What is a Peptidoglycan?

Peptidoglycans are found in the cell walls of Gram-positive and Gram-negative bacteria in true bacteria. It is a multilayer network structure composed of acetylglucosamine, acetylmuramic acid and four to five amino acid short peptides. An oligopeptide chain is derived from each N-acetyl muramic acid, which is connected to the N-acetyl muramic acid on the adjacent polysaccharide chain (see Figure 2), and two parallel sugar chains are connected laterally to form a network, which forms A layer of peptidoglycan.

Peptidoglycan

Peptidoglycan, also known as mucocomplex, murein. It is made up of disaccharide units,

The peptidoglycan backbone is composed of N-acetylglucosamine (G) and N-acetylmuramic acid

The disaccharide composed of G and M and the tetrapeptide linked to M are the basic structural units of peptidoglycan. The length of the glycan chain also varies depending on the species, as short as 9 disaccharide units and over 170 disaccharide units; one disaccharide unit is 10.3 angstroms, so the total length can be from 100 angstroms to 1700 angstroms. There is cross-linking between the individual tetrapeptide chains. In Gram-negative bacteria, most of the peptide chains of two peptidoglycans are directly cross-linked, and the cross-linked peptide chains account for 50% of the total number of peptide chains. Gram-positive bacteria are cross-linked through another peptide bridge. For example, the peptide bridge of Staphylococcus aureus is a pentapeptide composed of five glycines, and the cross-linked peptide chain accounts for 75-100% of the total number of peptide chains. Stereo cross-linking allows peptidoglycan to form a multi-layered network structure. Gram-negative bacteria have fewer layers of peptidoglycan (such as: E. coli is 3 layers and the thickness is about 2-3 nanometers), and the degree of cross-linking is low, so the network structure is loose; while Gram-positive bacteria, due to Peptidoglycan has many layers (such as about 10 layers of Staphylococcus aureus, about 20 layers of Bacillus subtilis, and the thickness can reach 10-50 nm). It has a high degree of cross-linking and a dense network structure. The differences in peptidoglycan content, thickness, layer, and degree of cross-linking of these two types of bacteria, plus the differences in other components of the cell wall, are reflected in the properties of staining, sensitivity to lysozyme, and material permeability. very big difference. A few such as: halophilic bacteria, special cell wall structure, does not contain peptidoglycan. ^[2]

Peptidoglycan Peptidoglycan is a multi-layered network-like macromolecular structure composed of disaccharide units, tetrapeptide tails, and peptide bridges. Heteropolysaccharides alternately connected with n-acetylglucamine (nag) and n-acetylmuramic acid (nama) are macromolecules formed by interleaving and connecting peptides of different compositions. Peptidoglycan is the primary component of the cell wall of many cocci. For example, the wall of Gram-positive cocci (g +) contains 50-80% of the dry weight of peptidoglycan. N-acetylpusamine and n-acetylmuramic acid pass through capsaicin to -1,4 glycosides. Bonded, the sugar chains are cross-linked by peptides to form a stable network structure. The length of the peptide chain varies depending on the species of coccus. The peptidoglycan contained in the wall of the Gram-negative cocci (g-) accounts for 5-20% of its weight is the same as that of gram-positive bacteria, but the third of the tetrapeptide tail is not l-lys but racemic diaminopimelate. Composition and structural characteristics 1.gm disaccharide unit; 2.tetrapeptide tail composition: Gram-positive bacteria (such as staphylococcus) are l-ala + d-glu + l-lys + d-ala, Gram-negative bacteria (Such as Escherichia coli) is l-ala + d-glu + m-dap + d-ala; connection method: -nh2 on n-l-ala in the tetrapeptide is connected to the carboxyl group of lactic acid in m; 3. Peptide bridge composition: Peptide bridges vary a lot to span 100 species. For example, in s. Aureus, the peptide bridge is a glycine pentapeptide. Connection method: The n-terminal -amino group of glycine pentapeptide is connected to the carboxyl group on the c-terminal d-ala of the tetrapeptide, and the c-terminal carboxyl group is connected to the l-lys -amino group on the other tetrapeptide. Gram-positive bacteria do not have special peptide bridges, so the d-ala and m-dap of the two peptide tails are directly connected.

Lysozyme can hydrolyze the -1,4 glycosidic bond between gm, make the cell wall appear holes, and break down to the base, thereby killing cocci. There are a large number of lysozymes in human tears, and some phages can secrete lysozyme when they infect the host. Eggs also contain a large amount of lysozyme. Antibiotics can inhibit the synthesis of living substances in peptidoglycans. Conditions for peptidoglycan synthesis: Two primary carriers are required, namely uridine diphosphate (udp) and polypentadiene phosphate (also known as coccoterpenol). Several antibiotics, such as penicillin, vancomycin, bacitracin, fosfomycin, cycloserine, and persistent mycin, act on different steps of peptidoglycan biosynthesis, inhibit peptidoglycan biosynthesis, and thus inhibit bacterial growth. ^[3]

Foreign medical circles have widely used ELISA to measure the titer of anti-peptidoglycan serum. This serum was used to identify the patient's disease. Studies have shown that peptidoglycan is an immune enhancer of the human immune system, and it can stimulate monocytes and endothelial cells to release immune regulatory substances. Such as tumor necrosis factor alpha (TNF-), interleukin (IL-1, IL-6, IL-8, IL-12) and interferon alpha. ^[4]