Advanced Microbial Physiology Lecture 2 Cell Wall Biosynthesis

Bacterial Cell Walls

Peptidoglycans of Cell Walls Peptidoglycan or murein is an enormous polymer composed of identical subunits It consists of long chains of repeating units of 2 sugar derivatives N-acetylglucosamine (NAG), and N-acetylmuramic acid (NAM) Sugar chains are cross-linked between tetrapetide chains originated from NAM

Most Gram negative bacteria lack the peptide interbridge made up by glycines Many Gram positive bacteria have the glycine interbridge

Synthesis of Peptidoglycan Murein biosynthesis involves a number of cytoplasmic, membrane and periplasmic steps NAG is first coupled with UDP A portion of UDP-NAG is converted into UDP- NAM, and the peptide chain is developed by sequential addition of amino acid NAM-pentapeptide binds to undecaprenyl- phosphate and merged with NAG of UDP-NAG Disaccharide-pentapeptide is adding to the growing chain of peptidoglycan chains

Gene Cluster for Cell Wall Syn

Penicillin-binding proteins Bacterial cells contain a variety of penicillin- binding proteins (PBPs) PBPs belong to the family of acyl serine transferases, which includes high-molecular- weight (HMW) PBPs, low-molecular-weight (LMW) PBPs, and b-lactamases. HMW PBPs are enzymes that are composed of two modules located on the outer surface of the cytoplasm membrane and anchored to the cytoplasmic membrane by an N-terminal, non- cleavable signal peptide.

Penicillin-binding proteins The C-terminal module is the penicillin-binding domain, which catalyzes the cross-linking of the peptidoglycan peptides HMW PBPs can be divided into 2 classes: Class A, possessing both transglycosylase and transpeptidase activity Class B, with only transpeptidase activity

Incorporation of New Cell Wall Rod-shaped bacteria have two modes of cell wall synthesis: New PG is inserted along a helical path, or New PG is inserted in a closing ring, leading to septum formation Rugby-ball-shaped Streptococcus elongate by inserting new cell wall at the equatorial rings, then forms division septum in the middle of the cell. Round cells do not seem to have an elongation mode of cell wall synthesis.

Proposed mode of insertion of new PG