Peptidoglycan

Peptidoglycan, also called murein, is the mesh-like polymer of sugars and short peptides that forms the cell wall of nearly all bacteria, giving them their shape and protecting them from bursting under internal pressure. Because animal cells have nothing like it, peptidoglycan is one of the most successful targets in medicine: penicillin and many related antibiotics work by blocking its construction.

The amount and arrangement of peptidoglycan also underlie the Gram stain, the century-old test that still divides bacteria into two broad groups.

Peptidoglycan is built from two components:

• Glycan strands: long chains of two alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), joined by β-1,4 bonds.
• Peptide cross-links: a short peptide of about four or five amino acids is attached to each NAM. Peptides on neighboring strands are linked together, turning the strands into a continuous, bag-shaped network called the sacculus that surrounds the whole cell.

The peptides are unusual. They contain D-amino acids, such as D-alanine and D-glutamate, which are rare in proteins. Many contain meso-diaminopimelic acid (DAP), found in most Gram-negative bacteria, or L-lysine, found in many Gram-positive bacteria. The D-amino acids make the wall resistant to most ordinary protein-cutting enzymes.

• Gram-positive bacteria, such as Staphylococcus and Streptococcus, have a thick wall of many peptidoglycan layers, often 20–80 nanometers thick, threaded with teichoic acids. This thick wall retains the crystal violet dye of the Gram stain, so the cells stain purple.
• Gram-negative bacteria, such as Escherichia coli, have only a thin peptidoglycan layer in the periplasmic space between the inner cell membrane and an outer membrane. The thin layer loses the dye, so the cells stain pink with the counterstain. The outer membrane is anchored to the peptidoglycan by lipoproteins.

Peptidoglycan synthesis takes place in three locations:

1. Cytoplasm: A series of Mur enzymes builds a NAM–peptide unit linked to the nucleotide carrier UDP.
2. Membrane: The unit is attached to a lipid carrier (bactoprenol), joined to NAG to form lipid II, and flipped across the membrane.
3. Outside the membrane: Glycosyltransferases add the new unit to growing glycan strands, and transpeptidases form the peptide cross-links, releasing the final D-alanine.

The transpeptidases are known as penicillin-binding proteins (PBPs). Because bacteria are constantly growing, they must cut and reinsert wall material without ever opening a hole that would let them burst.

• β-lactam antibiotics (penicillins, cephalosporins, carbapenems) mimic the D-alanyl-D-alanine end of the peptide and permanently block PBPs. Resistance often comes from β-lactamase enzymes, frequently carried on plasmids, or from altered PBPs, as in MRSA.
• Vancomycin binds the D-Ala–D-Ala end of lipid II, physically blocking both strand growth and cross-linking.
• Lysozyme, found in tears, saliva, and egg white, cuts the bond between NAM and NAG. Alexander Fleming discovered it in 1922, several years before he discovered penicillin.

The innate immune system also senses peptidoglycan fragments. The receptors NOD1 and NOD2 detect specific breakdown products (DAP-containing fragments and muramyl dipeptide, respectively) and trigger inflammation. Mutations in NOD2 are associated with an increased risk of Crohn's disease.

Peptidoglycan is a defining feature of bacteria. Archaea lack it; some archaea build an analogous polymer called pseudomurein, which uses a different sugar and linkage and is therefore immune to lysozyme and penicillin. Mycoplasma bacteria have lost their walls altogether, which is why β-lactam antibiotics do not work against them.

Remarkably, traces of the ancestral cell wall survive in some chloroplasts. Glaucophyte algae keep a peptidoglycan layer around their plastids, and mosses retain the genes needed to make one, relics of the cyanobacterial ancestor that gave rise to all chloroplasts.