Page 92 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Microbial-derived natural bioproducts for a sustainable environment 67

located in the cytoplasmic membrane. The glycosyl carrier lipid ensures and regulates the
structured assembly and extracellular transport of the polymerized chain.
Group IV: This group of enzymes located outside the cell wall are involved in the macro-
molecules polymerization on the carrier lipid. The backbone chain of the assembled
repetitive moieties is then translocated from the cell surface to form a loose slime or a
well-attached polysaccharide capsule.
The hydrophilic, high-molecular weight characteristics of the assembled poly-
mers make it a challenging task for the bacterium to export them from the cyto-
plasm to the cell membrane. In Gram-positive bacteria, the assembled
polysaccharide chain simply gets translocated to the cell surface where its elonga-
tion takes place. However, due to the presence of the wide diversity of the EPS
molecular structures in most Gram-negative bacteria, the elongation and transloca-
tion pathways have been generally believed to follow either the Wzx Wzy-depen-
dent pathway (Xanthan and gellan), the ABC transporter-dependent pathway
(generally present in CPS), or the synthase-dependent pathway (alginate, cellulose)
(Plank, 2005; Rehm, 2010; Freitas et al., 2011; Schmid et al., 2015). The draft
genome of X. campestris NRRL B-1459, several Pseudomonas, and Sphingomonas
strains were reported for the production of EPS via Wzx Wzy-dependent pathway
(Pollock, 2002; Wibberg et al., 2015). The first description of cellulose synthesizing
(celS) genes from Acetobacter xylinum (acsABCD) and alginate synthesis by
Pseudomonas and Azotobacter via synthase-dependent pathway were also reported
(Wong et al., 1990; Rehm, 2009). The most common extracellularly synthesized
polymer is dextran, whose polymerization mainly consisting of α-(1 6)-linked glu-
cose that takes place outside the cell with the help of the key enzyme dextransu-
crase (part of the glucosyltransferases) (Schmid et al., 2015). L. mesenteroides,
Streptococcus, and Lactobacillus have been reported to produce this homopolysac-
charide (Kralj et al., 2004; Majumder et al., 2009; Freitas et al., 2011).
Studies have reported that the alginate biosynthesis consists of a complex series
of gene clusters in Pseudomonas aeruginosa (single operon) and Azotobacter vine-
landii (3 operons) cells that controlled both EPS molecules production and its extra-
cellular secretion (Czaczyk and Myszka, 2007; Kumar et al., 2007; Whitney and
Howell, 2013). The regulatory system consisting of algA (GDP-mannose pyropho-
sphorylase), algD (GDP-mannose dehydrogenase) for precursor synthesis, algC
(phosphomannomutase), alg8 alg44 (polysaccharide synthase), and algK and algE
(both in alginate export) genes are linked to the polymerization and secretion of the
bacterial alginate macromolecules. The genes and enzymes responsible for succino-
glycan biosynthesis have been studied in Rhizobium sp. (Sutherland, 2001a,b).
Succinoglycan is regulated by a two-component system and consists of 30 genes
yielding larger repeating units (octasaccharide) of the two precursor sugar nucleo-
tides, UDP-glucose and UDP-galactose. However, the author also reported the key
enzyme succinoglycan depolymerase involved in the degradation of the EPS pro-
duced by the bacteria. The two genes, epsE and epsG, responsible for the EPS syn-
thesis in Streptococcus thermophilus Sfi39 have been reported by Kumar et al.
(2007). Four distinct EPS gene clusters involved in EPS synthesis have not only
been characterized till date but also suggested that a dozen or more unique EPS