Transferases in Polymer Chemistry - Enzymatic Polymerisation - page 40

gels - from enzymatically modified starch using amylomaltase were reported
recently [ 208 - 213 ] .
Amylomaltase (4-

α -glucanotransferase; systematic name: (1 → 4)- α - D -glucan:

(1 → 4)- α - D -glucan 4- α - D -glycosyltransferase; EC 2.4.1.25) catalyzes the glucan-

chain transfer from one -1,4-glucan (or to the 4-hydroxyl

group of glucose), or within a single linear glucan molecule to produce a cyclic-

α -1,4-glucan to another α

-
1,4-glucan [ 213 - 215 ] . The enzyme was first found in Escherichia coli , but seems
to be distributed in various bacterial species with different physiological functions.
In E. coli , amylomaltase is expressed with glucan phosphorylase from the same
operon. Amylomaltase is a member of the maltooligosaccharide transport and uti-
lization system and plays a role in converting short maltooligosaccharides into
longer chains, upon which glucan phosphorylase can act [ 216 ] . The name amyloma-
ltase is used for the microbial 4-

α

-glucanotransferases, whereas the plant counter-
parts are usually called disproportionating enzymes (D-enzymes) [ 212 , 214 , 215 ] .
Typically, enzymatic modification of starch employs hydrolyzing enzymes
such as

α

α -amylase, pullulanase, and glucoamylase. These hydrolyze the α -1,4- or

α -1,6-glycosidic bonds in amylose and amylopectin by first breaking the glycosidic

linkage and subsequently using a water molecule as acceptor substrate. Amylo-
maltases also initially break the glycosidic linkage but, instead of water, they use
another oligosaccharide as an acceptor substrate and form a new glycosidic linkage.
Amylomaltases can use high molecular weight starch as both donor and acceptor
molecule and can catalyze the transfer of long α -1,4-glucan chains [ 217 ] , or even

highly branched cluster units of amylopectin.

3.6 Hyaluronan Synthase

Hyaluronan (HA) is a nonsulfated non-epimerized linear glycosaminoglycan
(GAG) existing in vivo as a polyanion of hyaluronic acid and composed of
repeating disaccharide units of D-glucuronic acid and N -acetyl- D -glucosamine
[GlcA

→ →

4)] [ 218 - 220 ] . It is a major constituent of the extra-
cellular matrix (ECM) of the skin, joints, eye, and many other tissues and organs.
Despite the simple structure of this macromolecule, the complexity of its physico-
chemical properties and biological functions is tremendous. HA has extraordinary
hydrophilic, rheological, and signaling properties and is viscoelastic. This naturally
occurring biopolymer is dynamically involved in many biological processes, such
as embryogenesis, inflammation, metastasis, tissue turnover, and wound healing.
The isolation, purification, and identification of nearly pure HA have been the
center of scientific interest for many decades. The bacterial production of HA by
Streptococcus equi [ 221 ]and Streptococcus zooepidemicus [ 222 ] enabled it to be
produced in larger quantities than could be achieved by extraction methods alone.
HA produced by S. equi has a lower molecular weight than does HA produced
by S. zooepidemicus , which has a MW of about 1.8-2

β (1 3)GlcNAc β (1

10 6 Da with a yield of

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