W5- Lecture 20- Glycogen metabolism Flashcards
name 3 qualities of glycogen
Very large molecule
Highly-branched structure
Insoluble, but heavily hydrated
what are the two types of bonds between monomers in glycogen ?
how often does branches occur ?
α-1,4 links for straight chains
α-1,6 for branch points
every 8-12 residues
what is the protein core of glycogen called ?
Glycogenin
why is branching in glycogen important ?
Improves solubility
More available sites for synthesis and degradation
how is glycogen stored and with what other compounds ?
Stored in granules
contain enzymes and regulatory proteins required for synthesis and degradation
how does the liver use glycogen ?
to maintain blood glucose level
Released over long periods of time
Liver cells contain glucose-6-phosphatase, convert G-6-P to glucose for export into blood
how do the muscle cells use glycogen ?
provides energy for muscle
Released instantaneously
when required
Muscle cells lack glucose-6-phosphatase
glycogen overview
- Conversion of glucose to glucose-1-phosphate
- Activation of glucose via addition of UDP to G-1-P
- Addition of UDP-Glucose to glycogen molecule via α-1,4 bond
- Reversal of the above for catabolism
explain activation of glucose
ATP used in the process to generate UTP from UDP
Reaction of UTP + Glucose-1-phosphate produced UDP glucose + PPi
Catalysed by UDP-glucose pyrophosphorylase
ATP used in the phosphorylation of glucose to glucose-1-phosphate. Traps glucose in cell
Thus for every mole of glucose, 2 moles ATP consumed in this step.
explain Glycogenesis
Catalysed by glycogen synthase
Addition of glucose, via an α-1,4 linkage, to the non-reducing end of the growing glycogen chains
Release of the UDP molecule – regenerated to UTP for use in glucose activation
when a glycogen ring opens in solution what is the reducing end / non reducing end
reducing end -aldehyde group
Non-reducing end is at the other side of this structure (C4)
explain how glycogen becomes branched ?
After 10 or so glucose units have been added to glycogen α-1,6 branch point is formed (branching enzyme).
The enzyme breaks one of the α-1,4 bonds & transfers a block of residues (usually 7) to a more interior site in the glycogen molecule.
These are re-attached by an α-1,6 bond.
Branching enzyme also known as amylo-(1-4→1-6) transglycosylase
explain Glycogen phosphorylase action on glycogen molecule
Breaks α-1,4 glycosidic bonds from the non-reducing end of the glycogen molecule, using inorganic phosphate to allow production of glucose-1-phosphate
Processive enzyme – stays attached to the glycogen molecule and keeps working until it gets too close to a branch point (4 glucose residues)
Debranching enzyme then required
Explain action of debranching enzyme
debranching enzyme has dual activity
1) 3 residues transferred to via an α-1,4 linkage to a different non-reducing end (transferase activity)
2) The residue at the α-1,6 branching point is removed via debranching activity
Explain the formation of G-6-P
Glucose-1-phosphate cannot directly enter glycolysis
Phosphoglucomutase converts it to glucose-6-phosphate
