Pseudopeptidoglycan (also known as
pseudomurein;
[White, David. (1995) The Physiology and Biochemistry of Prokaryotes, pages 6, 12-21. (Oxford: Oxford University Press). .] PPG hereafter) is a major
cell wall component of some
Archaea that differs from
peptidoglycan in chemical structure, but resembles bacterial peptidoglycan in function and physical structure. Pseudopeptidoglycan, in general, is only present in a few methanogenic archaea. The basic components are
N-acetylglucosamine and
N-acetyltalosaminuronic acid (bacterial peptidoglycan containing
N-acetylmuramic acid instead), which are linked by β-1,3-glycosidic bonds.
Lysozyme, a host defense mechanism present in human secretions (e.g. saliva and tears) breaks β-1,4-glycosidic bonds to degrade peptidoglycan. However, because pseudopeptidoglycan has β-1,3-glycosidic bonds, lysozyme is ineffective. It was thought from these large differences in cell wall chemistry that archaeal cell walls and bacterial cell walls have not evolved from a Common descent but are only the result of a convergent evolution,
No archaeal enzymes are known that cleave the β-1,3-glycosidic bonds in pseudopeptidoglycan, but it can be degraded by pseudomurein endoisopeptidase encoded by two .
Structure
Pseudopeptidoglycan is composed of two sugars,
N-acetylglucosamine and
N-acetyltalosaminuronic acid. These sugars are made of different
Carbohydrates, and the peptide cross-links within pseudopeptidoglycan are formed with different amino acids. The
peptide bond is formed between the
lysine of a
N-acetyltalosaminuronic acid and a
glutamine of a parallel
N-acetyltalosaminuronic acid.
Pseudopeptidoglycan, like peptidoglycan in bacteria, forms a mesh-like layer outside of the
plasma membrane of the archaea.
Function
Only a few methanogenic archaea have cell walls composed of pseudopeptidoglycan. This component functions much like peptidoglycan in a bacterial cell.
Pseudopeptidoglycan is used by the archaeal cell to determine its shape and provide structure to the cell. It is also used to protect the cell from undesired molecules or anything harmful in its environment.
Biosynthesis
PPG is produced by enzymes of two gene clusters. Recent work on the peptide ligases show, surprisingly, a common origin with murein synthesis. The pathway is now known to include the
-to-bacteria CarB,
MurC/D (peptide ligase),
MurG,
MraY,
UppP, UppS, and
flippase presumably performing an analogous function, and two novel but conserved transmembrane proteins.
GlmM and
GlmU, which produce
UDP-GlcNAc in bacteria, are also present with phosphoglucomutase (PGM). Half of the species also have MurT and GatD, known to perform cell wall modifications in bacteria. No orthologous cross-linking enzymes have been identified. Notably, "formation of the disaccharide moiety of the glycopeptide monomer occurs before the transfer to membrane protein by MraY", as opposed to after in bacteria. Further work would be needed to connect these information into a coherent pathway.
Effects of different bacterial medicines on pseudopeptidoglycan
Lysozyme
Lysozyme is a natural defense mechanism in humans that has the ability to break down peptidoglycan in bacterial cells. It degrades the peptidoglycan by targeting the β-1,4-glycosidic bonds that connect the alternating
Amino sugar in which it is composed of.
This degradation of the glycosidic bonds within peptidoglycan cause the sugars to separate and inhibit the structural integrity of the peptidoglycan and the bacteria.
Pseudopeptidoglycan, however, is composed of a different acidic amino sugar, which is N-acetyltalosaminuronic acid. This difference is the reason that it has β-1,3-glycosidic bonds (as opposed to the β-1,4-glycosidic bonds in bacteria).
Penicillin
Penicillin is a group of
Antibiotic that have been effective against many bacterial infections. It attacks bacteria by targeting and inhibiting the transpeptidase that catalyzes the cross-linking of the amino sugars in peptidoglycan.
However, pseudopeptidoglycan contains different amino sugars, and therefore, a different catalysis enzyme is used. The different amino acids cause antibiotics, that target cell walls like penicillin, to be ineffective against pseudopeptidoglycan.
Taxonomic distribution
PPG is found in the archaeal orders of Methanobacteriales and
Methanopyrales.
Some genera under these orders are:
See also
Further reading
