Enzymes (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Enzymes & metabolism

• Enzymes are biological catalysts made from protein

  • Enzymes speed up chemical reactions in cells

  • They allow reactions to occur at much faster speeds than they would without enzymes at relatively low temperatures (such as human body temperature)

• Substrates temporarily bind to the active site of an enzyme

  • This leads to a chemical reaction and the formation of a product(s) which are released

• Enzymes remain unchanged at the end of a reaction, and they work very quickly

  • Some enzymes can process 100s or 1000s of substrates per second

Enzyme specificity diagram

How do enzymes work?

• Enzymes catalyse specific chemical reactions in living organisms

  • Usually one enzyme catalyses one particular reaction:

• The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)

• Enzymes have specific three-dimensional shapes because they are formed from protein molecules

  • Proteins are formed from chains of amino acids held together by bonds

  • The order of amino acids determines the shape of an enzyme

  • If the order is altered, the resulting three-dimensional shape changes

The lock & key model

• The ‘lock and key theory’ is a simplified model used to explain enzyme action

  • The enzyme is the lock

  • The substrate(s) is the key that can fit into the active site of the enzyme

  • The two are a perfect fit

1. Enzymes and substrates move about randomly in solution

2. When an enzyme and its complementary substrate randomly collide – with the substrate fitting into the active site of the enzyme – an enzyme-substrate complex forms, and the reaction occurs

3. A product (or products) forms and is released from the active site.

4. The enzyme remains unchanged and will go on to catalyse further reactions

The effect of temperature and pH on enzyme activity

The effect of temperature

• The specific shape of an enzyme is determined by the amino acids that make the enzyme

• The three-dimensional shape of an enzyme is especially important around the active site area

  • This ensures that the enzyme’s substrate will fit into the active site enabling the reaction to proceed

• Enzymes work fastest at an ‘optimum temperature’

  • In the human body, the optimum temperature is around 37°C

• Heating beyond the optimum temperatures will start to break the bonds that hold the enzyme together

  • The enzyme will start to distort and lose its shape

  • This reduces substrate binding and enzyme activity

• Eventually, the active site shape is lost completely

  • The enzyme is ‘denatured’

• Substrates cannot fit into denatured enzymes as the specific shape of the active site has been lost

Enzyme denaturation diagram

• Increasing temperature from 0°C to the optimum increases enzyme activity

  • Molecules have more energy, move faster and the number of collisions with the substrate molecules increases

  • This leads to a faster rate of reaction

• Low temperatures do not denature enzymes

  • At lower temperatures, there is less kinetic energy

  • This means that enzymes and their substrates collide at a lower rate

The effect of temperature on enzyme activity diagram

The effect of pH

• The optimum pH for most enzymes is 7

  • Some enzymes that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2)

  • Some enzymes that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)

• If the pH is too high or too low, the bonds that hold the amino acid chain together can be destroyed

• This will change the shape of the active site, so the substrate can no longer fit into it

  • This reduces the enzyme activity

• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

The effect of pH on enzyme activity diagram