AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 5: many proteins are enzymes

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Enzymes are globular proteins, which act as catalysts to specific biological reactions. In this sense, enzymes are responsible for the metabolic activity of a living organism - at a cellular and whole level. All enzymes are specific to a reaction, and susceptible to biokinetic change based on temperature and pH.
The active site of an enzyme is the region which binds to the substrate.

Enzymes can catalyse any number of reactions - whether intracellular or extracellular.
  • Catalase catalyses the breakdown of hydrogen peroxide into water and oxygen.
    • This is an extracellular reaction
  • Amylase and trypsin catalyse the breakdown of carbohydrate and protein respectively.
    • These are intracellular reactions

Catabolic enzymes break down bigger molecules by splitting them into smaller ones.
Anabolic build bigger and more complex molecules by binding together two or more smaller ones.

Lock and Key Hypothesis
  • Every enzyme has an active site which binds to a substrate
  • The enzyme’s active site is formed by its tertiary structure and is specific to the substrate
  • It will have no effect on a molecule that comes into contact with its active site but doesn’t relate to the enzyme’s function
  • The idea of active site specificity is called Lock and Key theory, because the active site acts like a lock and the substrate acts like a key.

Induced Fit Hypothesis
  • More widely accepted hypothesis in modern science.
  • The enzyme’s active site is not perfectly matched to the substrate before the two bind to form the enzyme-substrate complex
  • When the complex is formed, the tertiary structure of the enzyme changes slightly
It is this change in shape that enables the reaction to happen at a lower activation energy. ​
Once the enzyme-substrate complex has formed, the reaction takes place, and the complex becomes the enzyme-product complex. The enzyme and product become separate from each other and from the enzyme and released into their surrounding fluid.
EXAM TIP
  • Catabolic enzymes break down bigger molecules by splitting them into smaller ones.
  • Anabolic build bigger and more complex molecules by binding together two or more smaller ones.
Effect of pH on enzyme activity
  • The tertiary structure of an enzyme relies on hydrogen bonding.
  • Each enzyme has a tertiary structure that is optimal for most efficient action
  • changes in pH from the enzyme’s optimal pH change hydrogen bonding in tertiary structure
  • Enzymes are built to work optimally at the pH of their natural environment
    • e.g. trypsin, which works in the stomach, works best at low pH
    • arginase is often in the presence of ammonia which is alkaline, and this enzyme has a higher ‘optimal pH’. ​
Effect of temperature on enzyme activity
  • Small increase in temperature can increase enzyme activity as the kinetic energy of the enzyme and the substrate increase.
  • They are therefore more likely to come into contact with each other.
  • They also collide with greater force
  • Therefore are more likely to form an enzyme-substrate complex when they do meet.
  • As temperature increases, it will approach the enzyme’s optimal temperature: where the enzyme is working at maximum efficiency.
  • Beyond this temperature, the enzyme becomes denatured.
  • Bonds responsible for maintaining the enzyme’s tertiary structure become broken and the active site changes shape
  • Enzyme becomes totally inefficient
Effect of enzyme concentration on enzyme activity
  • Rate of reaction and enzyme concentration are directly proportional,
    • provided substrate molecules are in excess and enzyme availability is the only limiting factor.
  • Where substrate is in limited supply, as it all becomes reacted, less and less becomes available, so the rate of reaction curves off.
Effect of substrate concentration on enzyme activity
  • The rate of reaction and substrate concentration are directly proportional, up to a given point.
  • This point is called Vmax,
  • At Vmax, all the enzyme active sites are said to be ‘saturated’ or occupied.
  • There are no available active sites free to bind with substrates.
  • At Vmax, the amount of enzyme becomes the limiting factor.
Inhibitors have the effect of decreasing the rate of reaction of enzyme-controlled reactions. There are a few different types of inhibitors that you should be aware of.
Competitive
  • Can fit into the active site, but does not react
  • Competes with substrate for access to the active site
  • Effect is concentration dependent
    • Can be minimized by raising the concentration of substrate
  • Effects are reversible
Non-competitive
  • Binds to allosteric site
  • Changes the shape of the molecule, preventing it from working
  • This is permanent and cannot be rectified
  • Effect is independent of substrate concentration
Reversible inhibitors
  • Bind reversibly to the enzyme
  • Have no long-lasting effect since they detach themselves after a while
Irreversible inhibitors
  • Binds irreversibly
  • Even if in high competition with substrate, can  eventually inhibit a large proportion of available enzyme
  • Can have long-lasting effects
    • Until organism generates more enzyme and the inhibitor is outnumbered by enzyme
  • E.g. aspirin



Some poisons act as metabolic inhibitors. For example, potassium cyanide is a non-competitive inhibitor to the enzyme cytochrome oxidase. The effect of this poison is to decrease the body’s ability to respire using oxygen, so it begins to respire anaerobically, causing lactic acid accumulation in the blood.
EXAM TIPS
  • Show the examiner that you know the precise function of an enzyme: to lower the activation energy of a specific reaction so that it is spontaneous.
  • This might mean it can occur at body temperature while it might normally require higher temperatures, etc. ​