Mechanisms controlling lipid accumulation and polyunsaturated fatty acid synthesis in oleaginous fungi

Polyunsaturated fatty acids (PUFAs) are functional lipids that have been widely incorporated into several industrial sectors. Apart from animal- and plant-derived origins, oleaginous fungi belonging to Mucorales have been identified as promising alternatives for production of n-3 and n-6 PUFAs. It was found, in Mucorales fungi, that ATP:citrate lyase, acetyl-CoA carboxylase and malic enzyme trigger lipid overproduction, and biosynthesis of PUFA requires membrane-bound desaturases with fatty acyl substrate specificities. Accumulation of PUFAs in the cells is associated not only with the desaturation system, but it is also tightly bound with acyltransferases that facilitate the distribution of newly synthesized PUFA to individual lipid structures. Several physical parameters, such as temperature, aeration, and nutrient regimes, greatly affect either the lipid content or fatty acid composition among different Mucorales species. Conclusive evidence showed that the PUFA production yield of the fungi depends on the environmental control of “oleaginous” enzymes, and on the transcriptional expression of the desaturase genes. These valuable studies provide perspectives with biological rationale for microbial production of economically important lipids.

Regulation of lipid accumulation in oleaginous micro-organisms

A small number of eukaryotic micro-organisms, the oleaginous species, can accumulate triacylglycerols as cellular storage lipids, sometimes up to 70% of the biomass. Some of these lipids, particularly those containing high proportions of polyunsaturated fatty acids of nutritional and dietary importance, are now in commercial production; these are known as single-cell oils. The biochemistry of lipid accumulation has been investigated in yeasts and filamentous fungi and can now be described in some detail: lipid accumulation is triggered by cells exhausting nitrogen from the culture medium, but glucose continues to be assimilated. Activity of isocitrate dehydrogenase within the mitochondrion, however, now slows or even stops due to the diminution of AMP within the cells. This leads to the accumulation of citrate, which is transported into the cytosol and cleaved to acetyl-CoA by ATP:citrate lyase, an enzyme that does not occur in non-oleaginous species. This enzyme is therefore essential for lipid accumulation. The presence of this enzyme does not, however, explain why different species of oleaginous micro-organisms have different capacities for lipid accumulation. The extent of lipid accumulation is considered to be controlled by the activity of malic enzyme (ME), which acts as the sole source of NADPH for fatty acid synthase (FAS). If ME is inhibited, or genetically disabled, then lipid accumulation is very low. There is no general pool of NADPH which can otherwise be used by FAS. The stability of ME is therefore crucial and it is proposed that ME is physically attached to FAS as part of the lipogenic metabolon. ME activity correlates closely with lipid accumulation in two filamentous fungi, Mucor circinelloides and Mortierella alpina. When ME ceases to be active, lipid accumulation also stops. No other enzyme activity shows such a correlation.