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1.1.6. Example: Alginate
These rules are conveniently illustrated in the alginate case, where the
hexose
β
-D-mannuronic acid is converted (epimerised) into
α
-L-guluronic acid
(by enzymes, see also Chapter 1.2):
Hence, changing the configuration on C5 (only) results in a D-to-L transition,
but because of the rules in carbohydrate chemistry, also a renaming from
β
to
α
even if the chemistry at the anomeric carbon is unchanged (Remembering
this makes similar situations simple to understand)
1.1.7. Epimers and anomers.
Sugars differing in the stereochemistry of one or a few of the asymmetric
carbons are epimers. Some common examples:
O
COOH
OH
OH OH
HO
It is a D-sugar because C6 points up
It is a
β
-sugar beacuse the
anomeric -OH points up (for
a D sugar)
β
-D-mannuronic acid
(It is called a uronic acid beacuse C6 is -COOH)
O
COOH
OH
OH OH
HO
It becomes a L-sugar because C6 points down
It becomes a
α
-sugar. The
anomeric -OH (still) points
up, but the sugar is a L-
sugar.
α
-L-guluronic acid
Enzyme acting on C5 (C5-epimerase)