3 Case study: mechanism of ß-lactams

The β-lactam antibiotics target the bacterial cell wall (Figure 8) by inhibiting the enzymes responsible for cross-linking adjacent molecules in the peptidoglycan layer. The ß-lactam antibiotics bind to these enzymes, collectively known as penicillin-binding proteins (PBPs), and prevent them from forming cross-links. As the bacterial cell grows, the effect of the antibiotic is to progressively weaken the cell wall until the cell lyses as a result of osmotic damage.

Figure 8 Structure and arrangement of peptidoglycan chains in the bacterial cell wall. Peptidoglycan molecules consist of a backbone of carbohydrate units with sets of amino-acid residues attached (yellow). They are cross-linked by bridges (red), providing structure and strength.

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The diagram shows two adjacent chains of alternating N-acetylglucosamine and N-acetylmuramic acid units, represented here as large blue balls. Extending downwards from each N-acetylmuramic acid unit is a chain of four amino acid residues, shown here as small yellow balls. The ends of the amino acid chains on adjacent carbohydrate units are joined by glycine cross-bridges, shown here as red bars.

Figure 8 Structure and arrangement of peptidoglycan chains in the bacterial cell wall. Peptidoglycan molecules consist of a backbone of ...

In Section 2 you learned that the activity of penicillins and cephalosporins resides in the ß-lactam ring. Activity 3 looks more closely at this.

Activity 3 Mechanism of ß-lactam antibiotics

Timing: Allow about 15 minutes

First, watch the short video below which describes the inherent instability of the ß-lactam ring structure which makes it highly reactive. The video refers to penicillin, but the same is true of the ß-lactam ring in cephalosporins and all other antibiotics of this class.

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Transcript: Video 4 The inherent instability of the ß-lactam ring structure.

INSTRUCTOR:

This is a model of the four-membered ring, referred to as a beta-lactam ring in the text, that's part of a penicillin molecule. And if you take a look at it, you can see that it's composed of three carbon atoms and one nitrogen atom. But even more importantly, note that as a four-membered ring, the bond angle now between the carbons-- instead of being like the tetrahedral carbon bond angle of 109 degrees, it's constrained to be just 90 degrees.

That introduces what we call ring strain into the molecule. And it's a way in which molecules are encouraged, therefore, to react. And indeed, the reaction of penicillin with bacteria and an enzyme in bacteria is to open up this four-membered ring and release that ring strain. I think you can see before I demonstrate that that the bonds are already in the four-membered ring quite bent.

But watch what happens when I break one of these bonds and simulate the reaction. You can see how it will spring apart to release that ring strain. So another important feature in the understanding of why chemical reactions take place is to release this ring strain that we're seeing in the four-membered ring and is demonstrated when the enzymes in bacteria react with penicillin.

End transcript: Video 4 The inherent instability of the ß-lactam ring structure.

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Video 4 The inherent instability of the ß-lactam ring structure.

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Figure 9 shows what happens when a ß-lactam antibiotic, in this case penicillin, binds to an active PBP.

Figure 9 Reaction of penicillin with a PBP. The -NH₂ side chain of the PBP reacts with the ß-lactam ring of penicillin to form a new side chain. This reaction releases the strain in the ß-lactam ring which remains open.

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This is the equation for the reaction of penicillin with a side chain amine group, –NH₂, on a protein molecule. The bond between the nitrogen and the carbonyl carbon atom of the ß-lactam ring is broken, and this carbon forms an amide, –CONH–, linkage with the amine group on the protein. At the same time, one of the protein’s amine hydrogen atoms is transferred to the N atom that was in the ß-lactam ring. The reaction results in deactivation of the protein.

Figure 9 Reaction of penicillin with a PBP. The -NH₂ side chain of the PBP reacts with the ß-lactam ring of penicillin to form a new side...

The reaction shown in Figure 9 results in a new PBP side chain which is much larger than the original -NH₂ group and effectively deactivates the PBP. Can you suggest why?

Answer

The large side chain means that there is no longer sufficient space for the enzyme (PBP) to bind to its normal substrate during the peptidoglycan cross-linking process.

You will learn more about how disrupting the interaction between ß-lactam antibiotics and PBP contributes to antibiotic resistance mechanisms in Week 3. Next, however, you will see how antibiotics of the same class, and with the same mode of action, can have a different spectrum of activity and exert different effects.