Oleaginous Yeasts: Biochemical Events Related with Lipid Synthesis and Potential Biotechnological Applications

The last years there has been a significant rise in the number of publications in the international literature that deal with the production of oils and fats deriving from microbial sources (the so called “single cell oils – SCOs”) that could be used as precursors for the synthesis of bio-diesel or as “tailor-made” lipids amenable for the replacement of expensive fatty materials found in the plant or animal kingdom [1,2]. These lipids are produced by the so-called “oleaginous” microorganisms (microorganisms principally belonging to yeasts, fungi and algae and to lesser extent bacteria, capable of storing quantities of lipids higher than 20%, wt/wt, in their dry weight) [1,3-5]. Remarkable differences in biochemical and kinetic level exist between the process of lipid accumulation when glucose or similarly metabolized compounds are used as substrates (“de novo” lipid synthesis) compared with that performed when hydrophobic materials are used as substrates (“ex novo” lipid synthesis). De novo lipid biosynthesis in the oleaginous microorganisms is non-growth associated process, conducted due to change of intra-cellular concentration of various metabolites after nitrogen depletion into the culture medium. Nitrogen exhaustion leads to a rapid decrease of the concentration of cellular AMP, which is further cleaved in order for nitrogen to be offered to the microorganism. Cellular AMP concentration decrease alters the Krebs cycle function; NAD + - (and in various cases NADP + isocitrate) dehydrogenase, allosterically activated by intracellular AMP, loses its activity and the carbon flow, hence, is directed towards the accumulation of intra-mitochondrial citric acid. When the concentration of citric acid inside the mitochondria becomes higher than a critical value, it is secreted inside the cytoplasm. Then, citric acid is cleaved by ATP-citrate lyase, enzyme-key showing the oleaginous character of the microorganisms, into acetyl-CoA and oxaloacetate and acetyl-CoA, by virtue of the action of fatty acid synthetase generates cellular fatty acids and subsequently triacylglycerols (TAGs), that are the most common form of lipophilic compounds found in the oleaginous microorganisms [1,3-5]. In the non-lipid producing microorganisms, nitrogen exhaustions provokes secretion of the previously hyper-synthesized citric acid into the growth medium (case of the fungus Aspergillus niger and many of the strains of the yeast Yarrowia lipolytica) or results in a block in the level of 6-phosphofructokinase (with mechanisms similar to the ones related with the decrease of activity of NAD