Linking of Linear Polymers Occurs Outside the Plasma Membrane

Cross-Linking of Linear Polymers Occurs Outside the Plasma Membrane 

The cross-linking of peptidoglycan strands takes place outside the cell membrane at the site of the preexisting wall. 

Because neither ATP nor any other obvious energy source is available there, a mechanism independent of any external energy source has evolved for this reaction. The reaction is a transpeptidation in which the terminal amino end of an open cross-bridge attacks the terminal peptide bond in an adjacent strand to form a cross-link. The terminal alanine residue is thus eliminated from the strand that becomes cross-linked (fig. 16.19). 

The second stage of cell wall synthesis. An ATP-requiring amidation of glutamic acid that occurs between steps 2 and 3 has been omitted. Points of action of the antibiotic inhibitors bacitracin and vancomycin are indicated. 

The structure of undecaprenol phosphate. Isoprene phosphates such as undecaprenol phosphate are important carriers and activators in the synthesis of oligosaccharides. Note the similarities to dolichol phosphate. 

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