# Microbial Biosynthesis of Lactones: Gaps and Opportunities towards Sustainable Production
## 3 Pathways and Enzymes Involved in the Biosynthesis of Lactones
### Sporidiobolus salmonicolor
poae_ & [34] \\  & 12756 & C\({}_{7}\)COcount & & _S. salmonicolor_ & [40] \\  & 12756 & C\({}_{7}\)COcount & & _S. salmonicolor_ & [40] \\  & 12756 & C\({}_{7}\)COcount & & _S. salmonicolor_ & [40] \\  & 12756 & C\({}_{7}\)COcount & & _A. gossypii_ & [23] \\  & 12756 & C\({}_{7}\)COcount & & _T. viride_ & [12] \\ \hline \(\gamma\)-decalactone (6) & 12813 & C\({}_{10}\)H\({}_{18}\)O\({}_{2}\) & Fruity, peach & _S. salmonicolor_ & [38] \\  & 12756 & C\({}_{7}\)COcount & & _F. poae_ & [34] \\  & 12756 & C\({}_{7}\)COcount & & _A. gossypii_ & [23] \\  & 12756 & C\({}_{7}\)COcount & & _T. viride_ & [12] \\  & 12756 & C\({}_{7}\)COcount & & _T. vide_ & [34] \\  & 12756 & C\({}_{7  
production, it was clear that the enzymatic hydroxylation of oleic acid (C18:1) at carbon-12 occurred before four cycles of \(\beta\)-oxidation and lactonian. Alternative routes for the biosynthesis of the other lactones start with the desaturation of oleic acid, leading to the formation of linoleic acid. From oleic or linoleic acid, a 10-hydratase, a 9,10-epoxygenase and/or a 10-lipoxygenase should be responsible for the hydroxy fatty acids that would generate \(\gamma\)-dodecalactone and cis-6-y-dodecenolactone [44]. An epoxide pathway is also possibly present in the yeast, which for instance is able to epoxidize oleic and linoleic acid in the formation of \(\gamma\)-dodecalactone and cis-6- \(\gamma\)-dodecenolactone [45]. Indirect evidence of the presence of epoxyhydrolase in this yeast was later reported, with epoxy fatty acids derived from oleic acid being converted into \(\gamma\)-lactones [46]. Unlike _S. salmonicolor, Sporidiobolus ruinenii_ is not able to de novo biosynthesise lactones. This species, together with _S. salmonicolor_, was used to study the conversion of methyl ricinoleate into \(\gamma\)-decalactone [47]. _S. salmonicolor_ produced only \(\gamma\)-decalactone from this precursor, while _S. ruinenii_ produced both \(\gamma\)-decalactone and the corresponding free acid, 4-hydroxydecanoic acid. Since lactonian is assumed to occur spontaneously in acidic conditions, the pH of the productions was controlled to exclude any interference from the media. Nonetheless, it was not demonstrated whether the free acid was a precursor or a result of the \(\gamma\)-decalactone catabolism. Still, two alternatives for the differences observed in methyl ricinoleate biotransformation by these species were advanced: (i) 4-hydroxydecanoic acid production by _S. ruinenii_ might be the result of a thioesterafication reaction of the acyl-CoA released by the \(\beta\)-oxidation, even though the lactonian of this free acid to \(\gamma\)-decalactone without acidic conditions could not be explained; (ii) the 4-hydroxydecanoic acid was the first intermediate in lactone catabolism. _S. ruinenii_ would be able to open the lactone ring of \(\gamma\)-decalactone and circumvent its toxic effect to the cell, since lactones are more toxic than 4-hydroxydecanoic acid [47]. Both hypotheses still need further investigation. In sum, the studies performed on _S. salmonicolor_ by analyzing labeled precursors via gas chromatography and mass spectrometry allowed for the postulation of the biosynthetic routes for several lactones [45]. Overall, it can be assumed that after fatty acid biosynthesis, lactone formation starts with the hydroxylation of an unsaturated fatty acid. For instance, regarding the production of \(\gamma\)-lactones, key enzymatic activities were identified: hydroxy-lases, desaturases, hydrateases, epoxygenases, lipoxygenases and monooxygenases such as P450 systems. From linoleic acid, the yeast uses a 13-lipoxygenase-peroxidase pathway for \(\delta\)-decalactone production, while the same pathway is used for \(\delta\)-jasim lactone [45]. Data pointed out that these hydroxy acids were the product of unsaturated fatty acids that then entered the \(\beta\)-oxidation pathway before undergoing lactonization. However, the hypothesis that the oxygenated precursors could also appear from the \(\beta\)-oxidation pathway itself was also considered [40].