Enzymatic Synthesis of Heparin - Sustainable Biotechnology: Sources of Renewable Energy - page 260

heparin-induced thrombocytopenia ( HIT) [2]. Furthermore, since heparin is isolated
from animal tissues, it could be contaminated by harmful substances. Most recently,
the contaminated heparin drug made by SPL caused severe side effects including
life-threatening anaphylactic reactions manifesting in abnormal low blood pressure,
difficulty breathing and occasional vomiting [3]. Hundreds of patients worldwide
have suffered the severe reactions linked to this contaminated heparin. This acci-
dent resulted in a major heparin recall in the USA, countries of the European Union
and Japan. The “oversulfated chondroitin sulfate (OSCS)” was identified as a con-
taminant in heparin [3-5]. This tragic event suggested the heparin supply chain is
vulnerable. Thus, a synthetic heparin that can be manufactured in a confined facility
remains a high priority. Chemical synthesis is so far the most successful approach to
the synthesis of structurally defined heparin mimetic. This effort has led to the total
synthesis of the pentasaccharide drug, Arixtra, for the treatment of thrombotic dis-
eases [6-10]. However, chemical synthesis of this pentasaccharide is complicated
and costly. In this review, we will summarize the recent efforts of enzymatic and
chemoenzymatic synthesis of heparin/HS.

2 Background Research

2.1 Structures and Biological Functions of HS

HS is widely expressed on the cell surface and in the extracellular matrix in the form
of HS proteoglycans (HSPGs), which contain a core protein and polysaccharide
side chains. HSPGs are involved in numerous biological processes, including blood
coagulation, wound healing, embryonic development, regulating tumor growth, as
well as the assisting of viral and bacterial infections [11-21]. HS polysaccharides
play the essential role for the functions of HSPGs. The wide range of biological
functions of HS attracts considerable interest in exploiting HS-based anticoagulant,
antiviral and anticancer drugs. Heparin, an analog of HS, is a commonly used anti-
coagulant drug. HS/heparin belongs to the glycosaminoglycan family. Depending
on the structures of the disaccharide repeating units, glycosaminoglycans include
HS, chondroitin sulfate (CS), dermatan sulfate (DS), keratan sulfate (KS) and
hyaluronic acid (HA). HS consists of highly sulfated repeating disaccharide units of
1

4-linked glucosamine (GlcN) and glucuronic (GlcUA)/iduronic acid (IdoUA).
The majority of glucosamine residues are modified with either N -acetyl or N -sulfo
groups (for detailed structure, refer to Fig. 1) [22-24]. However, up to 7% of glu-
cosamine in HS is present in N -unsubstituted glucosamine (GlcNH 2 ) and may play
an important biological role [25]. For example, it is known that the GlcNH 2 unit
is involved in the binding of herpes simplex virus 1 (HSV-1) glycoprotein D (gD)
[18, 26]. 6- O -sulfo glucosamine (GlcN6S) and 2- O -sulfo iduronic acid (IdoUA2S)
are the most common sulfated monosaccharides, and these units play critical roles
in binding to fibroblast growth factors (FGFs), fibroblast growth factor receptors
(FGFRs) [27] and platelet factor 4 (PF4, a major cause for HIT) [28]. 3- O -sulfo

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