How an enzyme molecule might work to join two other molecules together and so form a more complicated substance (the product) is shown in Figure 2.

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 molecule in Figure 2(b) would not fit the enzyme molecule in Figure 2(a). The product (substance AB in Figure 2(a) 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 substance 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 40000 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.

(a) a ‘building-up’ reaction (anabolic)
(b) a ‘breaking-down’ reaction (catabolic)

Figure 2 (a) (b) – Possible explanation of enzyme action

Enzymes and temperature

A rise in temperature increases the rate of most chemical reactions; a fall in temperature slows them down. However, above 50C the enzymes, being proteins, are denatured and stop working.

Figure 2 shows how the shape of an enzyme molecule could be very important if it has to fit the substances on which it acts. Above 50C 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 to slowly to maintain life.

One way to test whether a substance is an enzyme is to heat it to a 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), as shown in Figure 2(c)

Figure 2(c) – The effect of pH on digestive enzymes

The protein-digesting enzymes in your stomach, for example, works well at an acidity pH 2. At the 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 system of animals exrtacellular 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:

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 Figure 2(a). When the enzyme combines with the substrate, an enzyme-substrate complex is formed temporarily.

Figure 2(b) shows an enzyme speeding up a chemical change, but this time it is a reaction in which the molecule of a substance is split into smaller molecules. 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. Figure 2(a) shows how the shape of an enzyme can control what substances it combines with. The enzyme in Figure 2(a) has a shape called the active site, which exactly fits the substances on which it acts, but will not fit the substance in Figure 2(b). 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 I)

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

Figure 2(d) shows the effect of temperature on an enzyme-controlled reaction.

Figure 2(d) – Graph showing the effect of temperature on the rate of an enzyme-controlled reaction

Generally, a rise of 10C will double the rate of an enzyme-controlled reaction in a cell, up to an optimum temperature of around 37C (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 fit together with the same 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 37C. 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 37C. An example of a protein denaturing is the cooking of egg-white (made of 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 fit together.


Chemical reactions are what make you work. And enzymes are what make them work.

Enzymes are catalysts produced by living things

  1. Living things have thousands of different chemical reactions going on inside them all the time. These reactions need to be carefully controlled – to get the right amounts of substances.
  2. You can usually make a reaction happen more quickly by raising the temperature. This would speed up the useful reactions but also the unwanted ones too….. not good. There’s also a limit to how far you can raise the temperature inside a leaving creature before its cells start getting damaged.
  3. So…. living things produce enzymes that act as biological catalysts. Enzymes reduce the need for high temperatures and we only have enzymes to speed up the useful chemical reactions in the body.
  4. Enzymes are all large proteins and all proteins are made up of chains of amino acids. These chains are folded into unique shapes, which enzymes need to do their jobs.

A CATALYST is a substance which INCREASES the speed of a reaction, without being CHANGED or USED UP in the reaction.

Enzymes have special shapes so they can catalyse reactions

  1. Chemical reactions usually involve things either being split apart or joined together.
  2. Every enzyme has an active site with a unique shape that fits onto the substance involved in a reaction.
  3. Enzymes are really picky – they usually only catalyse one specific reaction.
  4. This is because, for the enzyme to work, the substrate has to fit into its active site. If the substrate doesn’t match the enzyme’s active site, then the reaction won’t be catalysed.
  5. This diagram shows the ‘lock and key’ model of enzyme action. This is simpler than how enzymes actually work. In reality, the active site changes shape a little as the substrate binds to it to get a tighter fit. This is called the ‘induced fit’ model of enzyme action.

The substrate that an enzyme acts on is called the substrate

Enzymes need the right temperature and pH

  1. Changing the temperature changes the rate of an enzyme-catalyzed reaction.
  2. Like with any reaction, a higher temperature increases the rate at first. But if it gets too hot, some of the bonds holding the enzyme together break. This changes the shape of the enzyme’s active site, so the substrate won’t fit any more. The enzyme is said to be denatured.
  3. All enzymes have an optimum temperature that they work best at.
  4. The pH also affects the enzymes. If it’s too high or too low, the pH interferes with the bonds holding the enzyme together. This changes the shape of the active site and denatures the enzyme.
  5. All enzymes have an optimum pH that they work best at. It’s often neutral pH 7, but not always – e.g. pepsin is an enzyme used to break down proteins in the stomach. It works best at pH 2, which means it’s well-suited to the acidic conditions there.

