Enzymatic Synthesis of Heparin - Sustainable Biotechnology: Sources of Renewable Energy - page 264

2.4 Chemical Synthesis of Heparin/HS

Chemical synthesis is a powerful approach to obtain structurally defined hep-
arin/HS oligosaccharide. The most successful example is the total synthesis of an
AT-binding pentasaccharide, which is currently a marketed drug with the trade name
Arixtra used in clinics to prevent venous thromboembolic incidents during surgery.
However, Arixtra only inhibits factor Xa activity and the synthesis of Arixtra is com-
plicated and requires more than 60 steps with only a 0.5% yield [7-9]. Although
the approval of Arixtra endorses the success of chemical synthesis of HS oligosac-
charides, the high cost of Arixtra limits its application. In order to improve its
pharmacological efficacy, a heparin mimetic with 16 saccharide units has been syn-
thesized with both anti-Xa and anti-IIa activities. But this compound is a simplified
hybrid molecule of HS oligosaccharides and highly sulfated glucose units that are
not natural occurring heparan sulfate/heparin structure [6, 10]. The compound is
effective in baboon [6]; however, it has not been marketed. Although many efforts
continue to pursue the synthesis of heparin oligosaccharides, it has been difficult
to generate authentic HS structures larger than a hexasaccharide solely utilizing
chemical synthesis. HS biosynthetic enzymes offer a promising alternative approach
for the synthesis of large heparin/HS oligosaccharides with the desired biological
activities.

2.5 Enzymatic Synthesis of Heparin/HS

Several groups have reported the attempts to synthesize HS using biosynthetic
enzymes to produce a product with anticoagulant activity [36-39]. For exam-
ple, Rosenberg's group utilized HS biosynthetic enzymes to synthesize the HS
containing AT binding sites with anticoagulant activity [36]. Although only
microgram amounts of product were generated, this approach demonstrated the
feasibility, and the yield (~1.1%) was higher than chemical synthesis. Lindahl
and colleagues reported an alternative chemoenzymatic approach for the syn-
thesis of anticoagulant heparin mimic in gram quantities from heparosan, an
Escherichia coli ( E. coli ) capsular polysaccharide. The product was called

neoparin [37].
Our lab has significantly improved on the enzyme-based synthesis of HS in
three aspects. First, we enhanced the expression of enzymes and successfully
coupled the synthesis with a 3 -phosphoadenosine-5 -phosphosulfate (PAPS) regen-
eration system, allowing us to prepare HS in milligrams quantities [40]. Second,
we utilized this approach to identify a new anticoagulant HS, named Recomparin,
which has a simplified structure [41]. Third, our lab prepared an octasaccharide
that inhibited the infection of HSV-1 [42]. Here, we focus our attention on the
progress.