Peptidoglycan


Peptidoglycan, murein or mucopeptide is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid. Attached to the N-acetylmuramic acid is an oligopeptide chain made of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Peptidoglycan serves a structural role in the bacterial cell wall, giving structural strength, as well as counteracting the osmotic pressure of the cytoplasm. This repetitive linking results in a dense peptidoglycan layer which is critical for maintaining cell form and withstanding high osmotic pressures, and it is regularly replaced by peptidoglycan production. Peptidoglycan hydrolysis and synthesis are two processes that must occur in order for cells to grow and multiply, a technique carried out in three stages: clipping of current material, insertion of new material, and re-crosslinking of existing material to new material.
The peptidoglycan layer is substantially thicker in gram-positive bacteria than in gram-negative bacteria. Depending on pH growth conditions, the peptidoglycan forms around 40 to 90% of the cell wall's dry weight of gram-positive bacteria but only around 10% of gram-negative strains. Thus, presence of high levels of peptidoglycan is the primary determinant of the characterisation of bacteria as gram-positive. In gram-positive strains, it is important in attachment roles and serotyping purposes. For both gram-positive and gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.
It is difficult to tell whether an organism is gram-positive or gram-negative using a microscope; Gram staining, created by Hans Christian Gram in 1884, is required. The bacteria are stained with the dyes crystal violet and safranin. Gram positive cells are purple after staining, while Gram negative cells stain pink.

Structure

The peptidoglycan layer within the bacterial cell wall is a crystal lattice structure formed from linear chains of two alternating amino sugars, namely N-acetylglucosamine and N-acetylmuramic acid. The alternating sugars are connected by a β--glycosidic bond. Each MurNAc is attached to a short amino acid chain, containing L-alanine, D-glutamic acid, meso-diaminopimelic acid, and D-alanine in the case of Escherichia coli ; or L-alanine, D-glutamine, L-lysine, and D-alanine with a 5-glycine interbridge between tetrapeptides in the case of Staphylococcus aureus. Peptidoglycan is one of the most important sources of D-amino acids in nature.
By enclosing the inner membrane, the peptidoglycan layer protects the cell from lysis caused by the turgor pressure of the cell. When the cell wall grows, it retains its shape throughout its life, so a rod shape will remain a rod shape, and a spherical shape will remain a spherical shape for life. This happens because the freshly added septal material of synthesis transforms into a hemispherical wall for the offspring cells.
Cross-linking between amino acids in different linear amino sugar chains occurs with the help of the enzyme DD-transpeptidase and results in a 3-dimensional structure that is strong and rigid. The specific amino acid sequence and molecular structure vary with the bacterial species.
The different peptidoglycan types of bacterial cell walls and their taxonomic implications have been described. Archaea do not contain peptidoglycan. Some Archaea contain pseudopeptidoglycan.


Peptidoglycan is involved in binary fission during bacterial cell reproduction. L-form bacteria and mycoplasmas, both lacking peptidoglycan cell walls, do not proliferate by binary fission, but by a budding mechanism.
In the course of early evolution, the successive development of boundaries protecting first structures of life against their environment must have been essential for the formation of the first cells.
The invention of rigid peptidoglycan cell walls in bacteria was probably the prerequisite for their survival, extensive radiation and colonisation of virtually all habitats of the geosphere and hydrosphere.

Biosynthesis

The peptidoglycan monomers are synthesized in the cytosol and are then attached to a membrane carrier bactoprenol. Bactoprenol transports peptidoglycan monomers across the cell membrane where they are inserted into the existing peptidoglycan.
  1. In the first step of peptidoglycan synthesis, glutamine, which is an amino acid, donates an amino group to a sugar, fructose 6-phosphate. This reaction, catalyzed by EC 2.6.1.16, turns fructose 6-phosphate into glucosamine-6-phosphate.
  2. In step two, an acetyl group is transferred from acetyl CoA to the amino group on the glucosamine-6-phosphate creating N-acetyl-glucosamine-6-phosphate. This reaction is EC 5.4.2.10, catalyzed by GlmM.
  3. In step three of the synthesis process, the N-acetyl-glucosamine-6-phosphate is isomerized, which will change N-acetyl-glucosamine-6-phosphate to N-acetyl-glucosamine-1-phosphate. This is EC 2.3.1.157, catalyzed by GlmU.
  4. In step 4, the N-acetyl-glucosamine-1-phosphate, which is now a monophosphate, attacks UTP. Uridine triphosphate, which is a pyrimidine nucleotide, has the ability to act as an energy source. In this particular reaction, after the monophosphate has attacked the UTP, an inorganic pyrophosphate is given off and is replaced by the monophosphate, creating UDP-N-acetylglucosamine. This initial stage, is used to create the precursor for the NAG in peptidoglycan. This is EC 2.7.7.23, also catalyzed by GlmU, which is a bifunctional enzyme.
  5. In step 5, some of the UDP-N-acetylglucosamine is converted to UDP-MurNAc by the addition of a lactyl group to the glucosamine. Also in this reaction, the C3 hydroxyl group will remove a phosphate from the alpha carbon of phosphoenolpyruvate. This creates what is called an enol derivative. EC 2.5.1.7, catalyzed by MurA.
  6. In step 6, the enol is reduced to a "lactyl moiety" by NADPH in step six. EC 1.3.1.98, catalyzed by MurB.
  7. In step 7, the UDP–MurNAc is converted to UDP-MurNAc pentapeptide by the addition of five amino acids, usually including the dipeptide D-alanyl-D-alanine. This is a string of three reactions: EC 6.3.2.8 by MurC, EC 6.3.2.9 by MurD, and EC 6.3.2.13 by MurE.
Each of these reactions requires the energy source ATP. This is all referred to as Stage one.
Stage two occurs in the cytoplasmic membrane. It is in the membrane where a lipid carrier called bactoprenol carries peptidoglycan precursors through the cell membrane.
  1. Undecaprenyl phosphate will attack the UDP-MurNAc penta, creating a PP-MurNac penta, which is now a lipid. EC 2.7.8.13 by MraY.
  2. UDP-GlcNAc is then transported to MurNAc, creating Lipid-PP-MurNAc penta-GlcNAc, a disaccharide, also a precursor to peptidoglycan. EC 2.4.1.227 by MurG.
  3. Lipid II is transported across the membrane by flippase, a discovery made in 2014 after decades of searching. Once it is there, it is added to the growing glycan chain by the enzyme peptidoglycan glycosyltransferase. This reaction is known as transglycosylation. In the reaction, the hydroxyl group of the GlcNAc will attach to the MurNAc in the glycan, which will displace the lipid-PP from the glycan chain.
  4. In a final step, the DD-transpeptidase crosslinks individual glycan chains. This protein is also known as the penicillin-binding protein. Some versions of the enzyme also performs the glycosyltransferase function, while others leave the job to a separate enzyme.

