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Figure 4. C5 epimerization of M residues (enzymatic)
This chemical reaction, which occurs in several steps, changes the
configuration at carbon 5 such that the carboxyl (–COOH) group after the
reaction points downwards, and the C5 proton (–H) upwards. The reaction
leads to a new sugar, which is the C5 epimer of its origin. Note that this
process requires breaking bonds and forming new ones.
Since C6 points downwards following the epimerization, the resulting sugar
becomes an L-sugar, not L-mannuronic, but L-guluronic acid (abbreviated G).
Further, as a consequence of the (complex) nomenclature of carbohydrates,
the anomeric configuration (at C1), which is chemically unaltered, is renamed
from
β
to
α
.
Alginates thus contain two monomers linked by 1
→
4 linkages:
β
-D-mannuronic acid (M) and
α
-L-guluronic acid (G).
Figure 5. M:
β
-D-mannuronic acid. G:
α
-L-guluronic acid (the C5 epimer of M)
Little is so far (2013) known about C5-epimerases in seaweeds, but genes
corresponding to bacterial epimerase genes have been found. It is therefore
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
OH OH
COOH
O
O
OH OH
COOH
O
C5 epimerase
O
HO
OH
OH
OH
COOH
β
-D-mannuronic acid
Haworth
CHO
OH
OH
OH
OH
COOH
Fisher
O
HO
OH
OH
OH
COOH
CHO
OH
OH
OH
OH
COOH
α
-L-guluronic acid