Antibiotics

Question 1

What do all penicilins contain?

Answer 1

A Beta Lactam ring.

Question 2

What is a B-lactam?

Answer 2

A B-lactam is a 4-membered cyclic amide.

considered as derived from an amino acid

Question 3

Why is B-lactam ring an amide?

Answer 3

Amide because the ring contains a C=O-NH group.

Question 4

What was the first clinically used penicillin and what was the issue with it?

Answer 4

The first clinically used penicillin was benzyl penicillin aka Penicillin G.
This penicillin is acid unstable and rapidly degrades in stomach acid. Hence only used for topical use e.g. WW2.

Question 5

What are the 4 essential components required for penicillin to exert activity?

Answer 5

• B-lactam ring
• Free carboxylic acid - stereochemistry dashed lines (bond going into paper)
• Strained bicyclic ring - 4 membered ring fused to 5 membered ring
• Acylamino side chain - stereochemistry black wedge (bond towards you)

Question 6

What can be changed in a basic penicillin structure?

Answer 6

R group attached to the acylamino side chain can be changed. The rest of the structure of the molecule is needed to bind to the bacterium.

Question 7

What structure/compound can be used as a starting point to make penicillin antibiotics?

Answer 7

6-APA = 6-aminopenicillanic acid used as starting compound but it does not have the acylamino side chain.
Hence acyl side chain R group can be added synthetically via chemical reaction.

Question 8

What does the term ‘chemically strained’ mean in terms of the bicyclic ring in the penicillin structure?

Answer 8

The fusion of the 4 membered ring with a 5 membered ring causes an instability which exists between the bonds. This makes the molecule reactive hence has good activity against bacteria.

Question 9

How are semi-synthetic penicillins made?

Answer 9

General structure of penicillins are made using 6-APA with a suitable acetylating agent. 6-APA is made by fungi using fermenters. Then R group added synthetically by synthetic chemists.

Question 10

Bacteria produce transpeptidase. Where is this found? What is the function of this enzyme?

Answer 10

Transpeptidase is found at the cell membrane in large quantities. It is an enzyme which catalyses crosslinking of peptidoglycan essential to form a cell wall.

Question 11

What is the mechanism of action of B-lactam antibiotics?

Answer 11

B-lactam antibiotics bind and react with transpeptidase enzyme produced by bacteria to inhibit cell wall synthesis. No cell wall means that water can penetrate through the cell and cause it to lyse.

Question 12

What is the rule for R group size?

Answer 12

The smaller the R group, the greater the activity.

Question 13

Why would some penicillins be inactive against bacteria?

Answer 13

Bacteria can be resistant to some penicillins due to bacterial enzymes.
B-lactamase - breaks open the B-lactam ring via acid hydrolysis and deactivates the penicillin.
Penicillin amidase - cleaves the amide bond acyl side chain and deactivates the penicillin.

Question 14

What can be done to the structure of penicillin to prevent B-lactamase enzymes degrading the antibiotic?

Answer 14

Increasing the steric bulk/size of the R group decreases the ability of the B-lactamase enzymes to bind to the penicillin.

Question 15

What is the problem in using a bulky R group which prevents B-lactamase degrading the penicillin?

Answer 15

A large R group very significantly reduces the activity of the penicillin against transpeptidase.
So only use a bulky R group when required, to be effective.

Question 16

Big R group

Answer 16

Blocks B-lactamase which stops the bacterium from destroying the penicillin drug but it is important to retain its activity against transpeptidase to kill the bacterium.

Question 17

Describe the ideal penicillin analogue which can block B-lactamase activity while retaining activity against transpeptidase?

Answer 17

An ortho-disubstituted phenyl ring with small substituents is the ideal size.

Question 18

What does broad spectrum mean?

Answer 18

Works against both Gram+ and Gram-.

Question 19

What can happen to the B-lactam ring in acidic conditions?

Answer 19

The B-lactam ring is susceptible to an intramolecular ring opening reaction which deactivates the antibiotic. Thus, the ring cannot bind to transpeptidase.

Question 20

How can acid stability of an antibiotic be increased? How does this work?

Answer 20

Ensure that the R group is an electron-withdrawing group. The inductive effect will draw electron density away from the carbonyl oxygen - reduces its tendency to act as a nucleophile. Prevents the ring opening.

Question 21

Describe the intramolecular ring opening reaction which occurs in acidic conditions?

Answer 21

The degradation of the penicillin in acidic conditions involves the nucleophilic attack from the oxygen of the acylamino side chain carbonyl group at the carbon of the B-lactam ring carbonyl group. This causes a partial delta- charge at the oxygen and delta+ charge at carbon = a bond forms - forming a different ring structure. The B-lactam ring is opened leading to the destruction of the antibiotic.

Question 22

Why would some penicillins show a poor activity against Gram negative bacteria?

Answer 22

• G- bacteria have an inner and outer membrane and the transpeptidase is found in the inner membrane which is difficult to reach.
• G- bacteria have a coating on the surface of the cell wall which is impermeable to penicillins.
• Some G- bacteria produce large amounts of transpeptidase which must be broken down.
• It is common to find transpeptidase mutations in G- bacteria.

Question 23

How must penicillin reach transpeptidase in the inner cell membrane in the case of Gram- bacteria?

Answer 23

The penicillin must diffuse through channels in the outer membrane formed by porin proteins to reach the inner membrane.

Question 24

What are porin channels?

Answer 24

Porin channels are channels used by the bacterium to bring in nutrients or excrete waste materials.
The inner part of the porin channels are lined with charge hence very highly polar.

Question 25

Since penicillin must diffuse through porin channels to reach the transpeptidase in the inner membrane of Gram- bacteria, what two features must the penicillin possess?

Answer 25

A molecular weight of <650 gmol-1

A hydrophilic character (polar)

Question 26

Why should the penicillin be polar/hydrophilic to diffuse through porin channels and how can this be achieved?

These have broad spectrum activity (G- and G+)

Answer 26

The penicillin should be polar because the inner part of the porin channel is charged/polar.
This can be achieved by having a hydrophilic groups such as OH or NH2 directly attached to the carbon alpha to the carbonyl group of the acyl side chain.

Question 27

Why must some antibiotics be given via IV?

Answer 27

Because they are not acid stable.

Question 28

What is the effect of having hydrophobic groups on the side chain?

Answer 28

The penicillin will favour activity against G+ bacteria and poor activity against G-.

Question 29

What is the effect of having hydrophilic groups on the side chain?

Answer 29

The activity will be reduced slightly against G+ bacteria. Hydrophilic groups attached to the carbon alpha to the carbonyl group on the side chain have the greatest activity against G-.

Question 30

What 4 things do you need to look out for in terms of activity of penicillin?

Answer 30

1. Does it have all 4 features and the correct stereochemistry?
2. Does it have acid stability?
3. Does it have activity against B-lactamase and transpeptidase?
4. Is it a broad spectrum - against G- and G+?

Question 31

What happens if a larger R group is used? What effect does it have against transpeptidase?

Answer 31

Larger R group = activity against transpeptidase decreases due to the lowering of the binding constant between the B-lactam and the transpeptidase enzyme.

Question 32

What does cluvanic acid do?

Answer 32

It is a B-lactamase inhibitor.