Figure 2.

A schematic model of the cell envelope of M. leprae. The plasma membrane is covered by a cell-wall core made of peptidoglycan (chains of alternating GlcNAc and MurNGly, linked by peptide crossbridges) covalently linked to the galactan by a linker unit (-P-GlcNAc-Rha-) of arabinogalactan. Three branched chains of arabinan are in turn linked to the galactan. The peptidoglycan-arabinogalactan layer forms the electron-dense zone. Mycolic acids are linked to the termini of the arabinan chains to form the inner leaflet of a pseudo lipid bilayer. An outer leaflet is formed by the mycolic acids of TMM and mycocerosoic acids of PDIMs and PGLs as indicated. The pseudo-bilayer forms the electron-transparent zone. A capsule presumably composed largely of PGLs and other molecules such as PDIMs, PIMs and phospholipids surrounds the bacterium. Lipoglycans such as PIMs, LM and LAM, known to be anchored in the plasma membrane, are also found in the capsular layer as shown. Abbreviations are as used in the text and Box 1.