MICROBIAL METABOLISM CONTD...
Glycogen is the storage form of glucose. The synthesis and degradation of glycogen
are carefully regulated so that sufficient glucose is available for the body’s
energy needs. Both glycogenesis and glycogenolysis are controlled primarily
by three hormones: insulin, glucagon, and epinephrine.
Glycogen synthesis occurs after a meal, when blood glucose levels are high. It
has long been recognized that the consumption of a carbohydrate meal is followed promptly by liver glycogenesis. The synthesis of glycogen from glucose-6-phosphate involves the following set of reactions:
1.) Synthesis of glucose-1-phosphate: Glucose-6-phosphate is reversibly converted
to glucose-1-phosphate by phosphoglucomutase, an enzyme that
contains a phosphoryl group attached to a reactive serine residue.The enzyme’s phosphoryl group is transferred to glucose-6-phosphate, forming
glucose- 1,6-bisphosphate. As glucose-1-phosphate forms, the phosphoryl
group attached to C-6 is transferred to the enzyme’s serine residue.
2.) Synthesis of UDP-glucose: Glycosidic bond formation is an endergonic
process. Derivatizing the sugar with a good leaving group provides the driving
force for most sugar transfer reactions. For this reason, sugar-nucleotide
synthesis is a common reaction preceding sugar transfer and polymerization
processes. Uridine diphosphate glucose (UDP-glucose) is more reactive than
glucose and is held more securely in the active site of the enzymes catalyzing
transfer reactions (referred to as a group as glycosyl transferases).
Because UDP-glucose contains two phosphoryl bonds, it is a highly reactive
molecule. Formation of UDP-glucose, whose G value is near zero, is
a reversible reaction catalyzed by UDP-glucose pyrophosphorylase.
However, the reaction is driven to completion because pyrophosphate (PPi)
is immediately and irreversibly hydrolyzed by pyrophosphatase with a large
loss of free energy (G 33.5 kJ/mol) .
3.) Synthesis of glycogen from UDP-glucose: The formation of glycogen from
UDP-glucose requires two enzymes: (a) glycogen synthase, which catalyzes
the transfer of the glucosyl group of UDP-glucose to the non reducing ends
of glycogen , and (b) amylo-(1,4 →1,6)-glucosyl transferase
(branching enzyme), which creates the (1,6) linkages for branches in the
Glycogen synthesis requires a preexisting tetrasaccharide composed of
four (1,4)-linked glucosyl residues. The first of these residues is linked
to a specific tyrosine residue in a “primer” protein called glycogenin. The
glycogen chain is then extended by glycogen synthase and branching
enzyme. Large glycogen granules, each consisting of a single highly branched glycogen molecule, can be observed in the cytoplasm of liver
and muscle cells of well-fed animals. The enzymes responsible for glycogen
synthesis and degradation coat each granule’s surface.