Fig. 1.

The PIM biosynthetic pathway. AcPIM2 and AcPIM6 are dominant PIM species, while Ac₂PIM2 and Ac₂PIM6 are relatively minor secondary products. Other PIM species are considered as biosynthetic intermediates and may exist in lower quantities. Each step is mediated via a mannosyltransferase or an acyltransferase. While PimE mediates the fifth mannose transfer, the enzymes that mediate the third, fourth, and sixth mannose transfer have not been identified. The acyltransferase-mediating conversion of monoacyl PIMs to diacyl PIMs has not been identified (indicated by question marks). Enzymes that mediate LM/LAM biosynthesis are not shown. Ac₂PIM2, diacyl phosphatidylinositol dimannoside; AcPIM2, monoacyl phosphatidylinositol dimannoside; LAM, lipoarabinomannan; LM, lipomannan; PIM, phosphatidylinositol mannoside.