Page 165 - Color Atlas of Biochemistry
P. 165
156 Metabolism
Glycogen metabolism important degradative enzyme, glycogen
phosphorylase, cleaves residues from a non-
Glycogen (see p. 40) is used in animals as a reducing end one after another as glucose
carbohydrate reserve, from which glucose 1-phosphate. The larger the number of these
phosphates and glucose can be released ends, the more phosphorylase molecules can
when needed. Glucose storage itself would attack simultaneously. The formation of glu-
not be useful, as high concentrations within cose 1-phosphate instead of glucose has the
cells would make them strongly hypertonic advantage that no ATP is needed to channel
and would therefore cause an influx of water. the released residues into glycolysis or the
By contrast, insoluble glycogen has only low PPP.
osmotic activity. [5] [6] Due to the structure of glycogen
phosphorylase, degradation comes to a halt
four residues away from each branching
A. Glycogen balance
point. Two more enzymes overcome this
Animal glycogen, like amylopectin in plants, is blockage. First, a glucanotransferase moves a
a branched homopolymer of glucose. The glu- trisaccharide from the side chain to the end of
cose residues are linked by an α1 4-glyco- the main chain [5]. A 1,6-glucosidase [6] then
sidic bond. Every tenth or so glucose residue cleaves the single remaining residue as a free
has an additional α1 6bond to another glucose and leaves behind an unbranched
glucose. These branches are extended by chain that is once again accessible to phos-
additional α1 4-linked glucose residues. phorylase.
This structure produces tree-shaped mole- The regulation of glycogen metabolism by
cules consisting of up to 50000 residues interconversion, and the role of hormones in
7
(M > 1 10 Da). these processes, are discussed on p. 120.
Hepatic glycogen is never completely de-
graded. In general, only the nonreducing ends
of the “tree” are shortened, or—when glucose B. Glycogen balance
is abundant—elongated. The reducing end of The human organism can store up to 450 g of
the tree is linked to a special protein, glyco- glycogen—one-third in the liver and almost all
genin. Glycogenin carries out autocatalytic of the remainder in muscle. The glycogen
covalent bonding of the first glucose at one content of the other organs is low.
of its tyrosine residues and elongation of this Hepatic glycogen is mainly used to main-
by up to seven additional glucose residues. It tain the blood glucose level in the postresorp-
is only at this point that glycogen synthase tive phase(seep. 308). Theglycogen content
becomes active to supply further elongation. of the liver therefore varies widely, and can
[1] The formation of glycosidic bonds be- decline to almost zero in periods of extended
tween sugars is endergonic. Initially, there- hunger. After this, gluconeogenesis (see
fore, the activated form—UDP-glucose—is p. 154) takes over the glucose supply for the
synthesized by reaction of glucose 1-phos- organism. Muscle glycogen serves as an energy
phate with UTP (see p. 110). reserve and is not involved in blood glucose
[2] Glycogen synthase now transfers glu- regulation. Muscle does not contain any glu-
cose residues one by one from UDP-glucose cose 6-phosphataseand is thereforeunableto
to the non-reducing ends of the available release glucose into the blood. The glycogen
“branches.” content of muscle therefore does not fluctuate
[3] Once the growing chain has reached a as widely as that of the liver.
specific length (> 11 residues), the branching
enzyme cleaves an oligosaccharide consisting
of 6–7 residues from the end of it, and adds
this into the interior of the same chain or a
neighboring one with α1 6linkage.These
branches are then further extended by glyco-
gen synthase.
[4] The branched structure of glycogen al-
lows rapid release of sugar residues. The most
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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