Everything about Murein totally explained
Not to be confused with glycoprotein.
Peptidoglycan, also known as
murein, is a
polymer consisting of sugars and amino acids that forms a mesh-like layer outside the
plasma membrane of
eubacteria. The sugar component consists of alternating residues of β-(1,4) linked
N-acetylglucosamine and
N-acetylmuramic acid residues. Attached to the N-acetylmuramic acid is a peptide chain 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. Some
Archaea have a similar layer of
pseudopeptidoglycan. Peptidoglycan serves a structural role in the bacterial
cell wall, giving structural strength, as well as counteracting the
osmotic pressure of the
cytoplasm. A common misconception is that peptidoglycan gives the cell its shape; however, whereas peptidoglycan helps maintain the structure of the cell, it's actually the
MreB protein that facilitates cell shape.
Peptidoglycan is also involved in
binary fission during bacterial cell reproduction.
The peptidoglycan layer is substantially thicker in
Gram-positive bacteria (20 to 80
nm) than in
Gram-negative bacteria (7 to 8 nm), with the attachment of the
S-layer. Peptidoglycan forms around 90% of the dry weight of Gram-positive bacteria but only 10% of Gram-negative strains. In Gram-positive strains, it's important in attachment roles and sterotyping purposes. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.
Antibiotic inhibition
Some antibacterial drugs such as
penicillin interfere with the production of peptidoglycan by binding to bacterial enzymes known as
penicillin-binding proteins or
transpeptidases Mutations in transpeptidases that lead to reduced interactions with an antibiotic are a significant source of emerging
antibiotic resistance.
Considered the human body's
own antibiotic,
lysozymes found in tears work by breaking the β-(1,4)-glycosidic bonds in peptidoglycan (see below) and thereby destroying many bacterial cells. Antibiotics such as penicillin commonly target bacterial cell wall formation (of which peptidoglycan is an important component) because animal cells don't have cell walls.
Structure
The peptidoglycan layer in the bacterial cell wall is a
crystal lattice structure formed from linear chains of two alternating amino
sugars, namely
N-acetylglucosamine (GlcNAc or NAG) and
N-acetylmuramic acid (MurNAc or NAM). The alternating sugars are connected by a β-(1,4)-
glycosidic bond. Each MurNAc is attached to a short (4- to 5-residue)
amino acid chain, normally containing D-alanine, D-glutamic acid, and mesodiaminopimelic acid. These three amino acids don't occur in proteins and are thought to help protect against attacks by most peptidases.
Cross-linking between
amino acids in different linear amino sugar chains by an enzyme known as
transpeptidase result in a 3-dimensional structure that's strong and rigid. The specific amino acid sequence and molecular structure vary with the bacterial
species.
Further Information
Get more info on 'Murein'.
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