Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 194

producing PDO through fermentation of glucose for the manufacture of polytrimethylene
terephthalate under the brand name Sorona ® (da Silva et al ., 2009 ). PDO is produced from
glycerol by a reductive pathway consisting of a dehydration of glycerol followed by a
reduction by NADH (Yang et al ., 2007). To generate NADH for the reductive pathway, an
oxidative pathway oxidizes glycerol to eventually form pyruvate; thus, not all glycerol can
go to the formation of PDO.
Numerous bacteria naturally form PDO from glycerol, including Klebsiella oxytoca
(Yang et al ., 2007 ), Klebsiella pneumoniae (Lin et al ., 2005 ), Citrobacter freundii
(Pflugmacher and Gottschalk, 1994; Boenigk et al ., 1993 ), Enterobacter agglomerans
(Barbirato et al ., 1997 ), and C. butyricum (Boenigk et al ., 1993 ; Biebl, 1991 ). These
bacteria also produce numerous other products in conjunction with PDO that affect
their productivity and may inhibit growth due to the need to produce NADH. Efforts to
improve strains through adaptation or genetic manipulation strategies are ongoing to
increase PDO yields, reduce the effect of inhibitors, and utilize sugars for PDO
production.

7.6.6 Butanediol
2,3 Butanediol (BDO) is another industrially important chemical that has a long history of
production from fermentation. The first published report of microbial BDO production was
by Harden and Walpole (1906). Like butanol production, microbial BDO production was
also driven by necessity during the world wars, especially for the manufacture of synthetic
rubber (Syu, 2001). Current uses include food flavorings, solvent, antifreeze, and as an
ingredient for plastic manufacturing (Syu, 2001).
The main pathway used for the production of BDO from two pyruvate moelcules is
combination of these molecules to form carbon dioxide and α -acetolactate, followed by

decarboxylation of -acetolactate to form carbon dioxide and acetoin, and then reduction of

acetoin by NADH to form BDO and NAD + (Juni and Heym, 1956). The reduction of acetoin
is reversible (Syu, 2001). If acetoin is oxidized to form diacetyl instead of being reduced,
BDO can be formed by combining two diacetyl molecules to form acetate and acetylacetoin,
which is then reduced by NADH or NADPH to acetylbutanediol followed by another
reduction to form acetate and BDO (Juni and Heym, 1956). The sum of these reactions
forms a cycle known as the BDO cycle (Juni and Heym, 1956).
BDO is a chiral molecule with two chiral centers. Thus, it can be present in levarotatory,
dextrorotatory, or meso (optically inactive) forms (Syu, 2001). The stereoisomers depend on
the stereoisomers of acetoin present, the specificity of BDO dehydrogenase, and the
reducing agent specificity for the acetylacetoin reductase enzyme, which catalyzes the
reduction of acetylacetoin to acetylbutanediol (Voloch et al ., 1983 ; Ui et al ., 1998 ). Voloch
and co-workers (1983) proposed that there are two types of BDO dehydrogenase in
Klebsiella pneumoniae , a D form and an L form. The D form catalyzes reduction of D(-)
acetoin to meso-BDO and the L form catalyzes reduction of L(+)acetoin to levarotatory
BDO. They also proposed that an enzyme called acetoin racemase catalyzes the isomerization
of D(-)acetoin to L(+)acetoin, and vice versa (Voloch et al ., 1983 ). Other BDO producers
have other BDO dehydrogenases that catalyze the production of dextrorotatory BDO. In
addition, when the acetylacetoin reduction pathway is used, the reducing agent specificity
of acetylacetoin reductase determines the stereochemistry of BDO. If NADH is the reducing
agent, dextrorotatory BDO is formed. If NADPH is the reducing agent, meso BDO is formed
(Ui et al ., 1998 ).

α