Many bacteria use the anaerobic pathway for synthesizing unsaturated fatty acids. This pathway does not utilize oxygen and is dependent on enzymes to insert the double bond before elongation utilizing the normal fatty acid synthesis machinery. In Escherichia coli, this pathway is well understood.
* FabA is a β-hydroxydecanoyl-ACP dehydrase – it is specific for the 10-carbon saturated fatty acid synthesis intermediate (β-hydroxydecanoyl-ACP).
* FabA catalyzes the dehydration of β-hydroxydecanoyl-ACP, causing the release of water and insertion of the double bond between C7 and C8 counting from the methyl end. This creates the trans-2-decenoyl intermediate.
* Either the trans-2-decenoyl intermediate can be shunted to the normal saturated fatty acid synthesis pathway by FabB, where the double bond will be hydrolyzed and the final product will be a saturated fatty acid, or FabA will catalyze the isomerization into the cis-3-decenoyl intermediate.
* FabB is a β-ketoacyl-ACP synthase that elongates and channels intermediates into the mainstream fatty acid synthesis pathway. When FabB reacts with the cis-decenoyl intermediate, the final product after elongation will be an unsaturated fatty acid.
* The two main unsaturated fatty acids made are Palmitoleoyl-ACP (16:1ω7) and cis-vaccenoyl-ACP (18:1ω7).
Most bacteria that undergo anaerobic desaturation contain homologues of FabA and FabB. Clostridia are the main exception; they have a novel enzyme, yet to be identified, that catalyzes the formation of the cis double bond.
;Regulation
This pathway undergoes transcriptional regulation by FadR and FabR. FadR is the more extensively studied protein and has been attributed bifunctional characteristics. It acts as an activator of fabA and fabB transcription and as a repressor for the β-oxidation regulon. In contrast, FabR acts as a repressor for the transcription of fabA and fabB.