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Proteins · Paper 2 — Free Biology HL Practice Questions

What this topic covers

This theme is built on one repeated argument: structure exists because it does a job, and you can predict the job from the structure. Examiners test this exact logic at every scale — a motor protein's shape tells you which way it walks a microtubule, a red blood cell's missing nucleus tells you why it can pack more haemoglobin, a gill's countercurrent geometry tells you why it out-performs a lung-style parallel flow. Several chunks here — muscle and motility, enzyme kinetics, endosymbiotic theory, detailed phloem loading — exist only on the HL paper, and they're flagged below. Get the mechanism words right (troponin, not tropomyosin; ATP-binding, not hydrolysis) because that's where the marks actually sit.

Read the full Form and Function lesson notes →

1MasterySAQ-SEnzymes as biological catalysts5 marksPaper 2~8 min

A student investigates the effect of pH on the activity of the enzyme catalase, which breaks down hydrogen peroxide (

\text{H}_2\text{O}_2

) into water and oxygen. A

1\,\text{cm}^3

cube of potato (a source of catalase) is added to

10\,\text{cm}^3

of hydrogen peroxide solution at five different pH values. The volume of oxygen gas produced in the first 60 seconds is recorded. pH — Volume of

\text{O}_2

produced /

\text{cm}^3

3 — 2.5 5 — 8.0 7 — 15.0 9 — 6.5 11 — 1.0 All experiments are conducted at

25\,^\circ\text{C}

(a)

State the pH at which catalase activity is highest. [1 mark]

(b)

Describe the effect of pH on the volume of oxygen produced, using the data provided. [2 marks]

(c)

Explain why catalase activity is lower at pH 3 than at pH 7. [2 marks]

Solutions

2ChallengeSAQ-LStructure of proteins: Primary to quaternary structure9 marksPaper 2~14 min

Data

- Glutamic acid has a charged, hydrophilic R-group; valine has a non-polar, hydrophobic R-group. - HbA is highly soluble in the cytoplasm of red blood cells and exists a tetramer (

\alpha_2\beta_2

) with quaternary structure. - Under low-oxygen conditions, HbS molecules aggregate into long fibrous polymers that deform red blood cells into a sickle shape. - A second mutant, HbC, has Glu at position 6 replaced by lysine (Lys), which has a positively charged R-group. HbC does not form polymers under low-oxygen conditions. - A third mutant, HbS-Memphis, carries the same Glu

\rightarrow

Val substitution at position 6 as HbS, plus a second substitution at position 23 where glutamic acid is replaced by alanine (Ala), which is hydrophobic. HbS-Memphis forms polymers more readily than HbS under the same conditions.

A molecular biology laboratory is studying a mutant form of haemoglobin, HbS, which causes sickle cell anaemia. Normal adult haemoglobin (HbA) has the following primary structure at the beginning of its beta-globin chain:

\text{HbA: } \ldots\text{Val-His-Leu-Thr-Pro-}\underline{\textbf{Glu}}\text{-Lys}\ldots

In HbS, a single nucleotide substitution in the beta-globin gene causes glutamic acid (Glu) at position 6 to be replaced by valine (Val). Additional

(a)

State the level of protein structure directly altered by the Glu

\rightarrow

Val substitution, and identify two higher levels of protein structure that are consequently affected in HbS. [2 marks]

(b)

Explain how the single Glu

\rightarrow

Val substitution leads to polymer formation under low-oxygen conditions. [3 marks]

(c)

Evaluate the statement: "The quaternary structure of a protein is entirely determined by its primary structure," using evidence from at least three of the haemoglobin variants described above. [4 marks]

Solutions

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