Amphiphilic Molecules of Microbial Origin Classification, Characteristics, Genetic Regulations, and Pathways for Biosynthesis - Biosurfactants: Research Trends and Applications - page 41

(dTDP)-activated sugar synthesis. Biosynthesis of rhamnolipids occurs sequentially
in three steps (Burger et al., 1963). First step is the synthesis of the fatty acid dimer
moiety of rhamnolipids and free 3-(3-hydroxyalkanoyloxy) alkanoic acid (HAA),
catalyzed by RhlA enzyme. The fatty acid moiety required for the synthesis deviates
from the general fatty acid biosynthetic pathway at the level of the ketoacyl reduction
with the help of RhlG enzyme (Campos-Garcia et al., 1998). The next reaction is the
formation of monorhamnolipids from dTDP-l-rhamnose and an HAA, catalyzed by
RhlB (rhamnosyltransferase1). Finally, dirhamnolipids are formed by the addition of
dTDP-l-rhamnose to the monorhamnolipids, catalyzed by RhlC (rhamnosyltransfer-
ase2). dTDP-l-rhamnose is formed from dTDP-d-glucose, and the conversion takes
place upon the action of enzymes encoded by rmlA, rmlB, rmlC , and rmlD , which
form the rmlBCAD operon (Rahim et al., 2000).
The production of rhamnolipids in P. aeruginosa is regulated by rhl quorum
sensing system at the transcriptional level (Soberón-Chávez et al., 2005). Different
genes that encode enzymes involved in the rhamnolipid biosynthesis are rhlA, rhlB,
rhlR , and rhlI . The genes rhlA and rhlB are arranged as an operon and encode
RhlB rhamnosyltransferase, while rhlR and rhlI act as regulators of the rhlAB
expression. The rhlC gene that codes for RhlC rhamnosyltransferase is not linked
to other rhl genes and forms an operon with a gene whose function is not known
(Rahim et al., 2001). Another system of genes called las system regulates the rhl
system and in turn regulates rhamnolipid synthesis. The quorum sensing response
depends on certain autoinducers, which are responsible for the activation of rhlAB
transcription, synthesized by rhlI and lasI (Pesci et al., 1997). The quorum sens-
ing response is primarily expressed under nutrient-limited conditions (Maier and
Soberón-Chávez, 2000).
s oPhoroliPiDs
Sophorolipid production is most optimal when both a hydrophobic and a hydrophilic
carbon source are supplied in the production medium (Van Bogaert et al., 2011).
Synthesis of sophorolipids requires glucose and a common fatty acid, both of which
can be supplied as such in the medium. The fatty acid constituents can also be syn-
thesized de novo from acetate or by stepwise oxidation of alkanes in the growth
medium (Van Bogaert et al., 2008). The first step in the biosynthesis is hydroxyl-
ation of the fatty acid to a terminal (ω) or subterminal (ω-1) hydroxy fatty acid. Next,
nucleotide-activated glucose (UDP-glucose) is linked to the hydroxyl group of the
fatty acid upon action of glycosyltransferase I. Subsequently, another UDP-glucose
is coupled to the first glucose moiety by glycosyltransferase II to get acidic nona-
cetylated sophorolipid (Saerens et al., 2011). It could either be retained in the native
sophorolipid mixture or further modified by both internal esterification (lactoniza-
tion) and acetylation of the carbohydrate head. Lactonization occurs by an esterifica-
tion reaction between the carboxyl group of fatty acid and hydroxyl group of sugar
moiety while acetylation is carried out by acetyl-CoA-dependent acetyltransferase
enzyme (Sen et al., 2011). Sophorolipids are often formed as mixtures, which differ
in the degree of acetylation of the sugar moiety and fatty acid saturation and lactoni-
zation. The gene regulation of sophorolipid synthesis is being studied extensively.