Peptidogycan

Introduction

Most gram-positive and gram-negative bacteria have a peptidoglycan in the cell envelope, which is 20-80 and 7-8 nm thick, respectively. The peptidoglycan is also called murein and is a major component of most bacteria's cell wall. The peptidoglycan is a polymer that forms a sparse mesh-like structure, which constitutes  a single large three-dimensional giant molecule that stabilizes the cell envelope of the bacteria.

Composition

The peptidoglycan is made up of two alternating carbohydrate molecules containing amino groups: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Each NAM molecule has an oligopeptide consisting of 4-5 amino acids linked to it. The oligopeptide from one peptidogycan chain is cross-linked to another oligopeptide from another peptidoglycan chain (see figure) via a so-called amide group (-CO-NH-). Amino acids (except glycine) can exist in two different isomeric forms, L-form and D-form, which are mirror images of each other. Living organisms usually contain only the L-form of amino acids, but the oligopeptide in the cell walls of bacteria consists of both the L- and D-form of the constituent amino acids as well as an amino acid (meso-diaminopimelic acid), which is not a part of ordinary proteins. Different species of bacteria may have slightly different composition of the oligopeptide.

Function

In addition to being a mechanical protection and a protection against osmotic lysis, the cell wall has an important function in cell division. The bacterium extends longitudinally before cell division and then the cell wal has to grow by synthesizing more of the peptidoglycan. When the bacterial cell then divides, the peptidoglycan must be reshaped to seal the two daughter cells.