    Pseudopeptidoglycan

In some archaea, i.e. members of the Methanobacteriales and in the genus Methanopyrus, pseudopeptidoglycan has been found. In pseudopeptidoglycan the sugar residues are β- linked N-acetylglucosamine and N-acetyltalosaminuronic acid. This makes the cell walls of such archaea insensitive to lysozyme. The biosynthesis of pseudopeptidoglycan has been described.

Recognition by immune system

Peptidoglycan recognition is an evolutionarily conserved process. The overall structure is similar between bacterial species, but various modifications can increase the diversity. These include modifications of the length of sugar polymers, modifications in the sugar structures, variations in cross-linking or substitutions of amino acids. The aim of these modifications is to alter the properties of the cell wall, which plays a vital role in pathogenesis.
Peptidoglycans can be degraded by several enzymes, producing immunostimulatory fragments that are critical for mediating host-pathogen interactions. These include muramyl dipeptide, N-acetylglucosamine, or γ-d-glutamyl-meso-diaminopimelic acid.
Peptidoglycan from intestinal bacteria crosses the intestinal barrier even under physiological conditions. Mechanisms through which peptidoglycan or its fragments enter the host cells can be direct or indirect, and they are either bacteria-mediated or host cell-mediated. Bacterial secretion systems are protein complexes used for the delivery of virulence factors across the bacterial cell envelope to the exterior environment. Intracellular bacterial pathogens invade eukaryotic cells, or bacteria may be engulfed by phagocytes. The bacteria-containing phagosome may then fuse with endosomes and lysosomes, leading to degradation of bacteria and generation of polymeric peptidoglycan fragments and muropeptides.

Receptors

senses intact peptidoglycan and peptidoglycan fragments using numerous PRRs that are secreted, expressed intracellularly or expressed on the cell surface.

Peptidoglycan recognition proteins

are conserved from insects to mammals. Mammals produce four secreted soluble peptidoglycan recognition proteins that recognize muramyl pentapeptide or tetrapeptide. They can also bind to LPS and other molecules by using binding sites outside of the peptidoglycan-binding groove. After recognition of peptidoglycan, PGLYRPs activate polyphenol oxidase molecules, Toll, or immune deficiency signalling pathways. That leads to production of antimicrobial peptides.
Each of the mammalian PGLYRPs display unique tissue expression patterns. PGLYRP-1 is mainly expressed in the granules of neutrophils and eosinophils. PGLYRP-3 and 4 are expressed by several tissues such as skin, sweat glands, eyes or the intestinal tract. PGLYRP-1, 3 and 4 form disulphide-linked homodimers and heterodimers essential for their bactericidal activity. Their binding to bacterial cell wall peptidoglycans can induce bacterial cell death by interaction with various bacterial transcriptional regulatory proteins. PGLYRPs are likely to assist in bacterial killing by cooperating with other PRRs to enhance recognition of bacteria by phagocytes.
PGLYRP-2 is primarily expressed by the liver and secreted into the circulation. Also, its expression can be induced in skin keratinocytes, oral and intestinal epithelial cells. In contrast with the other PGLYRPs, PGLYRP-2 has no direct bactericidal activity. It possesses peptidoglycan amidase activity, it hydrolyses the lactyl-amide bond between the MurNAc and the first amino acid of the stem peptide of peptidoglycan. It is proposed, that the function of PGLYRP-2 is to prevent over-activation of the immune system and inflammation-induced tissue damage in response to NOD2 ligands, as these muropeptides can no longer be recognized by NOD2 upon separation of the peptide component from MurNAc. Growing evidence suggests that peptidoglycan recognition protein family members play a dominant role in the tolerance of intestinal epithelial cells toward the commensal microbiota. It has been demonstrated that expression of PGLYRP-2 and 4 can influence the composition of the intestinal microbiota.
Recently, it has been discovered, that PGLYRPs are highly expressed in the developing mouse brain. PGLYRP-2 and is highly expressed in neurons of several brain regions including the prefrontal cortex, hippocampus, and cerebellum, thus indicating potential direct effects of peptidoglycan on neurons. PGLYRP-2 is highly expressed also in the cerebral cortex of young children, but not in most adult cortical tissues. PGLYRP-1 is also expressed in the brain and continues to be expressed into adulthood.