A-LevelBiologyEnzymes9700/22 · May/June 2024 · Q36 Marks

Lysozyme is an antibacterial enzyme that was discovered in 1921 by Alexander Fleming, the scientist who discovered penicillin. Lysozyme catalyses the hydrolysis of glycosidic bonds present in peptidoglycan molecules to form smaller products, NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid).

(a)Before the induced fit hypothesis was proposed in 1958, scientists believed that the lock and key hypothesis explained how lysozyme catalyses the hydrolysis of peptidoglycan to its products. Draw labelled and annotated diagrams in the space provided to show how the lock and key hypothesis was used to explain the mechanism of action of lysozyme on peptidoglycan.[3]
(b)Lysozyme and penicillin can be described as antibacterial agents. Compare lysozyme and penicillin to show the similarities and differences between these two antibacterial agents.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[1] active site with a specific shape as part of a lysozyme molecule drawn and active site labelled or included in an annotation ;
[1] complementary shaped peptidoglycan drawn and, peptidoglycan / substrate, labelled or included in an annotation;
[1] enzyme-substrate complex drawn and labelled ;
[1] lysozyme with same shaped active site and, two products / NAM and NAG drawn (formed from hydrolysed peptidoglycan) ;
[1] AVP; e.g. water shown enzyme active site labelled 'lock' and substrate labelled 'key' max 2 if no ref. to specific example e.g. label for lysozyme or peptidoglycan or murein or the products NAG and NAM max 1 no labels but otherwise clearly lock and key or by gaining the enzyme-substrate complex mark if induced fit drawn
[1] cause cell, lysis / bursting; I ref.to destroying / killing, bacteria (both)
[1] detail; e.g. weaken / destroy / AW, cell wall act on peptidoglycan cell wall cell wall cannot withstand turgor pressure / AW
[1] (lysozyme is, a protein/an enzyme/a catalyst, but) penicillin is, an antibiotic / an enzyme inhibitor / a beta-lactam ;
[1] (lysozyme hydrolyses, glycosidic bonds/peptidoglycan molecules, but) penicillin, inhibits transpeptidase(s) ; A peptidase(s)
[1] penicillin, prevents formation of / AW, peptide / cross, bridges / linkages ; R penicillin breaks cross bridges A ref. to autolysins e.g. 'cannot repair holes made by autolysins'
[1] penicillin only effective when cell is, growing / synthesising cell walls, but lysozyme effective, at all stages / AW;
[1] AVP; e.g. lysozyme (much) larger molecule ref. to lysozyme denatured v penicillin less affected by heat idea of lysozyme more affected by factors affecting enzyme A examples lysozyme globular v penicillin contains (beta-lactam / 4-member) ring