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Part 3 – PM3A > (PM3A) Chemistry of Antimicrobial Drugs > Flashcards

(PM3A) Chemistry of Antimicrobial Drugs Flashcards

1
Q

With respect to antibiotics, which relevant cells have a cell wall and which do not?

A

Bacteria have a cell wall

Animal cells do NOT

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2
Q

What is the cell wall made from?

A

Co-polymer called peptidoglycan

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3
Q

What is peptidoglycan?

A

A co-polymer which forms the bacterial cell wall

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4
Q

How much peptidoglycan is there in a gram positive bacterial cell wall?

A

Thick layers

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5
Q

How much peptidoglycan is there in a gram negative bacterial cell wall?

A

1-2 molecular layers

Surrounded by a membrane

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6
Q

What are the sugar backbones in peptidoglycan?

A

(1) N-acetylglucosamine

(2) N-acetylmuramic acid

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7
Q

What is the composition of peptidoglycan?

A

(1) 2 sugar backbones

(2) Peptide chains composed of D-amino acids

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8
Q

What are the peptide chains in peptidoglycan composed of?

A

D-amino acids

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9
Q

What does a transpeptidase (TPase) enzyme do?

A

Catalyses a cross-linking reaction in the final stage of the biosynthesis of a bacterial cell wall

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10
Q

Which enzyme catalyses the cross-linking reaction in the final stage of bacterial cell wall biosynthesis?

A

Transpeptidase

TPase

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11
Q

Which type of antimicrobial inhibits an enzyme involved in the catalysis of cross-linking in the bacterial cell wall? What is this enzyme?

A

Penicillins

Transpeptidase (TPase)

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12
Q

Describe the structure of a penicillin.

A
  • Four membered beta-lactam ring

- Fused to a 5 membered thiazolidine ring

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13
Q

How can penicillins be manufactured?

A

(1) Chemical synthesis

(2) Fermentation

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14
Q

(1) Which bacteria are penicillins normally active against?

(2) Give an example.

A

(1) Non beta-lactamase producing gram-positive bacilli

(2) e.g. meningitis

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15
Q

What is the range of responses had by those experiencing an allergic reaction to penicillin?

A

Rash -> Anaphylactic shock

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16
Q

Which functional groups of penicillin are essential to its function?

A

(1) Amide
(2) Lactam
(3) Free carboxylate

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17
Q

(1) What state is the free carboxylate of penicillin normally in?
(2) What effect does this have on the administration of penicillin?

A

(1) Usually ionised

(2) Usually administered as a sodium/ potassium salt

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18
Q

Give an example of a penicillin that is sensitive to acid?

A

Penicillin G

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19
Q

What reasons are there for penicillin G being sensitive to acid?

A

(1) Ring strain

(2) Amide side chain can open the beta-lactam ring

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20
Q

What does ‘ring strain’ mean, with reference to penicillin sensitivity to acid?

A

(1) Acid-catalysed ring opening relieves strain by opening the more highly strained beta-lactam ring
(2) Carbonyl group of lactam is more prone to nucleophilic attack

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21
Q

How are aqueous solutions of antibiotics prepared if they contain a beta-lactam ring?

A

(1) Supplied as dry powder

(2) Reconstituted immediately before being dispensed

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22
Q

Why are antibiotics containing a beta-lactam ring reconstituted just before dispensing?

A

Beta-lactams have general instability in aqueous solutions

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23
Q

What are the storage requirements for an antibiotic containing a beta-lactam?

A

Fridge storage

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24
Q

How long can an antibiotic containing a beta-lactam be stored for?

A

7 days (refrigerated)

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25
Q

What effect can the amide side chain of a penicillin have on the beta-lactam ring in the presence of acid?

A
  • Can actively participate in a mechanism opening the beta-lactam ring in the presence of acid
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26
Q

(1) What happens if a penicillin is exposed to an acid?

(2) What effect does this have on the penicillin?

A

(1) Beta-lactam is destroyed

(2) Destroys functionality
ø Beta-lactam was essential for activity

27
Q

What is the effect on the beta-lactam ring of a penicillin if the amide is more electron poor?

A
  • Less of an ability to open the beta-lactam ring

- Amide is less nucleophilic

28
Q

What difficulty does the sensitivity of penicillins to acid present?

A

Oral delivery route (stomach acid)

29
Q

What is penicillin V?

A

Phenoxymethylpenicillin

30
Q

What is another name for phenoxymethylpenicillin?

A

Penicillin V

31
Q

Why is phenoxymethylpenicillin able to be administered orally, despite the sensitivity of penicillins to acid?

A
  • The nucleophilicity of the amide chain is reduced
  • Increases stability
  • Sufficiently stable to exist in stomach
32
Q

(1) Name 2 penicillins which can be given orally.

(2) Why can they be administered orally, where others cannot?

