Peptidoglycan

Peptidoglycan (PGN ), peptidoglycans, and murein called (from the Latin murus = Wall, Wall, protection), rare polysaccharide peptides are composed of sugars and amino acids, macromolecules that occur in the cell wall of bacteria ( murein sacculus ). Both gram-positive and gram- negative bacteria possess in their cell wall strength -imparting layer of peptidoglycan. The thickness of the shell varies, in Gram-positive 20 to 80 nm, in the case of gram negative less than 10 nm exceptions are belonging to the domain of the bacteria Mycoplasma and spiro plasmas have no cell wall, and therefore no Mureinhülle and the Planctomycetes, a protein have the S-layer cell wall.

Construction

Peptidoglycans consist of two strands of the β (1 → 4) glycosidic linked together sugar derivative molecules N- acetylglucosamine and N- acetylmuramic acid ( see Figure 1), which form a linear chain molecules of the backbone. From every N- acetylmuramic acid molecule is - bound to its Lactylgruppe - an oligopeptide to an N- acetylmuramic acid molecule of an adjacent strand ( see Figure 2). The composition of Gram- positive and Gram-negative bacteria is not always uniform. As most gram-positive cocci having instead diaminopimelic acid (DAP ) is L- lysine. Sometimes, the amino acid at position 2 is still additionally hydroxylated. An important characteristic of the tetrapeptide is the presence of D-amino acids such as D- alanine and D-glutamic acid, which are formed by enzymatically racemases from the corresponding L- amino acids.

Networking

The parallel-arranged strands are crosslinked. For this compound, an enzyme is needed which the peptide chains can associate with each other as transpeptidase. This is also referred to as a transpeptidase penicillin -binding protein ( PBP ) as it is the point of the beta -lactam antibiotics. You can set up different, and therefore be resistant to these antibiotics.

In Escherichia coli and other gram- negative bacteria, the two tetrapeptides are directly connected ( Fig. 2a). Here, the amino group of diaminopimelic acid of a peptide is linked to the carboxyl group of the terminal D -alanine of the neighboring peptide.

Staphylococcus aureus and gram- positive bacteria inter peptide bridge of five Glycinmolekülen ( syn. Pentaglycinbrücke, pentapeptide bridge ) connects two tetrapeptides ( Fig. 2b).

Since the connecting piece from organism to organism, can contain different amino acids, this contributes to the fact that more than 100 different types of peptidoglycans are also known. The variance of the amino acid sequence is stronger in Gram-positive bacteria than Gram negatives. Only histidine, arginine or proline were not detected in a connector.

By cross-linking the murein forms an extensive network that spans the surface of the bacterial cell ( murein sacculus, see Figure 3). In all cases, the sugar backbone is constructed the same.

Figure 2b: Schematic representation of the murein layer of Staphylococcus aureus MurNAc = N- acetylmuramic acid; GlcNAc = N- acetylglucosamine

Figure 3: Schematic representation of the murein layer ( peptidoglycan ) of Gram-negative bacteria

Gram-positive and Gram-negative bacteria

A bacterium is surrounded by a single murein macromolecule. Gram-negative bacteria ( the Gram stain is negative verhaltend ) have a thin, single layer Mureinhülle, which accounts for about 5-10 % of the dry weight of the bacterial envelope. In gram- positive bacteria, the thicker cell wall of a multilayer Mureinnetz and teichoic acids has been established. The Mureinanteil here can be up to 50 % of the dry weight of the bacterial envelope, which assumes basic dyes by Gram staining.

Function, expansion and importance

The Mureinhülle keeps the bacteria protoplasts against osmotic pressure together. The murein layer injured or dissolved, for example by the enzyme lysozyme, bursts the bacterium. The growth of a bacterium which Mureinnetz therefore must be extended without a larger gap. Mureinbausteine ​​are synthesized in the cytoplasm and with the help of the lipid carrier Bactoprenol (see also: Transportation (membrane protein) ) exported. In the out- of cytoplasmic Mureinnetz bonds are broken in the backbone strands and in the oligopeptides by specific lytic enzymes localized and the pre-made and exported Mureinbausteine ​​be inserted by specific enzymes. The expansion of the murein therefore requires a precise interplay of various enzymes. If this interaction is disturbed, the bacterium bursts also. Some of the most important antibiotics, such as vancomycin, penicillin, and inhibit the development of the peptidoglycan.