Essential Cell Biology 5th edition

72
PANEL 2–4 AN OUTLINE OF SOME OF THE TYPES OF SUGARS
MONOSACCHARIDES
Monosaccharides usually have the general formula (CH 2 O) n, where n can be 3, 4, 5, or 6, and have two or more hydroxyl groups.
O
They either contain an aldehyde group ( C ) and are called aldoses, or a ketone group ( C O ) and are called ketoses.
H
3-carbon (TRIOSES) 5-carbon (PENTOSES) 6-carbon (HEXOSES)
H
C
O
H
H
C
C
O
OH
ALDOSES
H
H
C
C
O
OH
H
H
H
C
C
C
OH
OH
OH
HO
H
H
C
C
C
H
OH
OH
H
C
OH
H
C
OH
H
C
OH
H
H
H
glyceraldehyde ribose glucose
H
H
H
C
OH
H
C
OH
C
O
KETOSES
H
H
C
C
OH
O
H
H
C
C
C
O
OH
OH
HO
H
H
C
C
C
H
OH
OH
H
C
OH
H
C
OH
H
C
OH
H
dihydroxyacetone
H
ribulose
H
fructose
RING FORMATION
In aqueous solution, the aldehyde or ketone group of a sugar
molecule tends to react with a hydroxyl group of the same
molecule, thereby closing the molecule into a ring.
H O
C
CH 2 OH
1
6
O
H C OH
5
2
H
OH
H
HO C H
4
1 glucose
3
OH H
H C OH HO
H
4
3 2
H C OH
5
H OH
CH 2 OH
6
H O
CH 2 OH
5
O
1C
OH
H C OH
2
4 H H 1 ribose
H C OH
3
H
H
3 2
H C OH
4
OH OH
CH 2 OH
5
Note that each carbon atom has a number.
ISOMERS
Many monosaccharides differ only in the spatial arrangement
of atoms—that is, they are isomers. For example, glucose,
galactose, and mannose have the same formula (C 6 H 12 O 6 ) but
differ in the arrangement of groups around one or two carbon
atoms.
CH 2 OH
O
HO
OH
H
OH H
H
H
CH 2 OH
O
H
OH
H
H
HO OH H
H OH
glucose
H OH
galactose
CH 2 OH
O
H
OH
H
HO OH OH
H
H H
mannose
These small differences make only minor changes in the
chemical properties of the sugars. But the differences are
recognized by enzymes and other proteins and therefore can
have major biological effects.