A

(1) Phenoxymethylpenicillin + ampicillin

(2) Greater acid stability due to decreased electron density (nucleophilicity) of the amide present in the penicillin

33
Q

(1) Give one example of an acid sensitive penicillin?

(2) Why is it acid sensitive?

A

(1) Penicillin G

(2) Amide opens the beta-lactam ring (essential for activity of penicillin) in the presence of acid

34
Q

What is the most common method of a bacteria gaining resistance to a penicillin?

A

Presence of beta-lactamase enzyme

35
Q

How does beta-lactamase affect beta-lactam

A

Hydrolyses it

36
Q

What is the penicillin called once the beta-lactam has been hydrolysed by an acid/ beta-lactamase?

A

Ring-opened penicillin

37
Q

What is the function of ring-opened penicillin?

A

None. It is inactive

38
Q

Where is beta-lactamase usually contained? Where else can it be observed and when?

A

(1) Usually present within the bacteria (gram-negative)

2) Can also be released into the surrounding environment (gram-positive

39
Q

What caused the critical bacterial resistance issue in the 1960s?

A
  • Over-use of penicillin G

- Led to an increase of penicillin-resistant Staph. aureus infections

40
Q

Following the increase of bacterial penicillin resistance in the 1960s, what method was used to counter-act this?

A

Steric shields

- To block penicillin from fitting into the beta-lactamase enzymes of the resistant bacteria

41
Q

What was the issue found when there was too much steric shielding implemented into the penicillin?

A

Inactivity of the penicillin

- Could not activate the transcriptidase target enzyme

42
Q

What was the first semi-synthetic penicillin that had resistance to the beta-lactamase enzyme of S. aureus?

A

Methicillin

43
Q

What is a beneficial change to the functional groups of penicillin to prevent binding of beta-lactamase?

A

Adding a spatially bulky functional group, can block binding to beta-lactamase

44
Q

What are the key factors to consider for an effective and useful penicillin analogue?

A

(1) Acid resistant/ stable

(2) Sterically shielded

45
Q

Why is methicillin not an ideal penicillin?

A

Has steric shielding (beta-lactamase is ineffective)

BUT is acid sensitive

46
Q

What approaches can be used to prevent action of beta-lactamase with penicillin?

A

(1) Steric shielding

(2) Administration alongside a beta-lactamase inhibitor

47
Q

What is the most effective beta-lactamase inhibitor?

A

Clavulanic acid

48
Q

What is clavulanic acid?

A

Most effective beta-lactamase inhibitor

49
Q

Why is clavulanic acid suitable as a beta-lactamase inhibitor

A

(1) Highly strained beta-lactam ring

50
Q

How can acid sensitivity of a penicillin be reduced?

A

Addition of an electron withdrawing group on the amide side chain

51
Q

What happens to a penicillin if the bulky side group added to prevent beta-lactamase binding (steric shielding) is too large?

A

Prevents binding to transpeptidase (TPase) target enzyme

52
Q

(1) What type of penicillin are the aminopenicillins?

(2) Give 2 examples

A

(1) Broad-spectrum

(2) Ampicillin + amoxicillin

53
Q

What is important, regarding penicillins, for good absorption through the gut wall?

A

No ionisation on side groups

54
Q

Name 2 common types of antibiotic.

A

(1) Penicillins

(2) Cephalosporins

55
Q

What is a similarity of cephalosporins to penicillins?

A

(1) Contain a 4 membered beta-lactam ring

56
Q

What is the beta-lactam ring of a cephalosporin bound to that the beta-lactam of a penicillin is not?

A

6-membered dihydrothiazine ring

57
Q

Which type of antibiotic has the greatest stability of the bicyclic system, cephalosporins or penicillins?

A

Cephalosporins

The adjacent 6-membered dihydrothiazide ring reduces the ring strain by a greater extent

58
Q

What is the mechanism of action of the cephalosporins?

A

Inhibitor of the transpeptidase (TPase) enzyme

59
Q

How does the activity of cephalosporin C compare to that of penicillin G?

A

Only about 1/1000 as active

60
Q

What is a benefit of cephalosporin C over penicillin G, regarding activity?

A

Cephalosporin C has more of a balance of activity with gram-positive and gram-negative bacteria

61
Q

Is cephalosporin C or penicillin G more resistant to beta-lactamase?

A

Cephalosporin C is more resistant to beta-lactamase

62
Q

Is cephalosporin C or penicillin G more resistant to acid hydrolysis?

A

Cephalosporin C is more resistant to acid hydrolysis

63
Q

What is a zwitterion?

A

A molecule with an equal number of negatively and positively charged functional groups

64
Q

(1) What was a functional group of cephalosporins which was commonly changed and gradually phased out with each generational change?
(2) Why?

A

(1) Esters

(2) Esters were commonly cleaved by esterases. This was not optimal for effective antibiotic activity

Part 3 – PM3A (28 decks)

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