(d) (i) the effects of pH, temperature, enzyme concentration and substrate…

The idea

Enzyme activity is measured as the rate of product formation or substrate disappearance. Increasing substrate concentration raises the rate until all active sites are occupied, giving a plateau (Vmax); further substrate has no effect because the enzyme is saturated. Increasing enzyme concentration raises the rate proportionally, provided substrate is not limiting. Temperature increases rate up to an optimum because molecules have more kinetic energy and more successful collisions; above the optimum, bonds holding the tertiary structure break, the active site loses its shape and the enzyme is denatured, so rate falls sharply. pH affects the ionisation of amino acids in the active site; each enzyme has an optimum pH, and extremes of pH alter charges and disrupt the active site, reducing activity. MCQ questions often test which factor is limiting at a given point on a graph.

What earns the marks

• Increasing substrate concentration increases rate until the enzyme is saturated (all active sites occupied), after which rate plateaus.
• Increasing enzyme concentration increases rate, provided substrate concentration is not limiting.
• Raising temperature increases rate up to an optimum due to greater kinetic energy and more frequent successful collisions.
• Above the optimum temperature, denaturation changes the tertiary structure and active-site shape, so rate falls.
• Each enzyme has an optimum pH; changes in pH alter active-site charges and shape, reducing activity away from the optimum.

Common mistakes and how to put them right

• Saying high temperature 'kills' the enzyme.

  Put it right by stating the enzyme is denatured because its tertiary structure and active-site shape are disrupted.

• Claiming rate keeps rising indefinitely with substrate concentration.

  Put it right by describing saturation and the plateau at Vmax.

• Confusing the effect of enzyme concentration with substrate concentration.

  Put it right by noting enzyme concentration raises rate only while substrate is available.

• Assuming all enzymes have the same optimum pH.

  Put it right by stating the optimum pH is specific to each enzyme.

Exam tips

• On a graph, identify which factor is limiting at the point in question before choosing an answer.
• Distinguish 'denatured' (irreversible loss of active-site shape) from 'less active' when temperature or pH moves away from the optimum.
• Check whether an option describes the initial linear rise or the plateau — they test different limiting factors.