Murein is a bacterial cell wall support biopolymer, also known as peptidoglucan. Murein is a heteropolymer (N-acetylglucosamine and N-acetylmuramic acid cross-linked through lactate residues with short peptide chains). As a determining substance for one of the three domains of living beings, of course, this polymer has its own structural and functional features. Let's try to sort them out.
Structure of a bacterial cell
Bacteria are a vast cluster of prokaryotic organisms. Their genetic apparatus is not enclosed in a nucleus separated by a membrane. Nevertheless, despite the evolutionary early appearance, these organisms have spread throughout all the environments of our planet. They can also live in oil fields, in the boiling water of geysers, in the cold waters of the northern oceans, in the stomach acids of animals. Resistance to negative environmental factors is achieved largely due to a special substance that forms the basis of the bacterial cell wall. The substance is murein.
A bacterial cell consists of 80-85% water, of the remaining 20%, as a rule, half are proteins, a fifth of RNA, 5% of DNA and some lipids. The cell wall accounts for 20% of the drysubstances (in some types of microorganisms even up to 50%). The thickness of this plate is about 0.01-0.045 micrometers.
Murein in the cell wall
The presence of a solid wall is characteristic not only for bacteria, but also for fungi and plants. However, only in prokaryotes it has a similar composition. The cell wall of a bacterium is a strong shell made of a complex murein polysaccharide molecule. The structure of a polypeptide consists of parallel polysaccharide chains interconnected by peptide residues. The modular unit is the disaccharide muropeptide (in it acetyl-D is connected to 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 with no gaps. The density of this wall is species-specific - in some species it is less dense (E. coli), in others it is more (Staphylococcus aureus).
In biology, murein is not just a polypeptide, but also its accompanying components of the bacterial cell wall. For example, Gram-positive bacteria also include polysaccharides, taichoic acids, proteins, or other polypeptides. Gram-negative bacteria have even more such inclusions. They are characterized by complex liposaccharides, lipoproteins, polypeptides.
The role of these substances in protection against bacteriophage viruses, as well as in protection against aggressive antibiotics and enzymes. Gram-positive bacteria have a fragile body. In Gram-negative bacteria due tothe 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 cell wall component found only in bacteria, structures similar to it also exist. For example, in the wall of some archaea (non-nuclear microorganisms that do not have organelle structures) and glaucocystophyte algae, pseudopeptidoglycan is formed. It performs the same functions and is similar in composition to murein.
Composition of murein, its structure
The structure is a cellular network formed by the components of n-acetylglucosamine and n-acetylmuramic acid. The bonds are formed by β1,4-glycosidic bonds. Cross-linking is carried out by means of peptide residues based on the action of the transpeptidase enzyme. Such a chain contains D-glutamic acid, L-lysine, D-alanine, L-alanine.
At the same time, the peculiarity 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, resistance and stability to the membrane.
Properties and functions
The properties of murein are determined by its structure. In addition to performing a mechanical and supporting function, it has antigenic properties. This determines its multifaceted protective role for bacteria.
One of the main functions of murein is to transport substances into and out of bacteria. This property determines the participationpeptidoglycan in eukaryotic chemo- and photosynthesis, 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.
At the same time, not only large molecules cannot bypass the cellular network of this substance. Murein is selectively permeable 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, which have a membrane structure and are 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 between the taxa of these microorganisms. In addition, according to the shape that murein gives to bacteria, we distinguish between their groups - cocci (round), rods, 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 done by detective staining.
Murein stability
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 the residues of acetylglucosamine and acetylmuramic acid, thereby causing hydrolysis of peptidoglucan and the death of bacterialcells.
Lysozyme is one of the enzymes in mammalian saliva, which determines its antibacterial properties. It also destroys the peptide chains of muroendopeptidase, thereby causing the destruction of the polymer. Created antibiotics (for example, penicillin, cephalosporin) disrupt the production of peptidoglycan. Cycloserine disrupts alanine synthesis.
In response to this exposure, bacteria respond to protection from antibiotics. Mutations in the genetic sequence responsible for the synthesis of lactamases, transpeptidase, lead to the emergence of strains that are resistant to antibiotics. Also, the evolutionary response of prokaryotes is a gradual change in the permeability of the membrane for cycloserine and other substances.
Murein in biology is a constantly 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.
