Antibiotics 1 W8

Question 1

What do we want to do

Answer 1

Kill the bacteria or stop them replicating

And design molecules to be as selective as possible as most processes with mammas and bacteria are very similar

Question 2

Differences between prokaryote and eukaryote

Answer 2

Prokaryote
- simple
- unicellular
- absent nucleus
- smaller
- circular DNA
- no memb bound organelles
- 70s ribosome

Eukaryote
- complet
- uni or multicellular
- present nucleus
- larger
- linear DNA
- memb bound organelles
- 80s ribosomes

Question 3

Antibiotics should be designed to …

Answer 3

interrupt biological processes which are not found in humans but are essential for survival of the bacteria. This way drug toxicity is minimised.

Question 4

Cell wall

Answer 4

protects the bacteria from mechanical damage and osmotic pressure.

Inhibition of cell wall synthesis leads to bacterial bursting and death.

Mammalian cells don’t have/need cell walls

Therefore Inhibition of cell wall biosynthesis is a good antibiotic drug target.

Question 5

Development year vs year of resistance observed

Answer 5

Penicillin :
- Early 1940’s
- 1945

Tetracycline :
- late 1940’s
- early 1950’s

Question 6

Gram + vs - cell wall

Peptoglycan layer, teichoic acids, outer membrane, periplasmic space, lipid content, examples

Answer 6

Peptidoglycan layer:
- positive = thick 20-80nm and multilayered
- negative = thin 2-10nm and single layered

Teichoic acids
- positive = present
- negative = absent

Outer membrane
- positive = absent
- negative = present containing lipopolysaccharides

Periplasmic space
- positive = usually absent or very small
- negative = present located between outer membrane and cytoplasmic membrane

Lipid content
- positive = low
- negative = high due to outer membrane

Examples
- positive = streptococus, staphylococcus
- negative = salmonella, E.coli

Question 7

Bacteria cell wall structure

Answer 7

Peptidoglycan structure made up of peptide and sugar units

Peptide chains bound to NAM sugars

Cell wall framework is meshed together by cross-linking between these peptide chains

Cross-link formation catalysed by transpeptidase enzyme

Question 8

What do D-amino acids in peptidoglycan do

Answer 8

render the cell wall resistant to the action of proteases which act on the more commonly occurring L-amino acids

Question 9

Penicillin

Answer 9

secondary metabolite of certain species of penicillium molds.

Produced when they are grown under stress

Question 10

Penicillin structre

Answer 10

All have same structure:
- B-lactam (4) ring
- thiazolidine (5) ring, containing one N and one S attached to the b-lactam ring
- variable side chain (R)

Took 20 years to take the fungi, colonise it and grow it and identified penicillin

Question 11

How does penicillin inhibit bacterial cell wall cross-linking

Answer 11

by targeting and inactivating penicillin-binding proteins (PBPs), which are enzymes involved in the final stages of peptidoglycan synthesis.

Penicillin disrupts bacterial cell wall synthesis by inhibiting PBPs, preventing the formation of peptide cross-links in the peptidoglycan layer. This weakens the bacterial cell wall, leading to cell death.

Question 12

features of penicillin structure that are essential for its antibiotic activity ; B-ring

Answer 12

1. β-Lactam Ring
   - Most crucial feature.
   - Mimics the structure of the D-Ala-D-Ala dipeptide substrate used by bacterial transpeptidiases during cell wall synthesis.
   - Ring bind covalently to the bact.transpep active site and in-activiating them, preventing cross linking

Question 13

Why is penicillin G acid sensitive

Answer 13

• primarily due to the high reactivity of the β-lactam ring under acidic conditions.
• The carbonyl group is vulnerable to nucleophilic attack, and in acidic environments, this can lead to ring opening, relieving strain and inactivating the antibiotic.
• The intramolecular participation of the neighboring acyl group further accelerates this process.

Question 14

Tackling the acid sensitivity problem

Answer 14

Attaching an electron withdrawing group to the carbonyl group pulls electron density away from the carbonyl oxygen. This reduces its tendency to act as a nucleophile thereby reducing/preventing neighbouring group participation

Question 15

Penicillin resistant bacteria produce …

Answer 15

B-lactamase enzyme which catalyse the same ring opening reaction of penicillins during acid hydrolysis

Question 16

Two approaches that can be used to tackle the problem of b b- lactamase sensitivity

Answer 16

1. Penicillin analogues which are not recognised by b-lactamases
2. Co-administration of b-lactamase inhibitors

Question 17

features of penicillin structure that are essential for its antibiotic activity ; carboxylic group

Answer 17

2. Carboxylic Acid Group
   - essential for binding to the active sites of PBPs
   - helps form H bonds with enzyme enhancing specificity and effectiveness

Question 18

features of penicillin structure that are essential for its antibiotic activity ; side chain R

Answer 18

3. Side chain R group
   - The R group attached to the β-lactam ring determines the spectrum of activity, resistance to β-lactamases, and pharmacokinetic properties (e.g., stability in acidic environments).
   - Modifications to the R group have led to the development of penicillin derivatives like amoxicillin and methicillin.

Question 19

features of penicillin structure that are essential for its antibiotic activity ; thiazolidine ring

Answer 19

4. Thiazolidine ring
   - S configuration ensures that the b-lactam ring is correctly positioned for interaction with PBPs