Page 91 - Refining Biomass Residues for Sustainable Energy and Bioproducts

66 Refining Biomass Residues for Sustainable Energy and Bioproducts

metabolite pathway, and polysaccharide production (Freitas et al., 2011). Generally,
the enzymatic steps involved in the production of bacterial EPS are carried out
intracellularly and then transported to the extracellular medium as macromolecules,
such as extracellular polysaccharides (EPS) and CPS, associated with cell surface
or lipopolysaccharides associated with the outer membrane. Synthesis of homo or
heteropolysaccharides precursors begins with the entry of the sugar substrate (active
or passive transport) into the bacterial cell catabolized through periplasmic oxida-
tion or intracellular phosphorylation (Gupta and Diwan, 2017). In the cytoplasm the
primary metabolites formed due to the glycolysis catabolization of the sugar sub-
strate or by the central carbohydrate metabolism pathway are used as precursors for
the synthesis of amino acids or monosaccharides. These monosaccharides are acti-
vated through phosphorylation and are turned into charged-energy-rich precursors
in the form of nucleoside diphosphate/monophosphate sugars which ultimately
involves in the manufacture of these polysaccharides (Kumar et al., 2007; Freitas
et al., 2011). The enzymes responsible for the formation of homopolysaccharides,
such as curdlan, dextran, and levan, are extracellular generally secreted from the
bacteria or located on the bacterial cell surface, whereas heteropolysaccharides,
such as alginate and xanthan, are synthesized using the intracellular sugar nucleo-
tide precursors formed in the cytoplasm where they are assembled and finally
exported outside the cell (Welman and Maddox, 2003; Madhuri and Prabhakar,
2014; Gupta and Diwan, 2017). Some of the precursors that are not involved in the
central metabolic pathways might act as a substrate for producing EPS. Different
environmental factors, such as carbon:nitrogen ratio (C:N), pH, temperature, and
different bacterial growth rates, also regulate the enzymatic activity of the EPS bio-
synthesis. Enzymes necessary for the formation of sugar nucleotide precursors and
enzymes as well as regulatory proteins associated with EPS production and secre-
tion are solely controlled by a separate group of genes. This fact was exceptional in
Xanthomonas campestris producing a single EPS, namely, xanthan and the genome
contained genes designated as rpf (regulation of pathogenicity factors) which con-
trolled the synthesis of extracellular enzymes including cellulase, polygalacturonate
lyase, amylase, and protease needed for the EPS precursor transformations as well
as directed the EPS production (Tang et al., 1990; Czaczyk and Myszka, 2007).
The intracellular machinery comprehends enzymes involved in the EPS synthesis
that may be classified into four different groups (Fig. 3.3)(Sutherland, 2001a,b;
Kumar et al., 2007; Freitas et al., 2011):

Group I: This group of enzymes is involved in the central metabolic pathway, such as
hexokinase, which involved in the phosphorylation of glucose to glucose-6-phosphate
(G-6-P) and phosphoglucomutase converts (G-6-P) to glucose-1-phosphate (G-1-P).
Group II: This group of enzyme contains uridine diphosphate-glucose (UDP-G)
pyrophosphorylase that catalyzes the conversion of G-1-P to UDP-G, the key sugar
nucleotide precursor required for EPS synthesis and also act as a source of
monosaccharides.
Group III: This group of enzymes constitutes glycosyltransferases located in the cell peri-
plasmic membrane and are associated with the transfer of activated UDP-G or UDP-Gal
sugar moieties to undecaprenyl phosphate (C 55 -P), identified as an isoprenoid alcohol