Suppression of Peptidoglycan Remodeling
By [Frank Zhao]
Peptidoglycan is a polymer made of sugars and amino acids that forms a layer outside the plasma membrane of bacterial cells. Peptidoglycans are cross-linked by peptides, serving as the cell wall that protects bacteria from the environment. Peptidoglycan limits the volume of the cell and thus it generates turgor pressure as the water rushes into the cell. Gram-positive bacteria obtain a thicker peptidoglycan layer than Gram-negative bacteria but this layer is crucial to the survival of both types of bacteria.
Many antibiotics target this component to kill the bacteria. For example, penicillin binds to penicillin-binding proteins and inhibits the synthesis of peptidoglycan, weakening the cell wall of bacteria. It has been discovered that a new mechanism could be used by antibiotics on peptidoglycan— blocking the action of autolysins, a peptidoglycan hydrolase that is essential for remodeling the bacterial cell wall during growth. Studying how this new mechanism works could give us insight into developing new antibiotics against resistant strains.
Structure and Function
The peptidoglycan layer is located in the bacterial cell wall, consisting of chains of two alternating amino sugars, N-acetylglucosamine (an amide derivative of the monosaccharide glucose) and N-acetylmuramic acid (a combination of N-acetylglucosamine and phosphoenolpyruvate). The alternating amino sugars are linked by Teichoic acids, which are chains of phosphodiester-linked glycerol or ribitol. Parallel chains of amino sugars can also be connected by cross-bridges made of extensions of peptides to form tetrapeptides, with m-Diaminopimelic acid binding D-Alanine.
Peptidoglycan sacculi cross-linked by peptides form a closed, sac-shaped structure that surrounds the cytoplasmic membrane. Peptidoglycan sacculi have been purified from Escherichia coli by incubation with boiling 4% sodium dodecyl sulfate. While a bacterial cell contains a significant number of different kinds of molecules, there is only one peptidoglycan molecule in each bacterial cell, which contributes to 0.8% of the mass of the whole cell.
The content of peptidoglycan sacculi could be different among bacterial species. In Gram-positive bacteria, such as Staphylococcus aureus, the tetrapeptide cross-bridges are made of L-alanine, D-glutamine, L-lysine, and D-alanine. In Gram-negative bacteria, on the other hand, the cross-bridges are constituted of L-alanine, D-glutamic acid, m-diaminopimelic acid, and D-alanine. Gram-positive bacteria typically have many layers (3-20) of peptidoglycan chains outside the cytoplasmic membrane, whereas Gram-negative bacteria often have one or two layers of peptidoglycan between the inner and outer membrane.
Peptidoglycan plays a crucial role in the survival and reproduction of bacteria. It is the main component of the bacterial cell wall, forming a strong net-like structure that maintains the shape of the cell. Since bacteria are unicellular organisms, it is extremely important to protect the only cell from the outside environment. When the bacteria experience a significant osmotic pressure difference across the cell membrane, the cell envelope could prevent the cell from shrinking or expanding too much so that it lyses and dies.
Peptidoglycan is also associated with binary fission, a reproductive process in which one bacterium divides into two bacteria. To successfully divide one cell into two, the peptidoglycan must grow as the bacterium elongates and quickly enclose both cells after division so that both of the progeny cells have complete cell envelopes.
First, glutamine would transfer an amino group to fructose 6-phosphate, producing glucosamine-6-phosphate. Then, acetyl CoA would donate an acetyl group to the amino group on fructose 6-phosphate to make N-acetyl-glucosamine-6-phosphate, which would then turn into N-acetyl-glucosamine-1-phosphate, a monophosphate. After the reaction between N-acetyl-glucosamine-1-phosphate and Uridine triphosphate, an inorganic pyrophosphate is substituted with N-acetyl-glucosamine-1-phosphate, producing UDP-N-acetylglucosamine.
Following the creation of the precursor for the N-acetylglucosamine in peptidoglycan, a lactyl group would be added to the glucosamine which converts UDP-N-acetylglucosamine into UDP-N-acetylmuramic acid. A phosphate from the alpha carbon of phosphoenolpyruvate would also be given off by the C3 hydroxyl group, creating an enol derivative that would later be reduced by NADPH. By adding five amino acids including D-alanyl-D-alanine, UDP-N-acetylmuramic acid can turn into a pentapeptide, completing the process of producing a precursor for N-acetylmuramic acid.
In the cytoplasmic membrane, a lipid carrier named bactoprenol carries peptidoglycan precursors across the cell membrane. The mechanism behind this transport remains elusive, but precursors would be added to peptidoglycan once they get out of the cell membrane.
Inhibitive Antibiotics Targeting Peptidoglycan
Corbomycin and Complestatin
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