Enzymes & Metabolism (AQA GCSE Biology: Combined Science)

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Enzymes & metabolism

  • Digestive enzymes work outside of cells; they digest large, insoluble food molecules into smaller, soluble molecules which can be absorbed into the bloodstream
  • Metabolism is the sum of all the reactions happening in a cell or organism, in which molecules are synthesised (made) or broken down
  • Enzymes are biological catalysts made from protein
    • Enzymes speed up chemical reactions in cells, allowing 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, which 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

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Enzymes are biological catalysts that work in cells, so they randomly move about wherever they are in the cell. They don’t ‘choose’ to collide with a substrate – collisions occur because all molecules are in motion in a liquid

How do enzymes work?

  • Enzymes catalyse specific chemical reactions in living organisms – usually one enzyme catalyses one particular reaction:

Enzyme specificity of catalase to hydrogen peroxide diagram

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The enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

  • 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 one simplified model that is used to explain enzyme action
  • The enzyme is like a lock, with the substrate(s) the keys that can fit into the active site of the enzyme with the two being a perfect fit

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Diagram showing the lock and key model

  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 from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

Enzymes: Temperature & pH

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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 their ‘optimum temperature’ – in the human body, the optimum temperature is around 37°C
  • Heating to high temperatures (beyond the optimum) will start to break the bonds that hold the enzyme together – the enzyme will start to distort and lose its shape – this reduces the effectiveness of substrate binding to the active site reducing the activity of the enzyme
  • Eventually, the shape of the active site is lost completely and the enzyme is described as being ‘denatured’
    • Substrates cannot fit into denatured enzymes as the specific shape of their active site has been lost

Enzyme denaturation diagram

Enzymes denature at high temperatures

Denaturation is largely irreversible – once enzymes are denatured they cannot regain their proper shape and activity will stop

  • Increasing temperature from 0°C to the optimum increases the activity of enzymes as the more energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
  • This means that low temperatures do not denature enzymes, but at lower temperatures with less kinetic energy both enzymes and their substrates collide at a lower rate

The effect of temperature on enzyme activity diagram

 graph-showing-the-effect-of-temperature-on-rate-of-enzyme-activity-igcse-and-gcse-biology-revision-notes

This graph shows the effect of temperature on the rate of activity of an enzyme

The effect of pH

  • The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some 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 to make up the protein can be destroyed
  • This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
  • Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

effect-of-ph-on-enzyme-activity-igcse-and-gcse-biology-revision-notes

If pH is increased or decreased away from the optimum, then the shape of the enzyme is altered

The effect of pH on enzyme activity diagram

graph-showing-effect-of-ph-on-rate-of-activity-for-an-enzyme-from-duodenum-igcse-and-gcse-biology-revision-notes

This graph shows the effect of pH on the rate of activity of an enzyme from the duodenum

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.