Enzymes as biological catalysts, the lock-and-key and induced-fit models, the effects of temperature, pH, substrate and enzyme concentration on rate, and the action of inhibitors.

What this dot point is asking

Edexcel wants you to explain that enzymes are biological catalysts, describe the lock-and-key and induced-fit models, explain the effects of temperature, pH, substrate and enzyme concentration on rate, and describe competitive and non-competitive inhibition. You should be able to interpret rate graphs and apply ideas about activation energy.

How enzymes work

Activation energy, $E_a$, is the minimum energy that colliding reactant molecules must have for a reaction to proceed. By providing an alternative route with a lower $E_a$, an enzyme lets a far higher proportion of molecules react at body temperature, increasing rate by factors of $10^6$ or more.

The lock-and-key model says the active site is a rigid shape exactly complementary to the substrate. The induced-fit model refines this: the active site is only approximately complementary at first, then changes shape as the substrate binds, putting strain on the substrate bonds and helping the reaction reach its transition state. The induced-fit model better explains why enzymes can act on a small range of similar substrates and why binding itself can catalyse the change.

The active site shape is determined by the enzyme's tertiary structure, which is held by hydrogen bonds, ionic bonds and disulfide bridges between R groups. Anything that disrupts these bonds changes the active site shape.

Factors affecting rate

• Temperature. Raising temperature increases kinetic energy, so enzyme and substrate collide more often and with more energy, raising the frequency of successful collisions and the rate. The temperature coefficient $Q_{10}$ is about $2$ over the physiological range, meaning rate roughly doubles for each $10$ degrees Celsius rise. Above the optimum, the extra vibration breaks the bonds holding the tertiary structure, the enzyme denatures, the active site changes shape and rate falls sharply.
• pH. Each enzyme has an optimum pH (about $7$ for amylase, about $2$ for pepsin). A change in $\text{H}^+$ concentration alters the charges on R groups, disrupts ionic and hydrogen bonds, and changes the active site shape, lowering rate. Extreme pH denatures the enzyme.
• Substrate concentration. Rate rises with substrate until all active sites are occupied as fast as they can turn over; rate then plateaus because enzyme concentration is now limiting.
• Enzyme concentration. With excess substrate, rate is directly proportional to enzyme concentration, because more active sites are available.

Inhibitors

• Competitive inhibitors have a shape similar to the substrate and bind reversibly to the active site, blocking it. They raise the substrate concentration needed to reach the maximum rate but do not lower the maximum rate, because adding enough substrate outcompetes them.
• Non-competitive inhibitors bind to an allosteric site away from the active site, changing the tertiary structure and so the active site shape. The substrate can no longer bind effectively, and adding more substrate does not reverse the effect, so the maximum rate is lowered.

End-product inhibition is a common non-competitive mechanism: the final product of a metabolic pathway binds back to an early enzyme, switching the pathway off when enough product is present (negative feedback).

Examples in context

Example 1. Catalase and the rate experiment. Catalase in liver tissue catalyses $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$. In a core practical, oxygen volume is collected over time. Plotting volume against time gives a curve that is steepest at the start (most substrate, most frequent collisions) and flattens as peroxide is consumed. Boiling the liver first abolishes activity because catalase is denatured, confirming the protein nature of the enzyme.

Example 2. Statins as competitive inhibitors. Statin drugs lower blood cholesterol by competitively inhibiting HMG-CoA reductase, the enzyme catalysing the rate-limiting step of cholesterol synthesis. Because the statin resembles the natural substrate, it occupies the active site and slows cholesterol production. This links enzyme inhibition directly to the Topic 1 work on cardiovascular disease and dietary cholesterol management.

Try this

Q1. Explain how an increase in temperature above the optimum reduces enzyme activity. [3 marks]

• Cue. Bonds holding the tertiary structure break, the active site changes shape, the substrate no longer fits, and the enzyme is denatured.

Q2. Explain why increasing substrate concentration can reduce the effect of a competitive inhibitor but not a non-competitive inhibitor. [3 marks]

• Cue. Competitive inhibitor binds the active site, so more substrate outcompetes it and rate recovers; non-competitive inhibitor binds elsewhere and changes active site shape, so extra substrate cannot displace it.

Q3. A reaction releases $24\ \text{cm}^3$ of gas in the first $40$ seconds. Calculate the mean rate in $\text{cm}^3\,\text{s}^{-1}$. [1 mark]

• Cue. $24 / 40 = 0.60\ \text{cm}^3\,\text{s}^{-1}$.