Describe the characteristics of an enzyme such as pepsin, and compare its catalytic behaviour to an inorganic catalyst such as iron.

Enzymes are affected by inhibitors. Lead ions are a non-competitive inhibitor, they have been linked to impaired mental functioning. Ritonavir^® is a drug used to treat HIV and acts as a competitive inhibitor. Compare the action of lead ions and Ritonavir^® on enzymes, and how they affect the initial rate of reaction of the enzyme with its substrate and the values of \({K_{\text{m}}}\) and \({V_{{\text{max}}}}\).

characteristics [2 max]

enzymes are proteins;

enzyme activity depends on tertiary and quaternary structure/the nature of the active site;

lock and key/induced fit hypothesis;

comparison [2 max]

enzymes function within a narrow pH range;

enzymes are denatured by high temps/temp above 40 °C and inorganic catalysts can be used at high temps/are less affected by conditions;

enzymes are very specific and inorganic catalysts often catalyse several reactions/non specific;

initial rates reduced;

lead binds to enzyme away from active site and changes shape of active site so substrate no longer fits / OWTTE;

ritonavir is a similar shape to the substrate and so fits inside active site instead of substrate / OWTTE;

lead: \({K_{\text{m}}}\) unchanged and \({V_{{\text{max}}}}\) lower;

ritonavir: \({K_{\text{m}}}\) higher and \({V_{{\text{max}}}}\) the same;

Accept competitive inhibitor for ritonavir and non-competitive inhibitor for lead.

In (a) the answers of many candidates showed that they had not properly read the question, and the comparative descriptions were not done well, candidates seldom including both the enzyme of the iron catalyst in their answers. Very few scored full marks here; the most common omissions were to mention that enzymes were proteins and that they had a tertiary and quaternary structure. Several referred to the denaturing and specificity of enzymes without comparing these features with iron and so failed to score marks for these correct statements.

In (b) candidates showed a good understanding of competitive and non-competitive inhibition and how the inhibitors attach to the enzyme. However many neglected to say that initial rates were reduced by both, and many candidates were confused about how \({K_{\text{m}}}\) and \({V_{{\text{max}}}}\) were affected by the two types of inhibition.