains of smaller molecules called amino acids, linked together by so-called peptide bonds, to form either large bundles or sheets), and catalyse (speed up the rate of) certain reactions, without getting affected by the reaction themselves. I.e. they help shove the reaction along but don't participate in it themselves. Enzymes are very specific, and a certain enzyme can only cataylse a certain type of reaction, and would be useless with another set of reagents. In this case, you are using pectinase. Pectin is another organic substance that is present in the cell walls of the apple, and along with cellulose, help give it rigidity and structure. When you make jams, perserves, and certain kinds of jelly (jell-o) by boiling fruit and cooling the concentrated juice, the gelatinous texture is actualy due to the pectin. What pectinase does is to speed up the break-down this pectin into simpler substances. Now, there are many factors affecting the rate of reaction of an enzyme. To ease your pain quickly, I'll start with temperature, but if you wish you could read on about the rest: 1. Temperature As you know, you can speed up most chemical reactions by increasing the amount of energy available to the reactants, especially by heating them. This is because as the temperature increases, the motion of the atoms and molecules increases too, and they collide with each other more frequently and with more force, hence increasing the rate of reaction. The same with enzymes. At low temperatures, they do not function well, and are 'sluggish'. However, if you gradually increase the temperature, the rate of reaction increases too, by about twice for every ten degrees celsius (centigrade). However, there reaches a maximum point where the rate of reaction which the enyzme catalyses is at its maximum. This is known as the optimal temperature. Beyond this point, the rate of reaction falls sharply. Why? Because the enzyme is getting denatured. Denaturation is when the enzyme loses its structure and starts to fall apart because the temperature is too high. An example of denaturation is the cooking of egg white. The proteins present in the egg white are denatured when heated and cooked, and hence undergo many changes (in texture, colour, and even taste). Hence, if you draw a graph of the rate of reaction (or enzyme activity) versus the temperature, it will gradually rise and reach a peak before falling off quite rapidly. The enzymes in our body work best at body temperature, and hence if you cool the body too much, or heat it too much, there will be unpleasant consequences when the enzymes vital to our body's function do not work as well. 2. pH Some enzymes require high pH (alkaline conditions), while some require low pH to function (acidic conditions). For example, the enzymes in your stomach require the presence of hydrochloric acid (HCl) to be converted into the right form to actually be able to do its work. (e.g. pepsinogen is converted into pepsin by HCl. Pepsinogen doesn't do anything, while pepsin breaks down the proteins in the food you eat) Like temperature, enzymes have an optimum pH at which they function best. At pH levels above and below this, they do not work as well. Hence, the graph of rate of reaction against pH will also be like a camel's hump, and the tip of the hump is pH where the rate of reaction is the highest. Too high or too low a pH will also cause a denaturation of the enzyme. An example would be how milk curdles when mixed with acidic substances (like sour juices or vinegar), and the proteins in the milk denature and change their properties and appearance. 3. Concentration Concentration that affects the rate of reaction is both the concentration of the enzyme and the substrate (the substance the enzyme works on). By increasing the concentration of either of them in the reaction, you increase the chances that (a) molecule(s) of substrate will bump into a molecule of enzyme and the reaction will take place. Hence, by increasing concentration, the rate of reaction increases too. However, it does not shoot off to the stratosphere. There are limits too. There reaches a point after which no increases in concentration will do any good. This is the 'saturation point'. This is when all the molecules of enzyme are occupied with their business (or the other way round), and there is simply too much molecules or substrate or enzyme and adding more won't make any difference to the reaction. Hence, the graph of rate of reaction against concentration of substrate(s) or enzyme will be like a slope going up before hitting a plateau, and just flattening out. 4. Co-enzymes Co-enzymes are substances that are not enzymes themselves, but work together with enzymes to cataylse a reaction. Examples of these include certain vitamins like B-complex vitamins. Not all enzymes require co- enzymes, though. I hope that this has been enough information for you to digest. Good luck with your work! Feel free to ask the Mad Scientists if you need any more help! Regards, Tan "Amylase" Thiam Hock
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