So, let’s talk about how it actually works.
It works by targeting peptidoglycan. Peptidoglycan
is a mixed heteropolymer of hexose sugars
and amino acids which effectively surrounds
a bacterium like a net. The sugars are made
up of N-acetylglucosamine, which is abbreviated
NAG, and N-acetylmuramic acid, abbreviated
NAM. And they’re linked alternately in a
chain or polysaccharide. Attached to the NAM
or N-acetylmuramic acid are short peptides.
So, let’s have a quick look of what this
means from a physiological perspective to
Here we’ve got an example of two types of
bacteria. On the left hand side, we have the
Gram-positive and on the right hand side, we
have the Gram-negative. And these are broad
classifications of bacteria that are encountered.
Gram-positive bacteria, for example, such
as Staphylococcus Aureus, and Gram-negative
bacteria, for example, Pseudomonas Aeruginosa.
So, let’s have a look at the differences
between the two.
If we zoom into the cell wall in both cases,
we see that the Gram-positive has a very thick
layer of peptidoglycan shown there in brown
that protects the cell membrane and stops
the cells from losing their integrity. If
we look on the right hand side, we have a
lipopolysaccharide or green layer which is
over the top of the brown peptidoglycan layer.
This does or has presented a problem with
the use of penicillins in the treatment of
Gram-negative bacterial infections. But, we’ll
come onto that a little later on.
So, being aware of this and being shown this
in cartoon form, what I’d like to now do
is drill down into the details of the structure
of the peptidoglycan. And this brings me to
the next slide.
Here we have balls representing the N-acetylmuramic
and the N-acetylglucosamine shown as NAM and
NAG respectively. Here we show that the NAM
units are linked together with NAM units on
adjacent chains by the short peptide bridges
shown in red. Now, this is a good cartoon
structure which makes you understand how the
layers of peptidoglycan are built up around
bacteria. But, it does nothing in terms of
explaining what’s happening on a molecular
level. So, let’s zoom in a little more.
If we zoom in a little more and we look at
our NAG-NAM backbone, which we showed as the
alternating green and blue balls of our sugar
backbone, we can see that the red linkers
are constituted of a variety of different
amino acids shown here: L-Alanine, D-Glutamate,
L-Lysine, followed by this pentaglycyl polypeptide
which links to, again, D-Alanine, L-Lysine,
D-Glutamate and L-Alanine. And it’s this
chain here which is the important point of
action for our target enzyme.
What do I mean by that? Irreversible inhibition
of the enzymes involved in cross-linking peptidoglycan
is the main mode of action for penicillins.
Transamidase, which is a penicillin-binding
protein cleaves between two D-Alanine-D-Alanine
amino acids at the end of the peptide from
the N-acetylmuramic acid (NAM). Attack by
the free amino end of a pentaglycyl unit of
an adjacent peptide thus forms a crosslink
between the two linking the chains together.
So, let’s have a look at a diagram since,
sometimes, a mechanism speaks a thousand words.
What’s happening in this particular case
is that the enzyme or penicillin-binding protein
activity is cleaving off that terminal D-Alanine
and activating the one next to it. What then
happens is the pentaglycyl unit from a neighbouring
chain creates an amide bond or peptide bond
between itself and that penultimate D-Alanine.
This results in the cross-linking of the two
chains. Now, we have two sugar backbones held
together by a polypeptide bridge and it’s
this structural integrity that the penicillin-binding
protein is responsible for. And it is this
part that is responsible... that penicillins
are shown to inhibit.
Okay. So, we’ve actually looked at it from
an amino acid perspective and that’s fine.
But, let’s drill down even further and start
looking at it from a proper molecular perspective.