The enzyme from Staphylococcus aureus catalyses the transfer of glycine from a charged tRNA to MurNAc-L-Ala-D-isoglutaminyl-L-Lys-D-Ala-D-Ala-diphosphoundecaprenyl-GlcNAc (lipid II), attaching it to the N6 of the L-Lys at position 3 of the pentapeptide. This is the first step in the synthesis of the pentaglycine interpeptide bridge that is used in S. aureus for the crosslinking of different glycan strands to each other. Four additional Gly residues are subsequently attached by EC 126.96.36.199 (N-acetylmuramoyl-L-alanyl-D-glutamyl-L-lysyl-(N6-glycyl)-D-alanyl-D-alanine-diphosphoundecaprenyl-N-acetylglucosamine:glycine glycyltransferase) and EC 188.8.131.52 (N-acetylmuramoyl-L-alanyl-D-glutamyl-L-lysyl-(N6-triglycine)-D-alanyl-D-alanine-diphosphoundecaprenyl-N-acetylglucosamine:glycine glycyltransferase).
FemX catalyzes the first step in the synthesis of the pentaglycine interpeptide bridge crosslinking different glycan strands in Staphylococcus aureus. FemX uses MurNAc-L-Ala-D-Glu-L-Lys-D-Ala-D-Ala-diphosphoundecaprenyl-M-acetylglucosamine, i.e. lipid II, exclusively as acceptor for the first glycine residue. Addition of glycine residues 2, 3 and glycine residues 4, 5 is catalyzed by enzymes FemA and FemB, respectively. None of the FemABX enzymes requires the presence of one or two of the other Fem proteins for activity, rather, bridge formation is delayed in an in vitro system when all 3 enzymes are present
surface protein is linked to tri- and monoglycyl cross-bridges of peptidoglycan isolated from femB and femA mutant staphylococci, respectively. No surface protein is found linked directly to the epsilon-amino group of lysyl within the cell wall of a femAX strain
in strains carrying mutations of FemA, femAB, or the femAX genes, the sorting reaction of surface proteins is significantly slowed. Strains carrying mutations in the fem genes display a decreased rate of surface protein precursor cleavage as compared with the wild-type strains, suggesting that the altered cross-bridges slow the anchoring of surface proteins