Structure and Function of Bacterial cell Component
Although a given bacterial species has a basic, characteristic PG structure, the PG layer remains in a dynamic state throughout a bacterium's life, and PG structure is the result of complex biosynthetic, maturation, and degradation reactions, which will be described below. Structural analysis of PG muropeptides using HPLC and mass spectrometry has allowed the identification of the nature of peptide cross-bridges, the degree of cross-linking, and the frequency of maturation and hydrolysis events. It has also revealed the existence of covalent PG modifications, such as -acetylation, -deacetylation, or amidation; these modifications may play essential roles in bacterial physiology. Detailed PG structure has been ascertained for several LAB, including L. lactis , L. casei , L. rhamnosus , and L. plantarum . The first three species were found to have D-Ala4-D-Asp/Asn-L-Lys3 cross-bridges, while the latter has a direct D-Ala4-mDAP3 cross-bridge (Figure ).
Biosynthesis as a multi-step process
PG synthesis can be divided in three general steps: the first step takes place in the cytoplasm and leads to the synthesis of lipid II, the second step involves the transfer of lipid II to the extracellular side of the membrane, and the third step results in the polymerization of the synthesized subunits into a macromolecule [
Schematic representation of the main steps of peptidoglycan and wall teichoic acid biosynthesis. Grey arrows denote the steps of PG biosynthesis, and brown arrows indicate the steps of WTA biosynthesis. The membrane-embedded undecaprenyl-phosphate carrier is represented by dark grey curved lines The glycerol-phosphate units are represented with green circles. The linkages formed by PBP and LCP are indicated with red arrows. The pre-existing PG is highlighted in gray. In the schematic, D-Asp is added to the lipid precursors; however, depending on the bacterial species, it may also be added to soluble precursors.
Assembly of lipid II starts with the synthesis of uridine diphosphate--acetyl glucosamine (UDP-GlcNAc) via the enzymatic conversion of glucosamine to energetically activated UDP-GlcNAc. UDP-MurNAc is then generated from UDP-GlcNAc, following two successive enzymatic reactions: the synthesis of enolpyruvate-UDP-GlcNAc and its subsequent reduction, which is catalyzed by MurA and MurB. Then, the UDP-MurNAc-pentapeptide precursor is assembled in a series of successive ATP-dependent enzymatic steps catalyzed by Mur ligases . MurC and Mur D catalyze the addition of L-Ala and D-Glu, respectively, and MurE the one of L-Lys or mDAP. Finally, in a single step, MurF adds two residues in the form of a dipeptide (D-Ala-D-Ala) or a depsipeptide (D-Ala-D-Lac), whose synthesis requires D-D-ligases (Ddl). Specific racemases convert the naturally occurring L-stereoisomer of Ala and Glu to the D-forms found in PG . In addition, in , which synthesizes precursors that terminate with D-Lac, D-Ala-D-Ala-dipeptidase (Aad) eliminates D-Ala-D-Ala dipeptides that are produced by the Ddl ligase, thereby preventing their incorporation into the precursors . PG precursors terminating with D-Ala-D-Lac instead of with D-Ala-D-Ala were successfully produced in when the Ddl ligase gene was heterologously expressed. Modification of the last residue of the stem peptides of PG precursors has been shown to result in significant changes to PG structure and cell morphology . The UDP-MurNAc-pentapeptide is then attached with a pyrophosphate link to the lipid transporter, bactoprenol (undecaprenyl-phosphate), by the membrane translocase MraY, a process that yields undecaprenyl-pyrophosphoryl-MurNAc-pentapeptide, or lipid I (Figure ). Finally, the glycosyl-transferase MurG adds GlcNAc to lipid I, forming undecaprenyl-pyrophosphoryl-disaccharide-pentapeptide, or lipid II, which is the basic subunit used in PG assembly .
Another important enzymatic step that takes place in the cytoplasm is the assembly of peptide side chains that are added either to the nucleotide MurNAc-pentapeptide or the lipid precursors, depending on the species . D-Asp, the amino acid most commonly included in LAB side chains and that is found in and in most lactobacilli, is added to the third amino acid (L-Lys) of the stem peptide by aspartate ligase (AslA) (Figure ) . This enzyme belongs to the ATP-Grasp family, which includes enzymes that catalyze ATP-dependent carboxylate-amine ligation reactions and that use activated D-Asp-in the form of β-aspartyl phosphate-as a substrate . D-Asp is produced from L-Asp by the aspartate racemase encoded by racD, which is located in the same operon as the aslA gene in [, ]. The L-amino acids of the PG side chains are transferred from aminoacyl-tRNA by specific transferases, identified as BppA1 and BppA2 in Enterococcus faecalis , a species that has L-Ala-L-Ala cross-bridges like S. thermophilus.
Lipid II (with or without a side chain) is then translocated outside the cytoplasmic membrane by a flippase (Figure ). The integral membrane protein FtsW has been shown to transport lipid-linked PG precursors across the membrane and is proposed to act at the septum level. The RodA homologous protein appears to be involved in lateral PG synthesis during cell elongation in ovococci and bacilli .