Enzymes as catalysts lowering activation energy through formation of enzyme-substrate complexes. The induced-fit model of enzyme action. The effects of temperature, pH, enzyme and substrate concentration, and competitive and non-competitive inhibitors on the rate of enzyme-controlled reactions.

What this dot point is asking

AQA wants you to explain enzymes as biological catalysts using the induced-fit model, show how they lower activation energy by forming enzyme-substrate complexes, and describe how temperature, pH, enzyme concentration, substrate concentration, and competitive and non-competitive inhibitors change the rate of reaction.

The answer

Lowering activation energy

For a reaction to occur, reacting molecules must have a minimum amount of energy - the activation energy ($E_a$). Enzymes lower this energy barrier, so reactions proceed rapidly at the low temperatures found inside cells. They do this by holding substrates in the right orientation and putting strain on bonds within the substrate, so the bonds break more easily.

The induced-fit model

The active site is a specific region of the enzyme formed by its tertiary structure.

1. The substrate is complementary in shape to the active site, but is not an exact fit to begin with.
2. When the substrate binds, the active site changes shape to mould closely around the substrate.
3. This forms an enzyme-substrate (ES) complex and places strain on the substrate's bonds, lowering activation energy.
4. Products form, leave the active site, and the enzyme returns to its original shape, ready to bind again.

This is why enzymes are specific: the tertiary structure (and so the active site) is complementary to only one substrate (or a small group of similar substrates).

Factors affecting the rate of reaction

Temperature

Raising temperature increases the kinetic energy of molecules, so substrate and enzyme collide more often with enough energy - rate rises up to the optimum.

Above the optimum, increased vibration breaks the hydrogen and ionic bonds holding the tertiary structure together. The active site changes shape and is no longer complementary to the substrate: the enzyme is denatured and the rate falls sharply.

A useful idea is the temperature coefficient $Q_{10}$, the factor by which rate increases for a 10 °C rise: $Q_{10} = \dfrac{\text{rate at }(T+10)}{\text{rate at }T}$. For many enzymes $Q_{10} \approx 2$ up to the optimum.

pH

Each enzyme has an optimum pH. A change in pH alters the concentration of $\text{H}^+$ ions, which interferes with the hydrogen and ionic bonds maintaining the tertiary structure. Beyond a narrow range the active site changes shape, fewer ES complexes form, and (at extremes) the enzyme denatures.

Enzyme concentration

If substrate is in excess, increasing enzyme concentration gives more active sites, so more ES complexes form per unit time and rate rises proportionally. If substrate becomes limiting, adding more enzyme no longer increases the rate.

Substrate concentration

Increasing substrate concentration increases the rate because more ES complexes form. Eventually all active sites are occupied (saturated); the rate plateaus at $V_{max}$ and adding more substrate has no effect - rate is then limited by enzyme concentration.

Inhibitors

Competitive inhibitors

• Have a shape similar to the substrate.
• Bind to (compete for) the active site, blocking substrate from binding.
• Effect is reduced by increasing substrate concentration - more substrate outcompetes the inhibitor, so the same $V_{max}$ is eventually reached (it just takes a higher substrate concentration).

Non-competitive inhibitors

• Bind to the enzyme at a site other than the active site (an allosteric site).
• This changes the tertiary structure, so the active site changes shape and substrate can no longer bind.
• Increasing substrate concentration does not overcome the inhibition, so $V_{max}$ is lowered.

Worked example

Why this matters across 3.1

Enzymes are proteins (their specificity comes from tertiary structure - see proteins). They act on substrates such as sugars (see carbohydrates) and lipids (see lipids), and DNA replication relies on enzymes such as DNA polymerase (see nucleic acids). The hydrogen and ionic bonds that hold active sites in shape depend on the aqueous environment (see water and inorganic ions).

Try this

Q1. Explain why an enzyme is specific to one substrate. [3 marks]

• Cue. Active site has a specific shape determined by the tertiary structure; only a complementary substrate can bind / induce the fit and form an ES complex; a substrate of a different shape cannot bind.

Q2. A reaction is run at a constant enzyme concentration. Sketch and explain the shape of a graph of rate against substrate concentration. [4 marks]

• Cue. Rate rises (steeply at first) as more ES complexes form; then levels off; plateau because all active sites are occupied/saturated; rate now limited by enzyme concentration ($V_{max}$).

Q3. Distinguish between the effects of competitive and non-competitive inhibitors, referring to substrate concentration and $V_{max}$. [4 marks]

• Cue. Competitive binds active site (similar shape to substrate), effect reduced by raising substrate, same $V_{max}$; non-competitive binds allosteric site, changes active site shape, not overcome by raising substrate, lower $V_{max}$.