FIG 1.
Chemical structure and degradation pathway of colistin. The amino acid positions are numbered in accordance with the discussion in the text. The amino acid at position 6 is d-Phe in polymyxin B. Both colistin and PMB are mixtures of structurally related compounds. Fatty acid, 6-methyloctanoic acid for colistin A and PMB1 and 6-methylheptanoic acid for colistin B and PMB2; Dab, α-γ-diaminobutyric acid; Leu, leucine; Thr, threonine. The sites of cleavage by alkaline protease Apr are indicated by the arrows.