IGCSE NOTES : Biology - Biological molecules

A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. An enzyme is a protein that functions as a biological catalyst.

Enzymes are proteins that act as catalysts. They are made in all living cells. Enzymes, like catalysts, can be used over and over again because they are not used up during the reaction and only a small amount is needed to speed the reaction upEnzyme action How an enzyme molecule might work to join two other molecules together and so form a more complicated substance (the product).

An example of an enzyme-controlled reaction such as this is the joining up of two glucose molecules to form a molecule of maltose. You can see that the enzyme and substrate molecules have complementary shapes (like adjacent pieces of a jigsaw) so they fit together. Other substrate molecules would not fit into this enzyme as they would have the ‘wrong’ shape. For example, the substrate moleculeFigu would not fit the enzyme molecule. The product (substance AB is released by the enzyme molecule and the enzyme is then free to repeat the reaction with more substrate molecules. Molecules of the two substances might have combined without the enzyme being present, but they would have done so very slowly (it could take hours or days to happen without the enzyme). By bringing the substances close together, the enzyme molecule makes the reaction take place much more rapidly. The process can be extremely fast: it has been found that catalase, a very common enzyme found in most cells, can break down 40 000 molecules of hydrogen peroxide every second! A complete chemical reaction takes only a few seconds when the right enzyme is present.

As well as enzymes being responsible for joining two substrate molecules together, such as two glucose molecules to form maltose, they can also create long chains. For example, hundreds of glucose molecules can be joined together, end to end, to form a long molecule of starch to be stored in the plastid of a plant cell. The glucose molecules can also be built up into a molecule of cellulose to be added to the cell wall. Protein molecules are built up by enzymes, which join together tens or hundreds of amino acid molecules. These proteins are added to the cell membrane, to the cytoplasm or to the nucleus of the cell. They may also become the proteins that act as enzymes.

Enzymes and temperature

A rise in temperature increases the rate of most chemical reactions; a fall in temperature slows them down. However, above 50 °C the enzymes, being proteins, are denatured and stop working.  Above 50 °C the shapes of enzymes are permanently changed and the enzymes can no longer combine with the substances.

This is one of the reasons why organisms may be killed by prolonged exposure to high temperatures. The enzymes in their cells are denatured and the chemical reactions proceed too slowly to maintain life. One way to test whether a substance is an enzyme is to heat it to boiling point. If it can still carry out its reactions after this, it cannot be an enzyme. This technique is used as a ‘control’ in enzyme experiments.

Enzymes and pH

Acid or alkaline conditions alter the chemical properties of proteins, including enzymes. Most enzymes work best at a particular level of acidity or alkalinity (pH).

The protein-digesting enzyme in your stomach, for example, works well at an acidity of pH 2. At this pH, the enzyme amylase, from your saliva, cannot work at all. Inside the cells, most enzymes will work best in neutral conditions (pH 7). The pH or temperature at which an enzyme works best is often called its optimum pH or temperature. Conditions in the duodenum are slightly alkaline: the optimum pH for pancreatic lipase is pH 8.

Although changes in pH affect the activity of enzymes, these effects are usually reversible, i.e. an enzyme that is inactivated by a low pH will resume its normal activity when its optimum pH is restored.

Rates of enzyme reactions

As explained above, the rate of an enzyme-controlled reaction depends on the temperature and pH. It also depends on the concentrations of the enzyme and its substrate. The more enzyme molecules produced by a cell, the faster the reaction will proceed, provided there are enough substrate molecules available. Similarly, an increase in the substrate concentration will speed up the reaction if there are enough enzyme molecules to cope with the additional substrate.

Intra- and extracellular enzymes

All enzymes are made inside cells. Most of them remain inside the cell to speed up reactions in the cytoplasm and nucleus. These are called intracellular enzymes (‘intra’ means ‘inside’). In a few cases, the enzymes made in the cells are let out of the cell to do their work outside. These are extracellular enzymes (‘extra’ means ‘outside’). Fungi and bacteria release extracellular enzymes in order to digest their food. A mould growing on a piece of bread releases starch-digesting enzymes into the bread and absorbs the soluble sugars that the enzyme produces from the bread. In the digestive systems of animals, extracellular enzymes are released into the stomach and intestines in order to digest the food.

An enzyme-controlled reaction involves three groups of molecules, although the product may be two or more different molecules:

substrate enzyme product

The substance on which an enzyme acts is called its substrate and the molecules produced are called the products. Thus, the enzyme sucrase acts on the substrate sucrose to produce the monosaccharide products glucose and fructose. Reactions in which large molecules are built up from smaller molecules are called anabolic reactions. When the enzyme combines with the substrate, an enzyme-substrate complex is formed temporarily.

Again, when the enzyme combines with the substrate, an enzyme-substrate complex is formed temporarily. Try chewing a piece of bread, but keep it in your mouth without swallowing it. Eventually you should detect the food tasting sweeter, as maltose sugar is formed. If starch is mixed with water it will break down very slowly to sugar, taking several years. In your saliva there is an enzyme called amylase that can break down starch to sugar in minutes or seconds. In cells, many of the ‘breaking-down’ enzymes are helping to break down glucose to carbon dioxide and water in order to produce energy.

Reactions that split large molecules into smaller ones are called catabolic reactions. Enzymes are specific This means simply that an enzyme which normally acts on one substance will not act on a different one. The enzyme in has a shape called the active site, which exactly fits the substances on which it acts, but will not fit the substance. So, the shape of the active site of the enzyme molecule and the substrate molecule are complementary. Thus, an enzyme which breaks down starch to maltose will not also break down proteins to amino acids. Also, if a reaction takes place in stages, e.g. starch maltose (stage 1) maltose glucose (stage 2)
a different enzyme is needed for each stage.

The names of enzymes usually end with -ase and they are named according to the substance on which they act, or the reaction which they speed up. For example, an enzyme that acts on proteins may be called a protease; one that removes hydrogen from a substance is a dehydrogenase.

Enzymes and temperature

Generally, a rise of 10 °C will double the rate of an enzyme-controlled reaction in a cell, up to an optimum temperature of around 37 °C (body temperature). This is because the enzyme and substrate molecules are constantly moving, using kinetic energy. The reaction only occurs when the enzyme and substrate molecules come into contact with each other. As the temperature is increased, the molecules gain more kinetic energy, so they move faster and there is a greater chance of collisions happening. Therefore the rate of reaction increases.

Above the optimum temperature the reaction will slow down. This is because enzyme molecules are proteins. Protein molecules start to lose their shape at higher temperatures, so the active site becomes deformed. Substrate molecules cannot fi t together with the enzyme, stopping the reaction. Not all the enzyme molecules are affected straight away, so the reaction does not suddenly stop – it is a gradual process as the temperature increases above 37 °C.

Denaturation is a permanent change in the shape of the enzyme molecule. Once it has happened the enzyme will not work any more, even if the temperature is reduced below 37 °C. An example of a protein denaturing is the cooking of egg-white (made of the protein albumin). Raw egg-white is liquid, transparent and colourless. As it is heated, it turns solid and becomes opaque and white. It cannot be changed back to its original state or appearance.

Enzymes and pH

Extremes of pH may denature some enzymes irreversibly. This is because the active site of the enzyme molecule can become deformed (as it does when exposed to high temperatures). As a result, the enzyme and substrate molecules no longer have complementary shapes and so will not fi t together.