74
1
General Principles
5.2 Properties and Occurrence of Glycogen
Glycogen is an α-glucan that is widely distributed in mammals in the liver, muscle, and brain
and in fish, insects, and some species of bacteria, fungi, protozoa, and yeasts, as a reserve
form of chemical energy. It is a high molecular weight polysaccharide (1 × 10 6 to 2 × 10 9 Da)
composed of D-glucopyranose residues linked together by α-(1→4) glycosidic linkages with
10–12% α-(1→6) branch linkages [36]. It has been compared with amylopectin and called
“animal starch.” But, it is quite different from amylopectin in that it has over twice as many
α-(1→6) branch linkages per molecule, giving the many chains an average chain length of 8 to
10 D-glucopyranose residues compared to 20 for amylopectin. Further, the branch linkages do
not occur in clusters, as they do in amylopectin, and are randomly distributed, giving glycogen
different chemical and physical properties from amylopectin. Glycogen does occur in particles or granules of about 25 nm, called β-particles [37]. The β-particles are further combined
into a larger mass, called α-particles, which consists of approximately 100 β-particles. Nevertheless, in contrast to starch granules, the glycogen particles are quite water-soluble, because
of the relatively high percent of branch linkages and the absence of intermolecular bonding,
giving the absence of crystallinity. Glycogen reacts poorly with triiodide, giving a light brown
color or no color, but never the blue color given by starch granules and amylose nor the maroon
color given by amylopectin.
Glycogen has a specific function in the liver of mammals, where its primary role is to maintain
the normal concentration of D-glucose in the blood. In humans, it can provide 100–150 mg
of glucose per minute over a sustained period of 12 h, if necessary [38]. In skeletal muscle,
its primary function is to provide immediate energy for muscle movement by being converted
into α-glucopyranose-1-phosphate and in the human brain, where glycogen normally provides
about 100 g of α-glucose-1-phosphate per day for energy used by the brain [38].
Blue-green algae, which are photosynthetic bacteria (cyanobacteria) and not eukaryotic
algae, synthesize glycogen as a reserve energy storage polysaccharide instead of synthesizing
starch [39]. Glycogen is also synthesized by nonphotosynthetic bacteria, such as Escherichia
coli that synthesizes it intracellularly from UDPGlc [39] and Neisseria perflava that synthesizes it extracellularly from sucrose by the enzyme, amylosucrase [40]. The function of
glycogen for these bacteria has been postulated to provide reserve energy in times of the
absence of nutrients and as a source of energy for the formation of spores [39].
5.3 Properties and Occurrence
of Dextrans, Alternan, Mutan, and Pullulan
Dextrans are a large family of bacterial polysaccharides that have a contiguous series of
D-glucopyranose residues linked α-(1→6) to each other [41]. Over 100 strains of Leuconostoc mesenteroides [42], Streptococcus mutans, S. sobrinus, and S. salivarius produce specific
enzymes, dextransucrases that synthesizes dextrans from sucrose [41]. All of the dextrans are
branched, primarily by α-(1→3) glycosidic linkages, but also by α-(1→2) and α-(1→4) linkages in specific L. mesenteroides strains. Differences in the number and arrangements of these
branches, such as having single glucose branches or long α-(1→6) linked branch chains, and
the order and frequencies of the branches, impart differences in the structures and properties [41,42]. The classic prototypical dextran is the commercial product synthesized by dex-
1
General Principles
5.2 Properties and Occurrence of Glycogen
Glycogen is an α-glucan that is widely distributed in mammals in the liver, muscle, and brain
and in fish, insects, and some species of bacteria, fungi, protozoa, and yeasts, as a reserve
form of chemical energy. It is a high molecular weight polysaccharide (1 × 10 6 to 2 × 10 9 Da)
composed of D-glucopyranose residues linked together by α-(1→4) glycosidic linkages with
10–12% α-(1→6) branch linkages [36]. It has been compared with amylopectin and called
“animal starch.” But, it is quite different from amylopectin in that it has over twice as many
α-(1→6) branch linkages per molecule, giving the many chains an average chain length of 8 to
10 D-glucopyranose residues compared to 20 for amylopectin. Further, the branch linkages do
not occur in clusters, as they do in amylopectin, and are randomly distributed, giving glycogen
different chemical and physical properties from amylopectin. Glycogen does occur in particles or granules of about 25 nm, called β-particles [37]. The β-particles are further combined
into a larger mass, called α-particles, which consists of approximately 100 β-particles. Nevertheless, in contrast to starch granules, the glycogen particles are quite water-soluble, because
of the relatively high percent of branch linkages and the absence of intermolecular bonding,
giving the absence of crystallinity. Glycogen reacts poorly with triiodide, giving a light brown
color or no color, but never the blue color given by starch granules and amylose nor the maroon
color given by amylopectin.
Glycogen has a specific function in the liver of mammals, where its primary role is to maintain
the normal concentration of D-glucose in the blood. In humans, it can provide 100–150 mg
of glucose per minute over a sustained period of 12 h, if necessary [38]. In skeletal muscle,
its primary function is to provide immediate energy for muscle movement by being converted
into α-glucopyranose-1-phosphate and in the human brain, where glycogen normally provides
about 100 g of α-glucose-1-phosphate per day for energy used by the brain [38].
Blue-green algae, which are photosynthetic bacteria (cyanobacteria) and not eukaryotic
algae, synthesize glycogen as a reserve energy storage polysaccharide instead of synthesizing
starch [39]. Glycogen is also synthesized by nonphotosynthetic bacteria, such as Escherichia
coli that synthesizes it intracellularly from UDPGlc [39] and Neisseria perflava that synthesizes it extracellularly from sucrose by the enzyme, amylosucrase [40]. The function of
glycogen for these bacteria has been postulated to provide reserve energy in times of the
absence of nutrients and as a source of energy for the formation of spores [39].
5.3 Properties and Occurrence
of Dextrans, Alternan, Mutan, and Pullulan
Dextrans are a large family of bacterial polysaccharides that have a contiguous series of
D-glucopyranose residues linked α-(1→6) to each other [41]. Over 100 strains of Leuconostoc mesenteroides [42], Streptococcus mutans, S. sobrinus, and S. salivarius produce specific
enzymes, dextransucrases that synthesizes dextrans from sucrose [41]. All of the dextrans are
branched, primarily by α-(1→3) glycosidic linkages, but also by α-(1→2) and α-(1→4) linkages in specific L. mesenteroides strains. Differences in the number and arrangements of these
branches, such as having single glucose branches or long α-(1→6) linked branch chains, and
the order and frequencies of the branches, impart differences in the structures and properties [41,42]. The classic prototypical dextran is the commercial product synthesized by dex-