The enzyme carbonic anhydrase has been found in a wide range of organisms and acts as a catalyst in many tissues. Studies have shown that there are differences in the protein structure of the enzyme and differences in the number and organisation of introns and exons of the gene coding for the enzyme. All carbonic anhydrase enzymes catalyse the same reversible reaction, shown in Fig. 4.1. [Figure 4.1]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

1. [2] carbonic acid;
2. [2] hydrogencarbonate (ions) ; A bicarbonate (ions)
3. [1] any one from: similar / same, active site ;
4. [1] have the same catalytic amino acids ;
5. [1] similar / same, tertiary structure ; A same quaternary structure
6. [3] allow pre-RNA for primary transcript either exon or intron statement required for mp2 to 7, one mark each row any three from: exons | introns 1 both, are (nucleotide sequences that are) transcribed/form part of the primary transcript ;
7. [3] 2 (DNA/RNA) coding (sequences) vs (DNA/RNA) non-coding (sequences);
8. [3] 3 (RNA exons) needed / involved / AW, in translation or for formation of, polypeptides / proteins / carbonic anhydrase vs not involved in translation / AW;
9. [3] 4 not removed, from primary transcript/from RNA/during gene splicing / during RNA splicing vs removed, from primary transcript/ from RNA /during gene splicing / during RNA splicing;
10. [3] 5 (join to) form mRNA / contain mRNA codons vs not part of mRNA/in primary transcript only;
11. [3] 6 not involved in regulating activity of genes / AW vs may have regulatory roles;
12. [3] 7 AVP e.g. (as part of mRNA) leave the nucleus/go to cytoplasm/go to ribosomes vs AVP e.g. remain in nucleus;
13. [2] allow Hb or hb or Hgb for haemoglobin two marks for points in left column or two marks for points in right column cytoplasm / cytosol; vs (bound to) cell surface membrane;
14. [2] close to haemoglobin (for H+ uptake) R in/on/with, hb or idea of being dispersed within cell because haemoglobin is distributed throughout ; vs close to, anion exchanger / transport protein, for (easy) exit of hydrogen carbonate (ions);
15. [2] quick for hydrogen carbonate (ions) to be transported out of cell / AW or H+ (from dissociation) binds rapidly to haemoglobin or cytoplasm provides water for reaction to form carbonic acid; vs able to, hydrate / react with, carbon dioxide as it enters cell;
16. [1] exocytosis;
17. [3] any three from: 1 acetazolamide / inhibitor, binds / attaches / AW, to, allosteric site / site other than active site ; R alternative active site
18. [3] 2 (which) changes, shape / tertiary structure / 3-D structure, of active site; A active site distorted I protein structure
19. [3] allow ecf for mp3 and 4 if competitive described (to max 2) 3 substrate(s) / CO2 / HCO3¯, cannot, enter / bind to / fit in / AW, active site ; A active site no longer complementary to, substrate / AW A enzyme substrate / ES, complexes cannot form A ESCs cannot form I 'substrate / CO2/HCO3¯ / 'cannot bind to enzyme' without a link to active site
20. [3] 4 product, not formed / released ; A named product
21. [3] 5 ref. to acetazolamide / inhibitor, leaves, allosteric site / enzyme ; I reversibly binds