State and explain how the rate of an enzyme-catalysed reaction is related to the substrate concentration.

When an inhibitor is added it decreases the rate of an enzyme-catalysed reaction. State the effect that competitive and non-competitive inhibitors have on the value of \({V_{\max }}\). Explain this in terms of where the inhibitor binds to the enzyme.

Competitive inhibitor:

Non-competitive inhibitor:

(i)     Sketch a graph to show the effect that a change in pH will have on the rate of an enzyme-catalysed reaction.

(ii)     Explain why changing the pH affects the catalytic ability of enzymes.

at low substrate concentrations/at first rate is (directly) proportional to (substrate) concentration / OWTTE;

Do not accept only qualitative statement such as “rate increases as concentration increases”.

at high substrate concentrations/eventually rate reaches maximum/levels off/becomes constant / OWTTE;

active sites become blocked/saturated / OWTTE;

Competitive inhibitor:

\({V_{\max }}\) same;

inhibitor occupies active site;

Non-competitive inhibitor:

\({V_{\max }}\) lower;

inhibitor binds elsewhere causing distortion in shape of active site / OWTTE;

In each part, explanation mark cannot be awarded without correct reference to V_max.

(i)     sketch graph with rate and pH labels and bell-shaped curve (showing rate has maximum);

(ii)     (at higher or lower pH value of) charges on enzyme/amino acid (residues) changes;

so (shape of) active site changes / tertiary structure lost / OWTTE;

Although most had some idea of what sort of answer part (a) required, it was rare to find full marks being awarded – the most common reasons were a qualitative answer for the first mark, and the absence of a reference to active sites for the third mark.

In (b), the distinction between competitive and non-competitive inhibitors was well known, although a surprising number of answers contained explanations without stating the effect on \({V_{\max }}\).

Most sketch graphs in (c) were sufficiently well drawn to score the mark, although many would have benefited from a scale that indicated a narrow pH range; the explanation was generally well known.