The Beta Lactam Antibiotics

Intended Learning Outcomes

This lecture will enable you to:• Understand how Nice Yellow Medicine that

you took in childhood works• Understand what Nice Yellow Medicine is

and how it is made• Understand why you can’t make Nice

Yellow Medicine in a practical class.

Bacterial targets for selective inhibition

• Cell wall• Cell membrane• DNA synthesis• Transcription• Protein synthesis in the

cytoplasm• (Other) metabolism

The bacterial cell wall• Bacteria are divided on the basis of cell

wall structure into Gram positive and Gram negative

iodine acetone

Gram negative acetone

Gram positive

The cell membrane

Cell wall structure and function

• The cell wall protects the cytoplasmic membrane from rupture –– Cell membrane is fragile– Osmotic pressure inside cell is high

• Cell wall maintains shape of cell• Gram negative bacteria have an outer

membrane–like structure.

Norcardiform bacteria• Cell walls of nocardiform bacteria

– These include the mycobacteria such as M. tuberculosis

– Nocardiform bacteria are Gm +ve but are difficult to stain.

– Peptidoglycan is covalently linked to the polysaccharide arabinogalactan.

– Lipids are esterified to arabinogalactan and make up 60% of the dry cell weight.

– Lipids confer resistance to staining and to antibiotics and biocides.

Peptidoglycan• The major structural component of GM+v and

GM_ve cell walls consists of repeating disaccharide unit of 2 different N-acetylated aminosugars.

• Short peptide chains are attached to alternate aminosugars and these crosslink to one another via peptide bonds.

• Some bacteria have many sheets of peptidoglycan and these are joined by the same type of crosslink

N-acetylglucosamine N-acetylmuramic acid L-alanine D-glutamic acid Meso-diaminopimelic acid D-alanine

The crosslinks between the polysaccharide chains are species dependent Example: E.coli peptidoglycan

Transpeptidation• Crosslinks are formed by a transpeptidation reaction, catalysed by a

membrae-bound transpeptidase.

SugarNH

HN

O COO-

O

NH

NH3+

COO-

HN

O

O

NH

COO-

Enz OH

H+

L-Ala D-Glu

DAP

D-Ala D-Ala

SugarNH

HN

O COO-

O

NH

NH3+

COO-

HN

O

O

O

L-Ala D-Glu

DAP

D-Ala

Enz

SugarNH

HN

O COO-

O

NH

NH2

COO-

HN

O

O

NH

COO-

L-Ala D-Glu

DAP

D-Ala D-Ala

H+

SugarNH

HN

O COO-

O

NH

NH3+

COO-

HN

O

OL-Ala D-Glu

DAP

D-Ala

SugarNH

HN

O COO-

O

NH

NH

COO-

HN

O

O

NH

COO-

L-Ala D-Glu

DAP

D-Ala D-Ala

Carboxypeptidase

• One of a number of other enzymes in the cell membrane that acts on cell wall synthesis.

R

HN

O

NH

COO-

D-Ala D-Ala

EnzOH

H+

OEnz

OHN

R

H2O

OH

OHN

R

ß-Lactam Antibiotics

• Penicillins c. 1940s• Cephalosporins c. 1960s• Carbapenems c. 1990s

ALL CONTAIN LACTAM RING

The PenicillinsPenicillins were discovered by Alexander Fleming in the late

1920s.

Staphylococcus lawnZone of inhibitionFungal growth

Fleming prepared an antibacterial broth from the Penicillium notatum fungus and discovered many of its properties. Penicillin G was later purified from the broth and found to be the active ingredient.

The Penicillins

Developed by Florey and Chain as part of the second world war effort.The main difficulty was to cultivate the fungus to produce more penicillin.

Recommended reading: The Mould in Dr Florey’s Coat by Eric Lax.

N

SHN

Ph

COO-

OO

Biosynthesis of ß-lactams

Origins

Valine

Cysteine

Phenylalanine

N

S

COOH

HHHN

O

R

O

Structural mimicry of D-ala-D-ala ?

ß-lactam action with Transpeptidase / Carboxypeptidase

N

S

COOH

HHHN

O

R

O

Enz OH

H+HN

S

COOH

O

HHN H

O R

O

Enz

Critical Parameters Affecting ß-Lactam Utility

Activity and usefulness of the -lactams is determined by:

• Ability to penetrate the cell wall • Resistance to -lactamases• Affinity of various Penicillin Binding Proteins• Resistance to stomach acid• Formulation, pharmacokinetics, toxicology

etc.

Semi-synthetic penicillins

N

S

COOH

HHHN

OO

HS

NH2H

COOH

L-cysteine

NH2

COOHH

D-valine

H

COOH

NH2

N

S

COOH

HHH2N

O

Penicillin G

6-aminopenicillanic acid

N

S

COOH

HHHN

OO

laboratory

chemistry

amoxicillin

L-phenylalanine

HO

H2N H

The Penicillin StoryPenicillin G: The original• Acid labile therefore injection, sensitive to ß-lactamase

degradationAmpicillin: Semisynthetic• Acid stable (oral), sensitive to ß-lactamase degradationAmoxicillin: Semisynthetic• Resistant to class 1 ß-lactamase enzymesMethicillin and flucloxacillin: Semisynthetic• Resistant to all ß-lactamases

Beta lactams - Penicillins

N

S

COOH

HHHN

O

HH2N

HO O

N

S

COOH

HHHN

OO

Ph

N

S

COOH

HHHN

OO

HH2N

Ph

Penicillin G

Ampicillin

Amoxycillin

N

S

COOH

HHHN

OO

OMe

OMeMethicillin

N

S

O

HN H

CH3

CH3

COOH

O

Flucloxacillin

N

O

CH3

Cl

F

Beta-lactams - cephalosporins• The cephalosporin nucleus is

biosynthesized in a closely similar way to the penicillin nucleus

• First generation cephalosporins are now seldom used

• Second generation cephalosporins have Gram negative activity and are administered orally

• Third generation cephalosporins have good Gram negative activity, are useful against very serious illness and are injected.

N

HHHN

O

R

O

S

COOH

CephalosporinsCefaclor – second generationGram negative activity and activity against Haemophilus

N

HHHN

OO

S

COOH

NOMe

N

SH2NS

NNH

N

OO

Ceftriaxone – third generationGiven by intramuscular injection, for serious infection, often in combination with a macrolide or aminoglycoside

N

HHHN

OO

HH2NS

ClCOOH

Other beta-lactams

Carbapenems• Very broad spectrum• Resistant to most

beta-lactamases

Monobactams• Just one so far• Active against Gram

negatives only

NO

COOH

SR

NO SO3H

HN

O R

Other beta-lactams

Imipenem, an example of a carbapenem. It is rapidly deactivated by the kidneys unless administered with a specific inhibitor, cilastatin.

Aztreonam, the only monobactam in use at the moment. N

O SO3H

HN

ON

S

N NH2

O COOH

N

COOH

HH

O

HO H

S

HNNH

ß-Lactams

• Inhibit cell wall synthesis• Combine the specificity of natural products

with the versatility of laboratory-based drugs and are incredibly successful as a result.

• Have a heroic and inspiring history that has informed the development of other drugs.