General Index
Polysaccharides
Index
The union of many residues of monosaccharides through glycosidic bonds give
Polysaccharides. They can reach an enormous grade of polymerization. Polysaccharides are not informational
polymers, like proteins or nucleic acids, or even oligosaccharides. Their structure is either Homopolymer
AAAAAAAA.... or Copolymer ABABABAB......... that can be considered as the repetition of a
disaccharide. Sometimes polysaccharides have branched structure, while other biopolymers, like proteins
or nucleic acid, are invariably linear.
Polysaccharides have two main functions in the biological media. One is energy
reserve; the other is structural. The anomeric type of the glycosidic bond has to do with these functions;
thus, reserve polysaccharides usually present a-glycosidic bonds, while
structural polysaccharides present b-glycosidic bonds. This difference is clearly
seen in the Glucans, that is, polysaccharides whose monomer is glucose.
Glucans
a-Glucans.
Starch constitutes the main energy reserve in Plants. Starch is a mixture of
two different polysaccharides, Amylose and Amylopectin.
Amylose is a linear polysaccharide formed by glucose residues bound in a (1,4).
As we have seen in the structure of maltose, the axis of succesive residues are at an angle of approximately
120 degrees. Thus amylose has a helicoidal structure with a period of six residues.The model presents a
molecule of 24 residues. Amylose can have several hundred residues.
the other component of starch is Amylopectin:
Amylopectin is formed by many (several thousands) residues of glucose bound by a (1,4)
glycosidic bonds with branches formed from a (1,6) bonds. Glycogen, the main
energy reserve in animal cells, has a similar structure, with a greater degree of branching. It also has
a very high molecular weight.
In some Bacteria, the main energy reserve is Dextran:
Dextran is a polymer formed by glucose residues bound by a-(1,6) glycosidic bonds and
variable degrees of branching.
b-Glucans
The b-glucans, that is, polysaccharides formed from glucose units
linked by b-glycosidic bonds have normally a structural function. Such is the case
of Cellulose:
Cellulose is a polysaccharide formed by glucose residues linked through b (1,4)
glycosidic bonds. As we saw in the case of cellobiose, the axis of the glucose residues linked in b (1,4)
are approximately parallel. Thus, b-glucans have a linear structure, giving
extended fibers, with a very high degree of polymerization.
Chitin is a structural polysaccharide that is encountered in the cuticle of
Arthropods and in the cell wall of Fungi.
Chitin is a linear polysaccharide formed by residues of N-acetylglucosamine linked by
b-(1,4) glycosidic bonds. The blocking of carbon 2 by the acetamido group
makes chitin even less reactive than cellulose.
Glycosaminoglycans
Glycosaminoglycans (GAG) are structural polysaccharides that constitute the main component
of the intercellular matrix of connective tissues and that form the glycoconjugates called Proteoglycans,
not to be confused with Glycoproteins. The structure of GAG is that of a copolymer ABABAB... with different
degrees of polymerization, in which A is an uronic acid (glucuronic or iduronic acid) and B is an
aminoderivative (N-acetylglucosamine or N-acetylgalactosamine), that very frequently appears sulfated.
The bond between A and B is b-(1,3) and the bond between B and A is b-(1,4),
thus alternating both types of bond along the chain, that never appears branched. Both the uronic acid and the
sulfate groups present electronegative charge, giving to the GAG a strong polyanionic character.
Hyaluronic acid is a GAG formed by alternating units of glucuronic acid and
N-acetylglucosamine.
Hyaluronic acid is very abundant in mesodermic tissues, in the vitreous body of the eye,
and in the Wharton jelly of the umbilical cord. Hyaluronic acid has not a definite form in space; it
extends forming a random coil in solution. The electronegative charges retain an important quantity
of solvation water, being an important barrier to the diffusion of macromolecules.
Chondroitinsulfates are GAGs formed by residues of glucuronic acid and
N-acetylgalactosamine. This appears sulfated in carbon 4, in the case of chondroitin-4-sulfate:
And Chondroitin-6-sulfate appears sulfated in carbon 6 of the N-Acetylgalactosamine residue:
Other GAGs are Dermatan sulfate and Queratan sulfate.
Dermatan sulfate has a structure similar to chondroitin-6-sulfate, except that the
uronic acid is L-iduronic acid instead of glucuronic acid.
Queratan sulfate is the only GAG that has no uronic acid. Its structure is
...D-Galactose b-(1,3) N-acetylgalactosamine 6-sulfate b-(1,4)....
All GAGs, in different proportions, appear in the matrix ot mesodermic tissues. Other important GAGs are Heparan sulfate
and Heparin. Heparin appear in the metachromatic granules of mast cells of conective tissue and is widely used as anticoagulant.