1.7 - Glycogen Metabolism Flashcards
linkages used to form glycogen (2)
- a-1,4 linkage (chain)
- a-1,6 linkage (branch)
where is glycogen predominantly stored? (2)
- liver
- muscle
use of liver glycogen
maintain plasma glucose levels between meals (exported to other tissues)
use of muscle glycogen
sustain muscle contraction (fuels own energy requirements via glycolysis)
Reaction of glycogenesis (anabolism)
Glucose-1-phosphate ⇌ Glucose-6-phosphate
glycogenesis enzyme
phosphoglucomutase
UTP/UDP
uridine triphosphate/ uridine diphosphate
enzyme that forms UDP-glucose from glucose, 1-P and UTP?
UDP-glucose pyrophosphorylase
enzyme that forms glycogen from UDP-glucose
glycogen synthase
requirements for glycogen synthase to add glucose units to a pre-existing chain (glycogen synthesis)
must be a pre-existing chain of more than four glucosyl residues
how does glycogen synthesis start?
addition of new UDP-glucose units onto end of existing glycogen chain (sometimes called primer)
protein that carries out glycogen synthesis priming function
glycogenin (found free in cell, remains at core of glycogen granule it has initiated)
glycogenin structure (2)
- protein dimer composed of 2 identical subunits of Mr 37,000
- oligosaccharide of four glucosyl units built up on each molecule (8 total)
how is the first glycosyl residue added to glycogenin (2)
- UDP-glucose donates first glucosyl residue to glycogenin
- carbon 1 of glucose forms covalent link with hydroxyl group of a specific tyrosine residue in glycogenin protein sequence
what glycosidic linkages can glycogen synthase catalyse the formation of?
a1:4-glycosidic linkages
how does glycogen branch? (2)
- another enzyme required to generate a-1:6 linkages needed to branch
- branching enzyme transfers block of 7 residues (including non reducing end) to more interior site, creating new a-1:6-glycosidic linkage
why is glycogen a good energy store?
can be mobilised very rapidly
how is glycogen mobilised rapidly? (2)
- enzyme: phosphorylase and glycogen synthase sensitive to regulation by hormones, stress and muscle contraction
- branched structure helps it mobilise rapidly
how does glycogen make muscles heavier?
glucose is hydrophilic, water associates with glycogen granules, increasing overall weight
mechanism of glycogen breakdown in cells
a1:4-linkages broken by phosphorolysis (enzyme: glycogen phosphorylase)
action of glycogen phosphorylase in glycogenolysis
enzyme gives one molecule of glucose 1-phosphate from each end
phosphorolysis
analogous to hydrolysis (phosphate acts like water in hydrolysis), ATP not involved
kinases and phosphorylases
enzymes involved in phosphorylation of a substrate in cellular processes
kinases role
catalyse transfer of phosphate groups from ATP to sugar or proteins
