glycogen metabolism I Flashcards
glycogen
long chain homopolymer of glucose molecules with branches
glucose molecules within linear chain linked together via
alpha-1,4 glycosidic bonds
branch points between glucose monomers of separate chains are formed via
alpha-1,6 glycosidic bonds
non-reducing ends contain
a terminal glucose with a free hydroxyl group at carbon 4
reducing end consists of glucose monomer connected to a protein called
glycogenin
glycogenin creates a short glycogen polymer on itself and serves as a
primer for glycogen synthesis
glycogen is degraded and extended from
non-reducing ends
glycogen is stored in
liver, muscle, and other tissues
glycogen is stored as
granules
granules contain not only glycogen but also
the enzymes needed for glycogen metabolism
function of liver glycogen
regulates blood glucose levels
function of muscle glycogen
provides reservoir of fuel (glucose) for physical activity
regulation of glycogen metabolism
allosteric control
covalent modification through reversible phosphorylation of key enzymes
hormonal control
glycogenesis
synthesis of glycogen
3 key steps of glycogenesis
trapping and activation of glucose
elongation of a glycogen primer
branching of glycogen chains
rate limiting enzyme of glycogenesis
glycogen synthase (catalyzes transfer of glucose from UDP-glucose to non-reducing end of glycogen chain)
forms an alpha-1,4 glycosidic bond between glucose molecules
when glycogen chain reaches ______ residues, a fragment of the chain (about 7 residues long) is broken off at an alpha-1,4 link and reattached elsewhere through
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alpha-1,6 link by glucosyl (4:6) transferase
what does branching achieve
increases solubility of glycogen and increases number of terminal non-reducing ends
increases rate at which glycogen can be synthesized and degraded
a new branch point must be at least _____ residues away from a preexisting branch
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glycogenolysis overview pathway
glycogen –> glucose 1-phosphate (via glycogen phosphorylase) –> glucose 6-phosphate (via phophoglucomutase) –> free glucose can enter glycolysis (muscle/brain), free glucose can enter blood (for use by other tissues), or can enter pentose phosphate pathway (resulting in ribose and NADPH)
glycogenolysis in liver
glycogen –> glucose 1-P (via glycogen phosphorylase with co-factor vitamin B6) (rate limiting step) –> glucose 6-P (via PGM) –> glucose (via glucose 6-phosphatase) –> free glucose released into blood to maintain blood glucose
glycogenolysis in muscle
glycogen –> glucose 1-P (via glycogen phosphorylase with co-factor vitamin B6) (rate limiting step) –> glucose 6-P (via PGM) –> pyruvate (via glycolysis) –> pyruvate kept in muscle to produce ATP
4 key enzymes of glycogenolysis:
- glycogen phosphorylase (rate limiting step): degrade glycogen to glucose 1-P)
- debranching enzyme transferase: transfers a block of 3 of the remaining 4 glucose residues to the non-reducing end of the main chain forming an alpha-1,4 bond
- alpha-1,6-glucosidase: cleaves the alpha-1,6 bond of the single remaining glucose residue to release the free glucose
- converts glycogen breakdown product suitable for further metabolism (glucose 6-Phosphatase or PGM)????
phosphorolysis of glucose residues (via glycogen phosphorylase) continues until GP gets within
4 residues of the alpha-1,6 linkage of a branch point
glycogen phosphorylase (GP) adds an ________ and releases a glucose residue as ________
orthophosphate
glucose 1-P
cofactor required for glycogen phosphorylase (GP)
pyridoxal phosphate (vitamin B6)
phosphoglucomutase
converts glucose 1-P to glucose 6-P
how does phosphoglucomutase (PGM) work
a phosphoryl group is transferred from the enzyme to the substrate, and a different phosphoryl group is transferred back to restore the enzyme to its initial state
debranching enzyme transferase
transfers a block of 3 of the remaining 4 glucose residues to the non-reducing end of the main chain forming an alpha-1,4 bond
debranching enzyme or alpha-1,6-glucosidase
cleaves the alpha-1,6 bond of the single remaining glucose residue to release the free glucose
glucose 6-Phosphatase is present in ____ cells not in ____ cells
liver
muscle
small amount of glycogenolysis occurs in lysosome via
lysosomal alpha-1,6 glucosidase (acid maltase)
defect in acid maltase (lysosomal alpha-1,6 glucosidase)
pompe disease
glycogenolysis yields ____ glucose 1-P to every ____ free glucose
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