6.3.12 Cell surface carbohydrates and blood groupings
Small polysaccharide chains, covalently bonded by glycosidic links to
hydroxyl groups on proteins (glycoproteins), act as biochemical markers
(i.e. antigenic determinants) on cell surfaces. The membrane of the red
blood cells (RBCs) contains glycoproteins/glycolipids, and the type of sugar
that combines with these proteins/lipids varies from person to person. This
gives rise to different blood groups (A, B, AB and O). Human blood group
compatibilities are presented in the following table.
Donor blood type
Acceptor blood type
A
B
AB
O
A
Compatible
Incompatible
Compatible
Incompatible
B
Incompatible
Compatible
Compatible
Incompatible
AB
Incompatible
Incompatible
Compatible
Incompatible
O
Compatible
Compatible
Compatible
Compatible
6.4 Glycosides
Compounds that yield one or more sugars upon hydrolysis are known as
glycosides. A glycoside is composed of two moieties: sugar portion
(glycone) and non-sugar portion (aglycone or genin). For example, the
hydrolysis of salicin produces a glucose unit and salicyl alcohol.
O
O
OH
O
H
O
H
O
H
O
H
O
OH
O
H
O
H
O
H
OH
O
H
O
H
Salicin, a glycoside
Hydrolysis
Glucose, a glycone (sugar)
+
Salicyl alcohol, an aglycone
Glycosidic link
Glycosides of many different aglycones are extensively found in the plant
kingdom. Many of these glycosides are formed from phenols, polyphenols,
steroidal and terpenoidal alcohols through glycosidic attachment to sugars.
Among the sugars found in natural glycosides, D-glucose is the most
prevalent one, but L-rhamnose, D- and L-fructose and L-arabinose also
occur quite frequently. Of the pentoses, L-arabinose is more common than
D-xylose and the sugars often occur as oligosaccharides.
The sugar moiety of a glycoside can be joined to the aglycone in various
ways, the most common being via an oxygen atom (O-glycoside). However,
this bridging atom can also be a carbon (C-glycoside), a nitrogen
(N-glycoside) or a sulphur atom (S-glycoside). By virtue of the aglycone
6.4 GLYCOSIDES
319
Small polysaccharide chains, covalently bonded by glycosidic links to
hydroxyl groups on proteins (glycoproteins), act as biochemical markers
(i.e. antigenic determinants) on cell surfaces. The membrane of the red
blood cells (RBCs) contains glycoproteins/glycolipids, and the type of sugar
that combines with these proteins/lipids varies from person to person. This
gives rise to different blood groups (A, B, AB and O). Human blood group
compatibilities are presented in the following table.
Donor blood type
Acceptor blood type
A
B
AB
O
A
Compatible
Incompatible
Compatible
Incompatible
B
Incompatible
Compatible
Compatible
Incompatible
AB
Incompatible
Incompatible
Compatible
Incompatible
O
Compatible
Compatible
Compatible
Compatible
6.4 Glycosides
Compounds that yield one or more sugars upon hydrolysis are known as
glycosides. A glycoside is composed of two moieties: sugar portion
(glycone) and non-sugar portion (aglycone or genin). For example, the
hydrolysis of salicin produces a glucose unit and salicyl alcohol.
O
O
OH
O
H
O
H
O
H
O
H
O
OH
O
H
O
H
O
H
OH
O
H
O
H
Salicin, a glycoside
Hydrolysis
Glucose, a glycone (sugar)
+
Salicyl alcohol, an aglycone
Glycosidic link
Glycosides of many different aglycones are extensively found in the plant
kingdom. Many of these glycosides are formed from phenols, polyphenols,
steroidal and terpenoidal alcohols through glycosidic attachment to sugars.
Among the sugars found in natural glycosides, D-glucose is the most
prevalent one, but L-rhamnose, D- and L-fructose and L-arabinose also
occur quite frequently. Of the pentoses, L-arabinose is more common than
D-xylose and the sugars often occur as oligosaccharides.
The sugar moiety of a glycoside can be joined to the aglycone in various
ways, the most common being via an oxygen atom (O-glycoside). However,
this bridging atom can also be a carbon (C-glycoside), a nitrogen
(N-glycoside) or a sulphur atom (S-glycoside). By virtue of the aglycone
6.4 GLYCOSIDES
319