Murein is a reference biopolymer of the bacterial cell wall, also known as peptidoglucan. Murein is a heteropolymer (N-acetylglucosamine and N-acetylmuramic acid, crosslinked through lactate residues by short chains of the peptide). As a determining substance for one of the three domains of living beings, of course, this polymer has its own structural features and functions. Let's try to parse them.
The structure of the bacterial cell
Bacteria are an extensive cluster of prokaryotic organisms. Their genetic apparatus is not enclosed in a membrane-separated core. However, despite their evolutionarily early appearance, these organisms have spread throughout all the environments of our planet. They can live in oil fields, in boiling water of geysers, in the cold waters of the northern oceans, in the acids of the stomach of animals. Resistance to negative environmental factors is achieved largely due to the special substance that forms the basis of the bacterial cell wall. The substance is murein.
The bacterial cell consists of 80-85% of water, of the remaining 20%, as a rule, half are proteins, a fifth of RNA, 5% DNA and some lipids. The cell wall accounts for 20% of dry matter (in some types of microorganisms, even up to 50%). The thickness of this plate is about 0.01-0.045 micrometers.
Cell wall murein
The presence of a solid wall is characteristic not only for bacteria, but also for fungi and plants. However, only in prokaryotes does it have a similar composition. The cell wall of the bacterium is a strong carapace of a complex molecule of the murein polysaccharide. The structure of the polypeptide is a parallel polysaccharide chains linked by peptide residues. The unit unit is the muropeptide disaccharide (in it acetyl-D is combined with acetylmuramic acid).
The main feature that determines the properties of the bag formed by murein is the presence of a closed network of polysaccharide chains. This forms a dense network without breaks. The density of this wall is species-specific - in some species it is less dense (E. coli), in others it is more dense (Staphylococcus aureus).
In biology, murein is not just a polypeptide, but also the accompanying components of the bacterial cell wall. For example, in gram-positive bacteria, polysaccharides, taichoic acids, proteins or other polypeptides are also included. Gram-negative bacteria have even more such inclusions. They are characterized by complex liposaccharides, lipoproteins, polypeptides.
The role of these substances in protecting against bacteriophage viruses, as well as in protecting against aggressive antibiotics and enzymes. Gram-positive bacteria have a fragile body. In gram-negative bacteria , due to the presence of a large number of additional inclusions, the murein skeleton is covered with a soft protective membrane of lipids.
Types of Peptidoglycan
Although murein is a component of the cell wall that is characteristic only of bacteria, structures similar to it also exist. For example, pseudopeptidoglycan is formed in the wall of some archaea (non-nuclear microorganisms without organelle structures) and glaucocystophytic algae. It performs the same functions and is similar in composition to murein.
The composition of murein, its structure
The structure is a mesh network formed by components of n-acetylglucosamine and n-acetylmuramic acid. The bonds are formed due to β1,4-glycosidic bonds. Crosslinking is done by peptide residues based on the action of the transpeptidase enzyme. Such a chain contains D-glutamic acid, L-lysine, D-alanine, L-alanine.
Moreover, this feature is that such D-structures are found only in prokaryotic cells. Thus, the formed polypeptide takes the form of a three-dimensional structure that forms the basis of the bacterial cell wall. It provides strength, stability and stability to the membrane.
Features and Functions
The properties of murein are due to its structure. In addition to performing mechanical and supporting functions, it has antigenic properties. This leads to its multifaceted protective role for bacteria.
One of the main functions of murein is the transport of substances into the bacterium and their removal to the outside. This property determines the participation of peptidoglycan in the chemo- and photosynthesis of eukaryotes, nitrogen fixation, and other important processes. All of them are associated with the interaction of the cell and the environment, which is provided by the cell wall.
Moreover, not only large molecules can not pass the mesh network of this substance. Murein has selective permeability with respect to, for example, antibiotic agents. This property arises in the process of evolution and artificial selection on the part of man.
The participation of this structure in cell movement is associated with the presence of villi and flagella with a membrane structure and tightly connected with the murein sac.
The composition of the peptide chains that make up the peptidoglycan is a systematic feature and helps to distinguish the taxa of these microorganisms. In addition, according to the form that bacteria gives to murein, we distinguish their groups - cocci (round), sticks, spirochetes, etc.
The quantity and quality of additional inclusions in the structure of the cell wall determines two large clusters of microorganisms: gram-positive and gram-negative bacteria. Separation is carried out by detecting staining.
Murein resistance
Since murein is part of the bacterial cell wall, it is a signaling substance for the immune system of both humans and other organisms. For example, the enzyme lysozyme cleaves beta 1,4-glycosidic bonds between residues of acetylglucosamine and acetylmuramic acid, thereby causing the hydrolysis of peptidoglucan and the death of a bacterial cell.
Lysozyme is one of the enzymes of the saliva of mammals, which determines its antibacterial properties. Also destroys the peptide chains of muroendopeptidase, thereby causing the destruction of the polymer. Antibiotics created (e.g., penicillin, cephalosporin) disrupt the production of peptidoglycan. Cycloserine disrupts the synthesis of alanine.
In response to this effect, bacteria respond to protection against antibiotics. Mutation in the genetic sequence responsible for the synthesis of lactamases, transpeptidases lead to the emergence of strains resistant to antibiotics. Also, the evolutionary response of prokaryotes consists in a gradual change in the permeability of the membrane for cycloserine and other substances.
Murein in biology is an ever-changing system. This explains the constant race "antibiotics-new strains of bacteria", where the receipt of new active drugs is inevitably associated with a gradual decrease in their activity.