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21
Figure 2. GDP-mannuronic acid: Precursor molecule
Note that D-mannuronic acid (abbreviated ManA = M) is closely related to D-
mannose, the difference being that C6 is –COOH (carboxyl) instead of –
CH
2
OH. The polymerization is simply:
n (GDP-D-ManA)
→
(ManA)
n
+ n GDP
The polymerization reaction is enzymatic (transferases involved). The
linkages between the M residues become
β
-1,4
(as in cellulose), providing the
following Haworth structure of the intermediate mannuronan:
Figure 3. Mannuronan = poly-(
β
-D-Mannuronic acid). Linkages are 1,4 (just as
in cellulose). Mannuronan is an intermediate in the biosynthesis of functional
alginates
Mannuronan chains are normally very long. They may consist of several
thousand M residues, i.e. DP
7
> 1000. Mannuronan is, however, a non-
accumulating intermediate, except in certain mutated bacteria having
mannuronan as the final product.
By studying the biosynthesis of bacterial alginates in
Azotobacter vinelandii
and
Pseudomonas aeruginosa
it was found that a family of (mostly)
extracellular enzymes – mannuronan-C5-epimerases – attack the
mannuronan chains and perform a highly special chemical reaction - an
epimerization - at carbon no 5:
7
Defiition of DP: Chapter 2.1.2
O
H
HO
H
HO
OH
H
H
H
COOH
GDP
O
HO
GDP
OH
OH
COOH
=
GDP-mannuronic acid
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
OH OH
COOH
O