Quick questions on Enzymes: induced fit, activation energy, and competitive vs non-competitive inhibitors - AQA A-Level Biology

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.

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

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.

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.

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.

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

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

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