Question: 1

(a) The enzyme DHPS is involved in the production of folic acid in bacteria.

• The substrate for DHPS is a molecule known as PABA.

• The enzyme DHPS is inhibited by the drug sulfonamide.

Fig. 3.1 shows the structure of PABA and that of sulfonamide.

(i) Diagrams X, Y and Z represent these enzyme molecules and their active sites.

State the letter, X, Y or Z, that most accurately represents the enzyme DHPS.

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(ii) Using the information in Fig. 3.1, explain why sulfonamide acts as a competitive inhibitor of DHPS.

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(b) Fig. 3.2 shows the effect of increasing the concentration of the substrate (PABA) on the rate of reaction.

• Curve A shows the rate of reaction without the presence of the competitive inhibitor sulfonamide.

• Curve B shows the rate of reaction in the presence of the competitive inhibitor sulfonamide.

Explain the effect of increasing the concentration of substrate on the rate of reaction with inhibitor.

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(2) Enzymes are involved in the production of mRNA in eukaryotic cells. One enzyme is inhibited by the toxin, α-amanitin.

Fig. 4.2 shows the effect when α-amanitin attaches to this enzyme.

(a) Explain how α-amanitin stops the formation of an enzyme-substrate complex during RNA production.

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