In every living organism, chemical reactions take place within every cell, called metabolic reactions. These reactions are mostly controlled by biological catalysts called enzymes. Enzymes can both break down molecules, such as salivary amylase which breaks down starch; and make large molecules from smaller ones, such as starch phosphorylase, which builds up starch in plants. Enzymes tend to be used a lot in digestion, which is the breakdown of large molecules into smaller molecules the body can absorb. Examples of these enzymes are protease, which breaks down proteins into amino acids, and carbohydrase which breaks down carbohydrates into simple sugars. Enzymes can also be found inside cells, such as catalase, which breaks down harmful hydrogen peroxide into water and oxygen.
In the course of a reaction involving an enzyme, one substance is changed into another. The substance present at the beginning is called the substrate, and the substance produced at the end is called the product. Enzymes themselves are all proteins, each with an exact shape. They also include a gap in the side of them, which is shaped so that the substrate molecule will fit into it. This gap or dent is called the active site. During the reaction, the substrate molecule fits into the active site, and the enzyme pulls at it and makes it split apart into the products. When the reaction has finished, the products leave the active site and the enzyme is ready to perform the reaction again.
Each enzyme has a specific shape of active site, and will only accept one type of molecule, so the enzymes are specialised depending on which reaction they have to perform. They can also only create products of a certain type for the same reason.
There are also other properties that all enzymes have, such as the conditions they work best in. Most enzymes work best at a temperature of about 37ºC. Usually, chemical reactions get faster the hotter the conditions are, because molecules move around faster and hit each other with more energy. However, because the enzymes are proteins, they are damaged by high temperatures, and once the temperature reaches about 40ºC the enzymes become denatured, and are no longer useful as catalysts. This is one of the reasons why the human body is kept at a temperature of 37ºC. As well as having an optimum temperature, enzymes also have an optimum pH at which they operate. Because they are proteins, they are damaged by very acidic or alkaline conditions. Therefore they work best at a pH of 7, or neutral.
Although enzymes facilitate and speed up reactions, they are not actually used up when the reaction takes place. This is because they are catalysts. Therefore the body does not need to replace them unless they become denatured by a high temperature or pH.
Inside the body is not the only place where enzymes can be found. They are also used for other purposes, such as in washing powders, and also in the food industry. If clothes are stained with substances from animals or plants, these substances are often made up of particles such as proteins or fats. The easiest way to remove these is by adding protease and lipase to the washing powder, which break down the proteins or fats causing the stain. If washing powders contain enzymes, they are called biological washing powders. Originally, they only worked at warm temperatures, because at anything above about 40ºC they became denatured. However, this problem can be avoided by using enzymes from bacteria that naturally live in hot areas such as hot springs.
In the food industry, enzymes are used for a variety of purposes. They can be used to help make fruit juice by breaking down the pectin in the fruit, a substance that helps stick plant cells together. If this is broken down, the fruit is easier to juice because it is softer. When enzymes are used for a function such as this, they are immobilised to stop them from getting in to the fruit juice itself. Since they are catalysts, and so can be used over and over again, to lose them in the fruit juice would be a waste. Therefore they are attached to a substance called alginate, which is like agar jelly. When the fruit juice is filtered off, the alginate is left behind, and the enzymes are attached to it.
The enzyme amylase is also used in the food industry to create sugar. Normally, sugar is obtained from sugar cane or beet. However it is possible to obtain it from starch by using amylase, which breaks up the starch into sugar. Plants containing starch, such as potatoes, are crushed with water, and then amylase is added. This then digests the starch to form maltose. Sugars can also be converted to different kinds of sugar by enzymes. For instance fructose, a sugar that is sweeter than glucose, can be obtained by adding isomerase to glucose.
Recently it has become possible to use enzymes as biosensors. A certain enzyme is used to catalyse glucose into hydrogen peroxide and an acid. This is useful when testing blood sugar levels. The change in oxygen level caused by the enzyme is detected by the biosensor, and produces an electrical output depending on the amount of oxygen detected. This can be measured, and displayed on a screen. This kind of biosensor is very useful for diabetics, who need to monitor their blood sugar levels. It makes use of the property the enzymes have of being specific. Since the enzyme only catalyses one type of molecule, in this case glucose, only changes in the glucose level will produce a response in the readout. Also, because enzymes are not used up, the biosensor never needs to be replaced.
Gritchka has also drawn my attention to another use for enzymes, this time in genetic research. In order for research to be carried out as to what certain genes do in a cell (a key step in research into HIV and AIDS, as one example), "molecular scissors" are used. More properly known as ribozymes, or catalytic RNAs, these scissors cut the RNA messages used to convey information from certain genes, so rendering the genes effectively useless.
I say enzymes when talking about ribozymes, but there seems to be some dissent as to whether ribozymes really count as enzymes, since they are not actually proteins (see above, where I say "Enzymes themselves are all proteins"(!) I suppose the problem occurs because when the term was defined, all such biological catalysts were proteins (ribozymes were discovered in 1987). It all really comes down to definition. If you define an enzyme as a biological catalyst, then ribozymes are enzymes. If you include being a protein in the requirements, then they are not. Either way, ribozymes exhibit enzyme-like properties, which is why they are included here.