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2.2: Sugars and Polysaccharides
François Baneyx
Department of Chemical Engineering, University of Washington
[email protected]
Carbohydrates or saccharides are abundant compounds that play regulatory and structural
roles and serve as cellular fuel and energy storage. Carbohydrates have for general
formula (CH2O)n, where n 3. They are synthesized from atmospheric CO2 and H2O via
photosynthesis and polymerized into more complex polysaccharides such as starch,
glycogen and cellulose.
The simplest carbohydrates are monosaccharides. These simple sugars contain three to
nine carbon atoms and are classified as aldoses or ketoses depending on whether their
carbonyl group is an aldehyde or a ketone. The smallest monosaccharides are trioses that
only contain three carbon atoms. Those sugars containing four, five, six, etc, carbon
atoms are called tetroses, pentoses, hexoses, etc, respectively. Table 1 shows the
chemical structure of common monosaccharides. A particularly important
monosaccharide is glucose which is used to power many cellular processes and serves as
a precursor for the synthesis of more complex carbohydrates. Fig. 1 shows that carbons 2to-5 of D-glucose are chiral, leading to 24 = 16 possible stereoisomers that correspond to
all possible aldohexoses (four of which are shown in Table 1). While glucose can be
found in both L and D forms, the L form is less abundant in Nature and L-glucose only
plays a minor role in biological systems. In solution, D-glucose adopts a ring structure
(known as pyranose for a 6-membered ring) that results from the reaction of its carbonyl
group and alcohol groups (Fig. 1). The denomination refers to the location of the OH
substituent on carbon 1 of the ring. If the hydroxyl were located on the same side of the
ring as the CH2OH group, the resulting molecule would be called -D-glucose.
H
O
CH2OH
1C
H C
2
OH
3
C
OH
H
OH
H C
OH
4
6
C
C
OH
HO
H C
5
H
6 CH2OH
OH
H
H
C
C
H
HO
C
H
O
4
5
C
HO
3
C
O
OH
H
C
2
C
H
HO
H
1
C
OH
-D-Glucose
(-D-Glucopyranose)
CH2OH
D-Glucose
Fig. 1 Formation of cyclic -D-Glucose from its linear form.
(linear)
NNIN Nanotechnology Open Textbook, Chapter 2 – Biochemistry Fundamentals
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Table 1 Structure of some common monosaccharides
Aldoses
D-Triose
CHO
H C OH
CH2OH
D-Glyceraldehyde
D-Tetroses
CHO
CHO
H C
OH
H C
OH
H C OH
CH2OH
CH2OH
D-Erythrose
D-Threose
OH
D-Pentoses
CHO
H C
OH
CHO
CHO
OH
C H
H C OH
H C OH
H C
OH
H C OH
H C
OH
H C OH
CH2OH
CH2OH
CH2OH
OH
C H
D-Ribose
D-Arabinose
D-Xylose
CHO
CHO
CHO
H C OH
D-Hexoses
C H
H C OH
H C OH
OH
C H
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
D-Allose
D-Glucose
H C
CHO
OH
OH
C H
OH
C H
OH
C
H
OH
C
H
H C OH
OH
H C OH
CH2OH
CH2OH
D-Galactose
D-Mannose
H C
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Table 1 (Continued)
Ketotriose
Ketoses
CH2OH
C
O
CH2OH
Dihydroxyacetone
Ketotetrose
CH2OH
C
O
H C OH
CH2OH
D-Erythrulose
D-Hexoses
Ketopentoses
CH2OH
C
CH2OH
O
H C OH
H C OH
C
OH
O
C H
H C
OH
CH2OH
CH2OH
D-Ribulose
D-Xylulose
CH2OH
CH2OH
CH2OH
CH2OH
C O
C O
C O
C O
OH
C H
H C OH
OH
C H
OH
C H
H C
OH
H C
OH
H C OH
H C
OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
CH2OH
CH2OH
D-Sorbose
D-Togatose
D-Psicose
D-Fructose
OH
C H
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D-ribose and D-deoxyribose are two other important monosaccharides that are structural
components of RNA and DNA, respectively. The structure of these five carbon ring
sugars is shown in Fig. 2.
5 CH2OH
5 CH2OH
OH
O
1
4
Fig. 2 Structures of D-ribose (left)
and D-deoxyribose (right). The two
molecules differ by the presence or
absence of a hydroxyl group on
carbon 2. Not all hydrogen atoms
are shown.
OH
O
1
4
3
2
3
2
OH
OH
OH
H
Monosaccharides can polymerize via condensation reactions to yield polysaccharides.
The simplest case involves two simple sugars and the generation of a disaccharide.
Depending on whether the reaction proceeds with the or form of the sugar, an -1,4
or -1,4 glycosidic bond is generated, giving rise to different disaccharides as illustrated
in the case of D-glucose in Fig. 3.
