CAIE A-Level Biology AS 3.1 Mode of Action Topic Practice

Saccharomyces cerevisiae is a unicellular fungus that is important in the brewing and baking industries.

Fig. 4.1 is a diagram of a transmission electron micrograph of S. cerevisiae.

The hydrolysis of trehalose is catalysed by two different enzymes produced by S.cerevisiae, regulatory trehalase and non-regulatory trehalase.

A study was carried out to compare regulatory trehalase and non-regulatory trehalase extracted from S. cerevisiae.

The results of the study showed that:
- regulatory trehalase had a higher $\mathrm{K}_{\mathrm{m}}$ value (Michaelis-Menten constant) than non-regulatory trehalase
- the optimum pH of regulatory trehalase was pH 7.0-7.8
- the optimum pH of non-regulatory trehalase was pH 4.5-5.0.

Explain whether both types of trehalase, regulatory and non-regulatory, can be described as intracellular enzymes.

Prostaglandins are small lipids produced in many tissues of the body. One role of prostaglandins is to cause inflammation at the site of an injury or infection. Inflammation is the normal first response of the immune system to injury or infection.

Cyclooxygenase (COX) is an enzyme that catalyses one of the steps in the reaction pathway for the formation of prostaglandins from phospholipids. The reaction pathway occurs in the smooth endoplasmic reticulum (SER) of cells. Part of the reaction pathway is shown in Fig. 1.3.

Sometimes inflammation can have side-effects, such as pain. Aspirin is a drug that can be used to reduce these side-effects.

Aspirin reduces the catalytic activity of the COX enzyme by modifying the R-group of one of the amino acids.

Suggest how modifying the R-group of an amino acid in the COX enzyme can reduce the catalytic activity of the enzyme.

In most plants, sucrose is the main sugar that is transported from sources to sinks.

Fig. 5.1 shows part of one pathway that is used in plant cells to synthesise sucrose. The enzyme sucrose synthase catalyses the transfer of glucose from UDPG (uridine diphosphate glucose) to fructose.

The structure of UDPG is shown in Fig. 5.2.

Sucrose synthase acts by using an induced fit mechanism rather than a lock and key mechanism.

With reference to sucrose synthase and the synthesis of sucrose, outline the difference between the induced fit mechanism and lock and key mechanism of enzyme action.