Keratin is the structural protein in feathers of birds. Keratin polypeptides are composed of a high proportion of cysteine amino acids, which have sulfur-containing R groups. Keratin polypeptides form filaments. The two main types of keratin in feathers are α-keratin, which consists of many α-helices, and β-keratin, consisting of many β-pleated sheets.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

1. [1] secondary (structure);
2. [1] any one from: idea that area to be, hydrolysed / AW, could be between same amino acids ; A ref. to (always), hydrolyses / breaks down / acts on, peptide bonds
3. [1] suggestion that active site has (some) flexibility for, hydrolysing / binding to / AW, similar substrates ;
4. [1] (proteins / substrates) similar shape(s) so fit active site;
5. [1] active site and, substrates / proteins, (still) have complementary shapes ; R 'complementary to the antigen or the shape'
6. [1] AVP; e.g. (large enzyme / enzyme complex, with) more than one active site
7. [3] any three from: fibrous; A not globular I structural
8. [3] insoluble / does not dissolve / does not dissolve easily;
9. [3] (high proportion of) disulfide bonds / disulfide bridges / covalent bonds (because of cysteines) ;
10. [3] high proportion of /many / AW, hydrogen bonds (between polypeptides) ;
11. [3] high proportion of /many, hydrophobic interactions ; A many, amino acids with hydrophobic R-groups / hydrophobic R-Groups
12. [3] tight/close, packing, of, chains / polypeptides / molecules; I triple helix
13. [3] AVP; e.g. arranged so peptide bonds are not accessible parts of molecule needing to bind to active site not accessible many amino acids with small R-groups/small amino acids I details specific to collagen e.g. gly every third amino acid amino acids
14. [4] K12 | 41–63 | 6.8/6.9–8.4 ;
15. [4] A22 | 36/36.5–57 ; A 57.1/57.2 | 6.0–9.0
16. [4] P3 | 29–56 | 5.3–7.5
17. [4] P3 ;
18. [4] A22 ;
19. [3] must have pH and °C and %, at least once max 2 if only Fig. 2.1 or only Fig. 2.2 used for the response.
20. [3] any three from: advantage from Fig. 2.1 (temperature) 1 A22 and K12, active / remove stains, at a wide range of temperature or stated (active) temperature range of, A22 is 20.5/21 °C or K12 is 22 °C or use of Fig. 2.1 to give % activity and stated temperature of, good / above 60%, activity at, medium / hotter, temperature washes for, A22 / K12 ; or K12 active at low temperature (washes) ; A data e.g. 37–40% activity at 20 °C/41–43% activity at 30 °C
21. [3] disadvantage from Fig. 2.1 (temperature) 2 low activity / poor stain removal, at 30 °C / 70 °C / 80 °C, for A22 or at 20 °C/30 °C/70 °C, for K12; A data extracted from Fig. 2.1
22. [3] advantage from Fig. 2.2 (pH) – activity in alkaline conditions 3 A22, greatest activity / most effective / AW, pH 7.5 – 8.0 or (at least 60%) activity between pH 7.5 – 9.0 ; A range of pH 6.0 – 9.0 but R if stated pH 6.0–7.0 is alkaline or K12, active between pH 7.5 – 8.0/8.5 or optimum activity at pH 8.0 or data for (good) activity at stated pH values from Fig. 2.2 for either A22 or K12 ;
23. [3] disadvantage from Fig. 2.2 (pH) 4 use of Fig. 2.3 to show a decrease in activity at higher pH values;
24. [3] 5 any wash feature in context of mp 1–4; e.g. have (some) activity at low temperatures so, more economic /less energy used (to heat water) A ora active at high temperatures but, more costly / needs (more) energy can wash at high(er) temperature so, can sanitise clothes / can kill insect pests / can kill pathogens / shorter time to remove stains comparison of K12 and A22 for washes e.g. K12 more, thermostable / stable in (wishes at) higher temperatures K12 has higher optimum temperature so more, costly / energy used K12 only useful at lower alkaline pH A22 has wide(r) working range of pH A22 more tolerant to / stable at, higher pH