Page 332 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Plants 295

The average distance between branch points is only about 10–15 ring units in. comparison to
amylopectin with about 20–30 units between branch points. Many glycogen particles contain small
amounts of protein to which the polysaccharide chains are covalently bonded. Glycogen reacts
weakly with iodine giving a yellow–orange color. It is believed that about 50 linear glucose units is
required to form the blue complex found for amylose and because of the high degree of branching,
few “runs” near 50 linear glucose units are found in glycogen.
Starch and glycogen are produced when the amount of glucose is high and are readily degraded
back to glucose when energy is needed. In plants, this degradation occurs mainly through the action of
two enzymes known as alpha- and beta-amylase. Interestingly, while the alpha-amylase can degrade
starch and glycogen completely to glucose, beta-amylase is not able to degrade the branch points.
In animals, glycogen degradation to give the glucose needed as an energy source or to increase
the blood sugar concentration begins with the action of phosphorylase. Phosphorylase occurs in
active, a, and inactive, b, forms. Phosphorylase b is converted into phosphorylase a by phosphor-
ylation that occurs at the end of a series of events initiated by an increased intercellular concen-
tration of cyclic adenosine monophosphatase (cAMP) and activation of the protein kinase (Figure
10.8). This is reversed by a phosphoprotein phosphatase whose activity is hormonally regulated.
Thus, phosphorylatin initiated by increased intracellular concentrations of cAMP inactivates gly-
cogen synthetase and activates phosphorylase. This is an example of the complex steps that are
moment-by-moment carried out in our bodies. Here, enzymes that are responsible for the glyco-
gen metabolism do not act directly on glycogen but regulate the activity of other enzymes.
Skeletal muscle glycogen delivers glucose primarily as a response to contractile stress. Regulation

occurs though both modification of the enzyme phosphorylase, primarily by the action of epineph-
rine-adrenaline-and allosteric regulation of phosphorylase related to a demand for adenosine tri-
phosphate (ATP).
Glycogen found in the liver seldom is utilized as a source of energy but rather is employed to
regulate blood sugar levels. Some tissues, such as nerve and brain tissue, rely solely of glucose as
their energy source so that a steady supply of sugar is essential to their well-being. It is also found
in some fungi and yeasts. Even some plants such as sweet corn synthesize a polysaccharide that is
similar to glycogen.
OH OH OH

R H H O H H O H H H O
OH H OH H OH
O H O H O H
H OH H OH H OH

OH OH OH O (9.19)
H O O
R H H H O H O
OH H OH H H OH H H H H
O H O H O H OH
H OH O H R
H OH H OH H OH
Glycogen

Dextrans are a high molecular weight, branched extracellular polysaccharide synthesized by
bacteria. These bacteria are found in many places, including the human mouth where they fl our-
ish on sucrose-containing food, which become trapped between our teeth. The generated dextrans
become part of the dental plaque and thus are involved in tooth decay. Dextran-causing bacteria can
also infect sugar cane and sugar beet after harvest and act to not only decrease the yield of sucrose

but also interfere with sugar refining, clogging filters and pipelines. These bacteria can also contam-
inate fruit juices and wines, in fact any ready source of glucose or sucrose.

9/14/2010 3:40:43 PM
K10478.indb 295
K10478.indb 295 9/14/2010 3:40:43 PM