Beta-lactams and vancomycin: enzyme versus substrate

Beta-lactams and vancomycin: one wall, two outcomes

A man with an infected surgical wound is given vancomycin, and he improves. His neighbour has a gram-negative infection, receives the same drug, and does not improve. Both bacteria have a cell wall for the drug to attack, so why does one respond and the other not?

The answer lies in how beta-lactams and vancomycin each block cell-wall synthesis, and why each has the reach it has. Both hit the same wall-building step, but beta-lactams grip the enzyme and vancomycin grips the substrate.^[1]

Peptidoglycan and the PBP cross-link

A bacterium is wrapped in peptidoglycan, a mesh of sugar chains. Each chain carries short peptides, and those peptides end in D-alanyl-D-alanine.^[1][12]

Enzymes called penicillin-binding proteins (PBPs), which act as transpeptidases, cross-link the peptides from one chain to the next. The cross-links make the mesh strong.^[1][11]

Block the cross-linking, and the wall fails. Beta-lactams and glycopeptides both inhibit cell-wall synthesis in this way.^[1][9]

How beta-lactams work: the ring mimics D-Ala-D-Ala

The beta-lactams are the penicillins, cephalosporins, carbapenems and monobactams. All share a four-membered beta-lactam ring.^[9]

1. The ring resembles the substrateThe ring resembles the D-alanyl-D-alanine end of the peptide, so the PBP mistakes the drug for its substrate and binds it.^[1][9]
2. The PBP is acylatedThe drug acylates a serine in the PBP active site, irreversibly. The enzyme is lost, and cross-linking stops.^[4][9][11]
3. The cell lysesPrecursors pile up, the cell's own autolytic enzymes keep digesting the old wall, and the cell lyses. So beta-lactams are bactericidal.^[1][9]

In gram-negative bacteria, the drug usually has to pass through porins in the outer membrane to reach the PBPs.^[1][5]

How vancomycin works: it binds the substrate, not the enzyme

Vancomycin works from the other side. It is a large glycopeptide, and it does not touch the enzyme. It binds the substrate itself: the D-alanyl-D-alanine end of the peptide precursor.^[1][3][10]

Because it covers the substrate rather than the enzyme, vancomycin never binds a PBP, so a change in the PBP does not affect it.^[1]

With that end covered, the precursor cannot be cross-linked, and cell-wall synthesis stops. The precursors are made inside the cell and carried across the membrane, so this block happens outside the cytoplasmic membrane, where the wall is assembled.^[10]

Why vancomycin's size limits its reach

Its size is also its limit. Vancomycin cannot cross the outer membrane of gram-negative bacteria, so it is used against gram-positive infections. That answers the opening puzzle.^[6][10]

Vancomycin is also poorly absorbed from the gut. For a bloodstream infection it goes intravenously; taken by mouth it stays in the gut, which suits infections of the colon.^[7][10]

MRSA versus VRE: altered enzyme versus altered substrate

The two drug classes can fail in different ways, because they hit different parts of the same step. Compare two resistant organisms.

Methicillin-resistant Staphylococcus aureus (MRSA) makes an altered PBP, called PBP2a, to which beta-lactams bind poorly. Vancomycin does not need that PBP at all, so it still works.^[1][2]

Vancomycin-resistant enterococci (VRE) instead swap the final D-alanine for D-lactate, and vancomycin binds the altered precursor with about 1,000-fold lower affinity.^[3]

Beta-lactams target the enzyme; vancomycin targets the substrate. So an organism that changes the enzyme escapes beta-lactams but not vancomycin, and an organism that changes the substrate escapes vancomycin.^[1][3]