N-Acetylmuramic acid (MURNAc) is part of the peptidoglycan polymer of Gram-positive bacterial cell walls. MURNAc is covalently linked to N-acetylglucosamine and may also be linked through the hyroxyl on carbon number 4 to the carbon of L-alanine (Figure 9.26). A pentapeptide composed of L-alanyl-d--isoglutaminyl-L-lysyl-D-alanine-D-alanine is added to the MURNAc in the process of making the peptidoglycan strands of the cell wall.