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The Lipid Target

Mycobacterial mycolic acids are long-chain 3-hydroxy fatty acids, substituted at the 2-position with a long aliphatic chain. These acids are mainly esterified (Besra and Chatterjee 1994) to the pentarabinose termini of an arabinogalactan polysaccharide (Figure 1), which is in turn linked to a relatively conventional cell wall peptidoglycan (Minnikin 1982; Brennan and Nikaido 1995). In M. tuberculosis, there are three classes of mycolates, the so-called alpha -, methoxy- and ketomycolates (Figure 1). The omnipresent mycolates are considered to interact together in a co-operative manner, as indicated in Figure 1, to produce a coherent monolayer which lipophilically binds a variety of unusual waxes and glycolipids (Minnikin 1982; Brennan and Nikaido 1995). This can provide an effective outer membrane permeability barrier, which is probably responsible for resistance to antibiotics. Mycobacterial mycolates can be distinguished from those of other actinomycetes by their chain length (C70-C90), their long side-chains, their unsaturations, their additional oxygen functions and their methyl branches (Minnikin and Goodfellow 1980; Minnikin 1982; Minnikin et al. 1993; Dobson et al. 1985).


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