- Gram Positive vs. Gram Negative Cell WallRecall that for a gram positive bacteria, the peptidoglycan layer is very thick, and therefore there are many peptidoglycan layers sandwiched together in a gram positive cell wall.
- Recall that for a gram negative bacteria, the peptidoglycan layer is very thin, so there may only be one layer of peptidoglycan in a gram negative cell wall.
- Antibiotic Inhibitors of Gram Positive Cell WallLet's assume we are talking about a gram positive cell with a thick peptidoglycan layer because antibiotics that inhibit cell wall synthesis work on gram positive bacteria.
- What is Peptidoglycan?Recall that the bacterial cell wall in the domain Bacteria is composed of a rigid, tight-knit molecular complex called peptidoglycan.
, also known as murein
, is a vast polymer consisting of interlocking chains
of identical peptidoglycan monomers. It functions to prevent bacterial osmotic lysis
- Peptidoglycan Synthesis: The Complete ProcessIn order for bacteria to divide by binary fission and increase their size following division, links in the peptidoglycan must be broken, new peptidoglycan monomers must be inserted, and the peptide cross links must be resealed.
- A group of bacterial enzymes called autolysins break the glycosidic bonds between the peptidoglycan monomers at the point of growth along the existing peptidoglycan.
- Autolysins also break the peptide cross-bridges that link the rows of sugar together. In this way, new peptidoglycan monomers can be inserted to enable bacterial growth.
are synthesized in the cytosol of the bacterium
where they attach to a membrane carrier molecule called bactoprenol
. The bactoprenols
, which are similar to a "lipid anchors," transport the peptidoglycan monomers across the cytoplasmic membrane and helps insert them into the growing peptidoglycan chains
- 1) Make peptidoglycan monomersFirst, N-acetylglucosamine (NAG) links up with uridine diphosphate (UDP) to form UDP-NAG. Some of the NAG is enzymatically converted to N-acetylmuramic acid (NAM) forming UDP-NAM.
- 2) Alanine Racemase StepFive amino acids are sequentially added to the UDP- NAM forming a pentapeptide. The last two are D-alanine molecules enzymatically produced from L-alanine, the usual form of the amino acid.
-pentapeptide is attached to the bactoprenol
carrier molecule in the cytoplasmic membrane, the energy being supplied by one of the high-energy phosphate groups of the UDP.
is attached to the NAM
-pentapeptide on the bactoprenol
to complete the peptidoglycan monomer.
- 3) Bactoprenols help Transportation & InsertionBactoprenols then insert the peptidoglycan monomers into the breaks in the peptidoglycan at the growing point of the cell wall.
- 4) Transglycosylation Step
- Transglycosidase enzymes catalize the formation of glycosidic bonds between the NAM and NAG of the peptidoglycan momomers and the NAG and NAM of the existing peptidoglycan.
- 5) Transpeptidation or Cross-Linking Step
- Finally, transpeptidase enzymes reform the peptide cross-links between the rows and layers of peptidoglycan to make the wall strong.