Biosynthesis of Rhamnolipids

Rhamnolipids are a class of glycolipid produced by Pseudomonas aeruginosa, amongst other organisms, frequently cited as bacterial surfactants.They have a glycosyl head group, in this case a rhamnose moiety, and a 3-(hydroxyalkanoyloxy)alkanoic acid (HAA) fatty acid tail, such as 3-hydroxydecanoic acid.

Application

Rhamnolipids are a class of biosurfactants which contain rhamnose as the sugar moiety linked to β-hydroxylated fatty acid chains. Rhamnolipids can be widely applied in many industries including petroleum, food, agriculture and bioremediation etc.

Rhamnolipids biosynthesis

The first genes discovered in a mutagenesis screen for mutants unable to produce rhamnolipids were rhlA and rhlB.They are arranged in an operon, adjacent to rhlRI, a master regulator of quorum sensing in Pseudomonas aeruginosa. The proteins encoded by rhlA and rhlB; RhlA and RhlB respectively, are expected to form a complex because of the operonic nature of the genes which encode these two proteins and because both proteins are necessary for production of rhamnolipids.Furthermore, it was supposed that the role of RhlA was to stabilise RhlB in the cell membrane and thus the RhlAB complex was labelled as the enzyme Rhamnosyltransferase 1 and is frequently cited as such although there is no biochemical evidence for this and RhlA has been shown to be monomeric in solution. RhlA was subsequently shown to be involved in the production of the precursor to RHLs, HAAs. RhlB adds a rhamnose group to the HAA precursor to form mono-rhamnolipid. Therefore, the products of the rhlAB operon, RhlA and RhlB, catalyse the formation of HAAs and mono-rhamnolipids respectively.

RhlA is an α, β hydrolase (analysis by Fugue structural prediction programme). This fold is a common structural motif in fatty acid synthetic proteins and RhlA shows homology to transacylases. It has been shown using enzyme assays that the substrate for RhlA is hydroxyacyl-ACP rather than hydroxyacyl-CoA suggesting that it catalyses the formation of HAAs directly from the type II fatty acid synthase pathway (FASII). Furthermore, RhlA preferentially interacts with hydroxyacyl-ACP with an acyl chain length of ten carbon residues.The hydroxyacyl-ACP substrate of RhlA is the product of FabG, a protein which encodes the NADPH-dependent β-keto-acyl-ACP reductase required for fatty acid synthesis.It is a member of the FASII cycle along with FabI and FabA, which synthesise the precursors utilised by FabG.

Another gene necessary for synthesis of di-rhamnolipids, rhlC, has also been identified. RhlC catalyses the addition of the second rhamnose moiety to mono-rhamnolipids forming di-rhamnolipids, hence is often labelled rhamnosyltransferase 2.Like rhlA and rhlB, rhlC is thought to be an ancestral gene controlled by the same quorum sensing system as rhlA and rhlB. The rhamnose moiety for mono- and di-rhamnolipids is derived from AlgC activity and the RmlABCD pathway, encoded on the rmlBCAD operon. AlgC produces sugar precursors directly for alginate and lipopolysaccharide (LPS) as well as rhamnolipids.In rhamnose synthesis, AlgC produces glucose-1-phosphate (G1P) which is converted to dTDP-D-glucose by RmlA followed by conversion to dTDP-6-deoxy-D-4-hexulose and then dTDP-6-deoxy-L-lyxo-4-hexulose by RmlB and RmlC respectively. Finally, dTDP-6-deoxy-L-lyxo-4-hexulose is converted to dTDP-L-rhamnose by RmlD.The rhamnose can then be used in the synthesis of rhamnolipids by RhlB and RhlC.

The complete pathway of biosynthesis of rhamnolipids has not been confirmed. In summary, mono- and di- rhamnolipids are produced by sequential rhamnosyltransferase reactions catalysed by RhlB and RhlC respectively.The substrate for RhlB is the fatty acid moiety of the detergent, produced by RhlA.

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