Introduction

Cell Wall Biosynthesis

Peptidoglycan is a heteropolymer credited for being the structural component for the cell walls of both Gram-positive and Gram-negative Bacteria (Bugg & Walsh, 1992) . In addition to rigidity and structural integrity, peptidoglycan also has an important role in being an anchor for much of the cell envelope components (Bouhss et al., 2008) . The cell wall is a cross-linked matrix of polysaccharide chains, composed of alternating N-acetylmuramic acid (MurNAc) and N-acetylglucosamine (GlcNAc), with the crosslinking occurring between the pentapeptide side chains of opposite MurNAc residues, see figure 1(Bugg & Walsh, 1992) 

Figure 1: Schematic representation of peptidoglycan layer in Bacterial cell walls. The X in the pentapeptide layer structure is usually meso-diaminopimelate (m-DAP) in Gram-negative bacterial cell walls, whereas in Gram-positive bacteria a L-Lys is present.

There are many different steps involved in the formation of peptidoglycan and its lipid-linked intermediates, some taking place in the cytoplasm of the bacteria and others in the periplasm. However, it has been identified that there are 3 main stages, see stage 1 and stage 3 for extra information. The MraY Translocase is fundamental in the second stage, and so only this area will be explored.

Stage 2

The second stage of peptidoglycan biosynthesis involves the formation of a lipid linked disaccharide pentapeptide. The hydrophilic phospho-MurNAc-pentapeptide group from UDP-MurNAc needs to cross the cytoplasmic membrane to the periplasm where the peptidoglycan layer is situated. However, due to entropic restrictions, a hydrophilic group cannot simply diffuse through the hydrophobic membrane. In 1965, Stominger and Neuhaus laboratories demonstrated the link between the cytoplasmic and periplasmic steps of peptidoglycan biosynethesis is carried out by the translocation reaction of Phosphate-MurNAc-Pentapeptide onto an undecaprenyl phosphate carrier lipid (Bactoprenol C55-P) present in the cytoplasmic membrane (Bouhss et al., 2008) . This lipid structure is then capable of transporting the hydrophilic GlcNAc/MurNAc peptide structures across the hydrophobic environment, although this exact mechanism is not fully understood. The translocation of P-MurNAc-pentapeptide onto C55-P yielding undecaprenyl-pryophosphoryl-MurNAc pentapeptide (Lipid 1) is catalyzed by phospho-MurNAc-pentapeptide translocase (MraY Translocase), see figure 2 (White, Drummond & Fuqua, 2007) . The exact mechanism, function and structure of MraY translocase will be further explored within this webpage.

Figure 2: Combined cartoon and schematic membrane model of MraY. This diagram shows the orientation of the MraY enzyme in the membrane relative to the cytoplasmic and periplasmic side, with the different secondary structures highlighted: Alpha helix in red, Beta-sheets in yellow and loops in green. Furthermore, the translocation process is shown as a phospho-MurNAc-pentapeptide is added to C55-P.