6 CH2OH
6 CH2OH
5
5
O
4
1
OH
HO
+
OH
3
O
4
1
HO
3
-D-Glucose
6 CH2OH
6 CH2OH
5
HO
+
4
2
H2O
OH
OH
OH
-D-Glucose
-D-Glucose
3
2
OH
6 CH2OH
6 CH2OH
5
5
O
4
1
OH
2
OH
2
Maltose (-1,4 glycosidic bond)
1
3
1
OH
OH
OH
HO
3
O
4
O
3
O
OH
1
1
HO
2
-D-Glucose
OH
5
O
4
OH
OH
4
5
OH
OH
O
6 CH2OH
OH
2
5
H2O
6 CH2OH
O
O
4
1
OH
HO
OH
3
2
3
OH
2
OH
Cellobiose (-1,4 glycosidic bond)
Fig. 3 Maltose and cellobiose both result from the condensation of two D-glucose
molecules. However, whereas maltose contains an -1,4 glycosidic bond, cellobiose
contains a -1,4 bond.
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In addition to reacting with carbon 4, the hydroxyl group of carbon 1 in one sugar may
also react with carbon 6 of the second sugar, leading to the formation of an -1,6
glycosidic bond (Fig. 4). Such bonds act as branch points in more complex
polysaccharides.
6 CH2OH
6 CH2OH
5
5
O
4
1
OH
HO
OH
3
2
OH
-D-Glucose
+
6 CH2OH
O
4
5
H2O
1
5
O
4
4
1
OH
HO
OH
2
OH
HO
HO
3
OH
-D-Glucose
O
O
1
OH
3
6 CH2
2
OH
3
OH
2
OH
Isomaltose (-1,6 glycosidic bond)
Fig. 4 Formation of an -1,6 glycosidic bond between two -D-Glucose molecules.
Cellulose, the major structural constituent of plants cell wall is one of the most abundant
molecules on earth, accounting for more than half the carbon in the biosphere. Cellulose
is a linear polymer of up to 15,000 D-glucose monomers linked through -1,4 glycosidic
bonds (Fig. 5a). The length of the polymer is variable and cellulose molecular mass
typically varies between 50-kDa and 1a
6 CH2OH
6 CH2OH
MDa. In wood, cellulose fibers are
embedded in a matrix consisting of
5
5
O
O
other polysaccharides and lignin (a
phenolic
polymer),
yielding
a
4
1 O 4
1
OH
OH
composite material that exhibits
remarkable mechanical properties.
3
2
3
2
Because it is tightly packed and
n
OH
OH
hydrogen-bonded, cellulose is difficult
to degrade into its constituent sugars.
6 CH2OH
6 CH2OH
In herbivores, this slow process is b
5
5
catalyzed by cellulases produced by
O
O
symbiotic organisms living in the
4
1 O 4
1
animal intestinal tract. A related
OH
OH
compound, chitin, is the main structural
component
of
invertebrates
n
3
2
3
2
exoskeleton although it is also found in
NHCCH3
NHCCH3
the cell wall of fungi and certain algae.
Chitin is a linear polymer of N-acetylO
O
D-glucosamine residues linked by - Fig. 5 Structures of cellulose (a) and chitin (b).
1,4 glycosidic bonds (Fig. 5b).
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In addition to playing a structural role, polysaccharides may also serve as nutritional
reservoirs. Starch, which is synthesized as insoluble granules by plants, is composed of a
mixture of -amylose (a linear polymer of thousands of -D-glucose molecules linked to
each other through -1,4 glycosidic bonds) and amylopectin (which consists of -1,4linked glucose chains branching every 25-to-30 residues via -1,6 glycosidic bonds).
a
6 CH2OH
6 CH2OH
6 CH2OH
6 CH2OH
5
5
5
5
O
4
1
4
OH
2
b
1
2
OH
6 CH2OH
6 CH2OH
5
5
4
1
OH
2
O
3
2
OH
O
1
OH
2
O
OH
3
2
OH
6 CH2OH
6 CH2
5
5
O
4
1
OH
3
1
OH
OH
4
O
3
O
4
O
3
OH
O
1
OH
O
3
O
4
OH
O
3
O
1
OH
3
OH
5
O
4
O
2
6 CH2OH
O
4
1
OH
O
2
OH
3
O
2
OH
Fig. 6 Structures of amylose (a) and amylopectin (b). Although both are polymers of
glucose joined through -1,4 glycosidic bonds, amylopectin branches through -1,6
glycosidic bonds.
The molecular mass of amylose, which makes up about 20% of starch, varies between a
few thousands Da to 500-kDa (Fig. 6a). Amylopectin, one of the largest molecules in
Nature, has a molecular mass that ranges from 1- to 2-MDa (Fig. 6b). In animals, the
corresponding storage polysaccharide is glycogen which is chemically similar to
amylopecin but much more highly branched (every 8-to-12 glucose residues on the
average; Fig. 7b). In contrast to cellulose, amylose, amylopectin and glycogen adopt a
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more amorphous structure because the -1,4 glycosidic bonds do not cause successive
glucose residues to flip 180º with respect to each other as -1,4 glycosidic bonds do.
Thus, all these compounds can be easily digested allowing for a rapid infusion of glucose
in times of metabolic need (Fig. 7).
a
b
Fig. 7. Schematic representation of the structural organization of amylopectin (a)
and glycogen (b). In amylopectin, side branches are clustered while glycogen adopts
a tree-like structure.
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