A-LevelBiologyEnzymes9700/22 · Oct/Nov 2017 · Q312 Marks

Hydrolytic enzymes can function within the cell or can be secreted by the cell, where they are able to catalyse reactions.

(a)State the term used to describe an enzyme that functions within the cell.[1]
(b)(i)The rates of reaction of two different hydrolytic enzymes, enzyme G and enzyme H, were measured at different substrate concentrations. The results are shown in Fig. 3.1. The two enzymes have different values of the Michaelis–Menten constant (Km). [Figure 3.1] The Km value of enzyme G is 5mmol dm⁻³. Use Fig. 3.1 to derive the Km value for enzyme H. Show your working.[2]
(b)(ii)With reference to Fig. 3.1, explain how the values of Km for these enzymes provide information about the relationship between the enzyme and their substrates.[2]
(c)Name the vesicles in the cell that contain hydrolytic enzymes.[1]
(d)The cell has internal chemical messengers that signal when an old or damaged mitochondrion should be broken down. Suggest one feature involving mitochondrial structure or function that could lead to the release of these internal signals.[1]
(e)Glucocerebrosidase is an enzyme synthesised by macrophages to hydrolyse glucocerebroside, a glycolipid found in cell membranes. The GBA gene codes for glucocerebrosidase. There are a number of different mutations of the GBA gene. Some mutations result in a single amino acid change in the enzyme molecule produced, and this causes a disorder known as Gaucher's disease. Two of these mutations, at different positions in the GBA gene, are shown in Table 3.1. [Table 3.1] With reference to Table 3.1, • outline how a mutation leads to a single amino acid change in the enzyme glucocerebrosidase and • suggest why the two different amino acid changes, Asn to Ser and Leu to Pro, will result in differences in the tertiary structure of the enzyme.[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] intracellular (enzyme) ; R interacellular
[2] 8.5 mmol dm⁻³ ;; A 8–8.7 max 1 if no units
allow one mark if only half Vmax stated half Vmax = 0.5 (au)
[2] two from
(Km is the) affinity, of enzyme for its substrate ;
G / low Km enzyme, has a, high(er) affinity for its substrate (than H) ; ora
A binds more easily note that if the term 'affinity' is used, then this is also mp1
G / low Km enzyme, needs a lower concentration of substrate to reach, Vmax / maximum activity / ½ Vmax (than, H / enzyme with high Km) ; ora
G / low Km enzyme more likely to be saturated with substrate ;
(so) variations in substrate have less effect on rate of reaction (for G) ;
[1] lysosomes ; treat as neutral Golgi vesicles R lysozyme R if any other organelle named
[1] any one relevant e.g.
leakage (of substances) through / damage to, (mitochondrial) membranes A ref. to fewer cristae
or impaired uptake of substances through transport proteins / AW
or no / impaired, ATP production / aerobic respiration / oxidative phosphorylation
or no / low, protein / enzyme, synthesis (from mitochondrial ribosomes)
or change to, number / distribution / presence, of membrane proteins
or no mitochondrial replication occurring ;
[5] five from
1 change in nucleotide / base, sequence (of, DNA / gene / GBA) ; must be in context of DNA, ignore if in context of RNA
2 (because of) base substitution ; A substitution of a base
3 altered / AW, mRNA codon ; A mRNA triplet R genetic code I triplet code
4 idea that a, codon / triplet, specifies a particular amino acid ; in context of DNA or RNA
5 (different) tRNA with different amino acid (brought to ribosome) / tRNA brings Ser instead of Asn / tRNA brings Pro instead of Leu ; R tRNA makes a different amino acid
6 altered, primary structure or altered, sequence / order / arrangement, of amino acids ; R if describing result of frameshift, deletions or insertions e.g. all amino acids changed from mutation on / missing amino acid / added amino acids
7 affects (folding into) / different, secondary structure ;
different tertiary structure
8 ref. to different interactions between, R groups / side chains (because of changed primary structure) ; A idea of different bonds forming (if R-groups not stated) I peptide bonds change
9 idea that differences give different shapes of active site if shape not stated, allow point if linked to idea of 'tertiary structure changes shape' or idea of change to complementarity to substrate
10 mutation 1 / asparagine (Asn) to serine (Ser), change less effect on, active site shape / catalysis or mutation 2 / leucine (Leu) to proline (Pro), change greater effect on, active site shape / catalysis ;