Revision of the sophorolipid biosynthetic pathway in Starmerella bombicola based on new insights in the substrate profile of its lactone esterase

Abstract

Background: Sophorolipids (SLs) are a class of natural, biodegradable surfactants that found their way as ingredients for environment friendly cleaning products, cosmetics and nanotechnological applications. Large-scale production relies on fermentations using the yeast Starmerella bombicola that naturally produces high titers of SLs from renewable resources. The resulting product is typically an extracellular mixture of acidic and lactonic congeners. Previously, we identified an esterase, termed Starmerella bombicola lactone esterase (SBLE), believed to act as an extracellular reverse lactonase to directly use acidic SLs as substrate. Results: We here show based on newly available pure substrates, HPLC and mass spectrometric analysis, that the actual substrates of SBLE are in fact bola SLs, revealing that SBLE actually catalyzes an intramolecular transesterification reaction. Bola SLs contain a second sophorose attached to the fatty acyl group that acts as a leaving group during lactonization. Conclusions: The biosynthetic function by which the Starmerella bombicola ‘lactone esterase’ converts acidic SLs into lactonic SLs should be revised to a ‘transesterase’ where bola SL are the true intermediate. This insights paves the way for alternative engineering strategies to develop designer surfactants.