Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Pyrophosphate (PPi) hydrolyses into orthophosphate:
PPi + H 2O → 2 Pi (16.5)
Activated glucose units are transferred on the terminal hydroxyl groups at C-4 of
glycogen being attached thereto by means of α-1,4 glycosidic linkages. This process
is catalyzed by glycogen synthase:
Glycogen + UDP-glucose → Glycogen + 1 + UDP (16.6)
Glycogen synthase attaches these subsequent glucose moieties to the polysaccharide
chains with at least four sugar units.
Synthesis of new glycogen requires initiation from glycogenin, a 37-kDa protein
carrying an oligosaccharide moiety composed of glucose units linked with one
another through α-1,4 glycosidic bonds.
Glycogen synthase catalyzes selectively the formation of α-1,4 glycosidic linkages.
Branching takes place after formation of a sufficiently long, linear polymer.
A branching enzyme, amylo-(1,4-1,6)-transglucosylase, splits the α-1,4 glycosidic
bond and forms the α-1,6 glycosidic bond instead. This process produces a considerable
number of nonreducing ends open for interaction with glycogen synthase and
phosphorylase. Activity of glycogen synthase in liver depends on concentration of
glucose in blood and the activity of that enzyme in muscles depends on the level of
G-6-P. In either case, a high concentration of these factors is beneficial. Glycogen
synthase is inactive when phosphorylase is active, and, vice versa, when glycogen
synthase is active the phosphorylase is not. Extracellular factors such as epinephrine
or glucagon and intracellular factors such as calcium ions, which stimulate phosphorylase
activity, inhibit the synthesis of glycogen.
16.2.2 GLYCOGENOLYSIS AND GLYCOLYSIS
Degradation of glycogen into G-6-P and pyruvate under aerobic and anaerobic
conditions and transformation of pyruvate into lactate taking place exclusively under
anaerobic conditions is called glycogenolysis. 4 Decomposition of G-6-P to pyruvate
under aerobic and anaerobic conditions and into lactate under anaerobic conditions
exclusively is known as glycolysis.
Antemortem glycolysis reactions in animal tissues are reversible except in processes
catalyzed by phosphofructokinase and pyruvate kinase. In the presence of
oxygen, pyruvate decarboxylates and irreversibly turns into acetyl coenzyme A
(acetyl-CoA). Acetyl-CoA is oxidized in the citric acid cycle into CO 2 and H 2O. In
each case, electrons are accepted by NAD + . Oxidized NAD + is recovered after
electrons from NADH are transferred to the oxygen molecule.
After exsanguination, neither oxygen is transported to the tissues nor are metabolites
removed from them. Postmortem complex of pyruvate dehydrogenase is inactive.
Reduction of pyruvate into lactate takes place rather than its transformation
into acetyl-CoA. Accumulation of lactate in tissues reduces their pH value. The pH