EduNinja

[Maximum number: 5]

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.

Fig. 4.1

(a)

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.

[ 2 ]

(i)

Explain what is meant by a higher $\mathrm{K}_{\mathrm{m}}$ value.

[ 2 ]

any two from (higher $K_{m}$ enzyme)

has a lower affinity for its substrate ;
A binds substrate less easily needs a higher concentration of substrate to reach, $\mathrm{V}_{\text {max }} /$ maximum activity $/ \frac{1}{2} \mathrm{~V}_{\text {max }}$ ;
less likely to be saturated with substrate ;
variations in substrate have greater effect on rate of reaction ;

(b)

Saccharomyces boulardii is a strain of S. cerevisiae. It has been researched for its possible health benefits for some gut diseases.

Researchers investigating trehalase extracted from S. boulardii concluded that only one type of trehalase was present in the extract.

Fig. 4.3 shows the effect of pH on the activity of the trehalase extracted from S. boulardii.

Fig. 4.3

With reference to Fig. 4.3 and to the two different types of trehalase enzyme produced by S. cerevisiae, state and explain what can be deduced about the type of trehalase present in S. boulardii.

[ 3 ]

[Maximum number: 4]

Fig. 2.1 is a transmission electron micrograph of a cell from a leaf.

Fig. 2.1

(a)

Some plant cells produce a polypeptide called systemin.

[ 4 ]

(i)

Systemin stimulates plant cells to produce enzyme inhibitors known as serpins.

One of these serpins is a competitive inhibitor of some protease enzymes. It inhibits the protease enzymes found in herbivores, but does not inhibit the proteases in plants.

Suggest how this serpin inhibits only the protease enzymes in herbivores but not those in plants.

[ 3 ]

(ii)

The presence of competitive inhibitors, such as serpins, increases the Michaelis-Menten constant ( $\mathrm{K}_{\mathrm{m}}$ ) for the enzymes they inhibit.

Explain why the $\mathrm{K}_{\mathrm{m}}$ value increases.

[ 1 ]

Affinity for substrate has decreased.
The enzyme needs a higher concentration of substrate to saturate active sites.

Accept needs a higher concentration to reach $\frac{1}{2}V_{\max}$ / $V_{\max}$ .
Accept idea that inhibitor prevents substrate from binding to the active site, so more substrate is required to compete.

CAIE A-Level Biology AS 3.2 Factors Affecting Enzyme Action Topic Practice

Question 1

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Question (b)

Question 2

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Question (ii)