You can investigate the effect of pH on enzyme activity

The enzyme amylase catalyses the breakdown of starch to maltose. It’s easy to detect starch using iodine solution – if starch is present, the iodine solution will change from browny-orange to blue-black. This is how you can investigate how pH affects amylase activity:

  1. Put a drop of iodine solution into every well of a spotting tile.
  2. Place a Bunsen burner on a heat-proof mat, and a tripod and gauze over the Bunsen burner. Put a beaker of water on top of the tripod and heat the water until it is 35C (use a thermometer to measure the temperature). Try to keep the temperature of the water constant throughout the experiment.
  3. Use a syringe to add 1 cm3 of amylase solution and 1 cm3 of a buffer solution with a pH of 5 to a boiling tube. Using test tube holders, put the tube into the beaker of water and wait for five minutes.
  4. Next, use a different syringe to add 5 cm3 of a starch solution to the boiling tube.
  5. Immediately mix the contents of the boiling tube and start a stop clock.
  6. Use continuous sampling to record how long it takes for the amylase to break down all the starch. To so this, use a dropping pipette to take a fresh sample from the boiling tube every 30 seconds and put a drop into a well. When the iodine solution remains browny-orange, starch is no longer present.
  7. Repeat the whole experiment with buffer solutions of different pH values to see how pH affects the time taken for the starch to be broken down.
  8. Remember to control any variables each time (e.g. concentration and volume of amylase solution) to make it a fair test.

You could use an electric water bath, instead of a Bunsen and a beaker of water, to control the temperature

You could use a pH meter to accurately measure the pH of your solutions.

Here’s how to calculate the rate of reaction

  1. Its often useful to calculate the rate of reaction after an experiment. Rate is a measure of how much something changes over time.
  2. For the experiment above, you can calculate the rate of reaction using the formula:

E.g. At pH 6, the time taken for amylase to break down all of the starch in a solution was 90 seconds. So the rate of the reaction = 100 ÷ 90 = 11 s-1 (2 s.f.)

  1. If an experiment measures how much something changes over time, you calculate the rate of reaction by dividing the amount that it has changed by the time taken.

The units are in s-1 since rate is given per unit time.


  1. Enzymes are biological catalysts that speed up the rate of chemical reactions without being altered in the reaction. They are made up of proteins.
  2. Enzymes work by lowering the activation energy of a chemical reaction. Activation energy is the amount of energy needed for a reaction to take place.
  3. Enzymes allow biochemical reactions to take place without drastic conditions such as high temperatures because less heat energy is required to start a reaction.
  4. Enzymes can break down or build up biological molecules.
  5. Enzymes are required in small amounts because they remain unchanged in the chemical reactions they catalyse and can be reused.
  6. They are substrate-specific. Substrates are the reactants that an enzyme acts on. Each enzyme can only act on the particular substrate of the reaction they are supposed to catalyse. Foe example, amylase can only digest starch and not cellulose even though they are both polymers of glucose.
  7. Therefore, each enzyme catalyses a different reaction. This is due to its unique 3-dimensional structure.

‘Lock and key’ hypothesis

  1. The ‘lock and key’ hypothesis relates enzyme specificity to the presence of active sites. An active site is the region on an enzyme molecule that the substrate binds to. It is usually a pocket or groove on the surface of the enzyme that is part of the enzyme’s unique 3 dimensional structure.
  2. The shape of the active site conforms to the substrate. Only the correct substrate is able to fit into the active site.
  3. The process begins when the substrate molecule binds to the active site of the enzyme to form an enzyme-substrate complex.
  4. The reaction is then catalysed at the active sites to convert the substrate into product molecules.
  5. The product molecules depart from the active site, leaving the enzyme free to catalyse another reaction.
  6. The diagram below illustrates the ‘lock and key’ hypothesis for a reaction in which an enzyme breaks down a substrate molecule into 2 product molecules:
Process of an enzyme-catalysed reaction

Effects of temperature on the rate of enzyme-catalysed reactions

  1. The effects of temperature on the rate of enzyme-catalysed reactions is shown in the graph below:
Effect of temperature on the rate of reaction
  1. At low temperatures, enzymes are inactive and the rate of reaction is very low. Substrate and enzyme molecules have little kinetic energy, hence the frequency of collision is low. In addition, most substrate molecules do not contain sufficient energy to overcome the activation energy require to start a reaction.
  2. As temperature increases, the rate of enzyme activity increases. Enzyme activity doubles with every 10C rise in temperature. This is because the reactants have higher levels of energy, and the substrate molecules are able to collide with active sites more frequently.
  3. At the optimum temperature, enzyme activity is the highest.
  4. As the temperature increases beyond the optimum temperature, enzyme activity drops sharply. This is because enzymes are made of proteins, which are denatured at high temperatures. The enzyme loses its 3-dimensional structure and active site conformation due to the breaking of the weak bonds that hold the structure together.
  5. At extremely high temperatures, the enzyme is completely denatured and the rate of reaction drops to zero.

Effects of pH on the rate of enzyme-catalysed reactions

  1. The graph showing the effects of pH on the rate of enzyme-catalysed reactions is shown in the graph below:
Effect of pH on the rate of reaction of amylase
  1. Enzyme activity is the highest at the optimum pH of the enzyme.
  2. As the pH increases or decreases from the optimum, enzyme activity sharply decreases. This is because the hydrogen bonds and ionic bonds that hold the 3-dimensional structure are disrupted. The shape of the active site is changed as the enzyme is denatured.
  3. At extreme pH levels, the enzyme is completely denatured and the rate of reaction drops to zero.
  4. The optimum pH for each enzyme differs. For example, pepsin works best under the acidic conditions in the stomach, while intestinal enzymes work best under alkaline conditions.