A catalyst is a substance that speeds up the rate of reaction but remains unchanged at the end of the reaction. This means that the catalyst can be reused in future reactions.

Enzymes are made of protein.

An enzyme is a biological catalyst – this means they speed up a reaction without being used up themselves. Once a reaction has been completed, they can be used to catalyse the same type of reaction again.

Properties of enzymes - speed up chemical reactions, not changed by reaction.

Enzymes are involved in many reactions in living things and your body. For example, they:

•They can build large molecules from smaller one e.g., Protein synthesis.

•Break down large molecules into smaller ones e.g., digestion – breakdown of food molecules.

Enzymes are proteins and so, are made up of long chains of amino acids. These are folded together to form a specific shape. This specific shape is particularly important as this is where other molecules bind to the enzyme. This is known as the active site. The substance that binds to the enzyme is called the substrate. When it binds to the active site, the substrate fits into the enzyme to form an enzyme-substrate complex. Then the reaction happens quickly, and the product/s are released from the enzyme. The enzyme is then ready to catalyse another reaction.

How do enzymes work?

The molecules involved in a chemical reaction in a cell are – Substrate- The substance that an enzyme acts on. Enzyme- The catalyst that speeds up the reaction. Product- The substance(s) produced.

Do enzymes bind with all molecules?

Enzymes are highly specific. This means they can only bind to one type of substrate molecule.

One enzyme = one substrate.

The substrate must fit exactly into the active site. If it does not the molecule cannot bind, and the reaction cannot occur.

The enzyme is like a lock, and the substrate is like a key. Only one key will fit the lock and be able to unlock the door. This is known as the “lock and key hypothesis.”

Practical 1: Catalase activity in different tissues

Hydrogen peroxide is a harmful chemical found in all cells. An enzyme called catalase speeds up the breakdown of hydrogen peroxide into oxygen and water. Hydrogen Peroxide arrow (above the arrow catalyse) Oxygen + Water

Practical 1: Method

1.Place 5ml of hydrogen peroxide into each boiling tube.

2.Add 2 drops of detergent to each boiling tube.

3.Add equal sized pieces of food (source of enzyme) into each boiling tube.

4.After 2 minutes carefully measure the maximum height of froth in the tube.

5.Test for oxygen with glowing splint.

6.Record your results in a table.

The specificity of enzymes for their substrates

An enzyme is specific to its substrate. This means that each enzyme can only act on one substrate. E.g., the enzyme amylase will break down starch but not protein. This is because the shape of the active site is complementary (matches exactly) its specific substrate(s).

Practical 2: Specificity of enzymes

Catalase and pepsin are types of enzymes. Egg white suspension is cloudy because it has large protein molecules in it. When protein is broken down the suspension turns clear.

Practical 2: Method

1. Use separate droppers for each substance.

Tube A - 2 cm3 hydrogen peroxide solution and 1 cm3 of catalase and 2 drops of detergent.

Tube B - 2 cm3 hydrogen peroxide solution and 1 cm3 of pepsin and 2 drops of 1 mol hydrochloric acid

Tube C - 2 cm3 egg white suspension and 1 cm3 of catalase and 2 drops of detergent.

Tube D - 2 cm3 egg white suspension and 1 cm3 of pepsin and 2 drops of 1 mol hydrochloric acid.

2. Place your tubes in the water bath at 37 °C and leave for

   10 minutes

3. Examine the contents of tubes A and B for the presence of

   bubbles. Check the contents of tubes C and D to see if

   They are cloudy or clear.

Enzyme activity is affected by temperature and ph. Changes in temperature and pH can cause changes in the shape of an enzyme’s active site.

Enzyme activity, A change in the shape of the active site will affect the activity of the enzyme and therefore the rate of the reaction. If the shape of the active site changes too much the enzyme will be denatured - it will be inactive.

Enzyme activity is affected by the PH scale. Enzymes are active over a narrow range of pH values. Each enzyme will work best at a specific ph. This is known as the optimum. Enzymes work around ph7 anything above and below optimum change the enzyme shape meaning it has denatured.

Substrate concentration increases, rate of reaction increases. Until all the enzyme molecules are bound to substrate molecules. No further increase in rate of reaction.

Enzyme concentration increases, rate of reaction increases.

Limited by substrate concentration.

If no new substrate molecules are added the reaction will stop.

Practical 3: Enzymes and temperature

Method

1. Collect 3 test tubes.

2. Add 2cm3 egg white to test tube A.

3. Add 2cm3 hydrogen peroxide to test tube B.

4. Add 1ml Liver solution to test tube C.

5. 5. Place all three tubes at 0ºC, 20ºC, 40ºC, 60ºC or 80oC

   for 2 mins.

What factors affect enzyme-controlled reactions?

1.Temperature

2. pH

3. Concentration of enzymes and substrates affect enzyme-controlled reactions.