Fig. 3

Revised pathway for sophorolipid (SL) biosynthesis by S. bombicola. A All genes, except for the extracellularly bound SBLE, are located in a biosynthetic gene cluster on the second chromosome. B In a first step, a fatty acid (either exogenous or endogenous) is hydroxylated on the terminal (ω) or subterminal (ω-1) position by a P450 monooxygenase CYP52M1 (1). The UDP-glucosyltransferases UGTA1 and UGTB1 consecutively attach one glucose to the free hydroxyl to form a sophorose unit linked to a fatty acyl chain, known as acidic SLs. Through the subsequent action of the same glucosyltransferases, a sophorose unit is attached on the carboxylic end to form a glycosidic ester linkage (2, 3). It must be noted that a simplified version of these glucosylation steps is displayed in this figure, since the exact order is still unknown. These bolaform SLs (bola SLs) can be acetylated to varying degrees (n = 1–4) (4). Through the action of an MDR transporter, bola SLs are secreted in the extracellular space (5). SBLE performs a transesterification reaction resulting in ring closure and the formation of (acetylated) lactonic SL with the release of (acetylated) sophorose (6). For low-acetylated bola SLs (n = 1–2), a hydrolysis route is the preferred mode of action of SBLE and (acetylated) acidic SLs are formed with the release of (acetylated) sophorose