Question

How the different polymers of the cell wall are synthesised and incorporated into the cell wall.

Answer 1

Ans.The cell wall protects bacteria against osmotic lysis.bacteria cell wall almost always are bound by a chemically complex cell wall .The cell wall chemically composed of peptidoglycan. (Murein).

Peptidoglycan are unique to bacterial cells.

Two types.1in gram positive bacteria. Peptidoglycan single 20-80 nm thick homogeneous layer lying outside the plasma membrane.

2 in gram negative bacteria cell wall consists 2 to nm thick peptidoglycan layer cover by a 7to8 nm thick outer membrane.

Peptidoglycan is a polymer containing two sugar derivatives N-acetylglucosamine (NAG ) and N-acetylmuramic acid (NAM).joined through beta -1,4 glycosidic bond.

NAM is NAG with lactic acid attached by ether linkage.

The tetrapeptides side chain is composed of alternatinD-- and L - amino acid.Amino acid persent in tetrapeptides include L - alanine, D - alanine, D - glutamic acid,either L - lysine or meso -diamino -pimelic acid (DAP).Two tetrapeptides chains lying side by side may be linked together directly to each other or indirectly by a peptide intebridge, consisting of a short chain of amino acid.the reactions that forms the peptides cross links during peptidoglycan synthesis is called transpeptidation.

gram positive bacteria have a acidic substance called teichoic acid in their cell wall .teichoic acids are polyol phosphate polymers .They can be covalently linked to NAM of the peptidoglycan layer or to the lips of the plasma membrane. Teichoic acid and lipids are referred to as lipoteichoic acids. But teichoic acid are absent in gram negative bacteria. .

The biosynthesis of peptidoglycan can be devided into the following stages. The first stage occurs in the cytoplasm and leads to the synthesis of the nucleotide sugar linked precursors UDP -NAM and UDP -NAG.

In the second stages, sequential addition of amino acids to UDP -NAM occurs which results into the formation of NAM- pentapeptide ,

In the third stages. Which takes at the cytoplasmic membrane, the UDP NAM -pentapeptide make a complex with bactoprenol and forms lipid 1.NAG from UDP -NAG is added to lipid1 and lipid 2.

Fourth stages of peptidoglycan biosynthesis, outer side of the cytoplasmicmembrane, involves the polymerization of the newly synthesized disaccharide -peptide units and incorporation into the growing peptidoglycan.