The Biosynthesis,Activities And Applications Of Mannosylerythritol Lipids

Biosurfactants are mainly produced by a variety of microorganisms, and they are amphiphilic compounds secreted by microorganisms to regulate their metabolism. These extracellular compounds receive much concern due to their advantages over synthetic surfactants, including biodegradability, low or none toxicity, excellent surface-activity, and stability at various conditions. With the enhancement of the energy saving and environment protection consciousness, the trend that biosurfactants replace chemical synthetic surfactants becomes a reality. It is also an efficient way to achive bioconversions of the green renewable resources. Mannosylerythritol lipids (MELs) are glycolipid biosurfactants. They not only have excellent emulsifying activity, good stability at various conditions, and self-assembling properties, but also exhibit pharmacological activities. They have potential applications in food, cosmetic, medicine and agricultural fields. However, the MEL biosynthesis pathway has nt been clear so far. The industrialization of MELs is limited due to its low yield, complicated structure and the unclear activity mechanisms. Therefore, studies on the fermentation production of MELs and their activities were performed in this dissertation. The main contents and results in this study were showed as follows:(1) Pseudozyma aphidis is a potential strain for MELs biosynthesis. To enhance the productivity of MELs biosynthesis from vegetable oil by Pseudozyma aphidis DSM70725, Plackett-Burman design and response surface methodology were applied to optimize the submerged culture medium. The optimum compositions were developed and proved to possess a good stability through the verifications. The fermentation processes of MELs secretion by Pseudozyma aphidis DSM70725 from the optimal conditions and the non-optimal conditions were compared and analyzed. Furthermore, the products derived from the optimized culture medium were purified by solvent extraction and slica-gel column chromatography, and then identified by LC-MS, GC-MS and NMR strategies. The results revealed that MEL-Aaccounted for a ratio of 78.3%, and was the major types. They were incorporated medium and short fatty acid chains from C8 to C14 with no, one or two double bonds.(2) Sodium nitrate was the vital inorganic nitrogen source in the MELs metabolism by P. aphidis DSM70725. MELs and CLs were biosynthesized by P. aphidis DSM70725 under the restriction of nitrogen source. The combination method of RNA-Seq de novo analysis and digital gene expression spectrum was firstly used for analyzing the expression spectrum of MELs and CLs under the condition without sodium nitrate by P. aphidis DSM70725, and the differential genes and the key metabolism pathways involved in MEL and CL synthesis were summaried based on the GO and KEGG analysis. By comparison of the differential genes regarding the synthesis on MELs and CLs, the metabolism genes of MELs had a declined tendency.(3) The surface activity of MEL-A was stable on extreme temperature, high saline density and acid and alkaline conditions. In the water solution, this glycolipid could form an emulsion at nanometer level. The thermodynamically stable MEL-A/DLPC had been further studied. It showed that MEL-A/DLPC could be used for embedding hydrophobic compounds, such as betulinic acid, alpha linolenic acid and vitamin E. What's more, MEL-A was observed to induce B16 melanoma cell apoptosis, and the mechanism was associated with the endoplasmic reticulum stress (ERS) signaling pathway.