Ch. 24: Glycogen Degredation

glycogen

a highly branched homopolymer of glucose

it is mobilized quickly (under anerobic conditions) and it supplies glucose to the brain and other organs

what makes glycogen a vital energy reserve

maintain blood glucose homeostasis

goal of the liver in terms of glycogen degredation

provide ATP for muscle contraction

goal of skeletal muscle in terms of glycogen degredation

Step 1 of Glycogen Degredation

glycogen is hydrolyzed to cleave the alpha-1,4 glycosidic bond to generate glucose 1-phosphate

glycogen phosphorylase

cleaves one glucose molecule at a time from glycogen from the non-reducing end in the first step of glycogen degredation

pyridoxal 5’ phosphate

derivative of pyridoxine (Vitamin B6) where 5’ phosphate serves as a proton donor and acceptor

Step 2 of glycogen degredation

remodeling the branching point in glycogen with debranching enzymes

transferase

enzyme in step 2 of glycogen degredation that shifts a block of 3 glycosyl residues from 1 outer branch to the end

alpha-1,6 glucosidase

enzyme in step 2 of glycogen degredation that cleaves the alpha-1,6 glycosidic bond at the branching point, which releases the free glucose

Step 3 of glycogen degredation

generation of glucose 6-phosphate

glycogen phosphorylase

enzyme in step 3 of glycogen degradation that generates glucose 1-phosphate

phosphoglucomutase

enzyme in step 3 of glycogen degradation that converts glucose 1-phosphate to glucose 6-phosphate

reversible phosphorylation of glycogen phosphorylase

regulation by hormones: b is dephosphorylated (less active form) and a is phosphorylated (more active)

allosteric regulation of glycogen phosphorylase

regulation by metabolites: t (tense/less active) r (relaxed/more active)

liver glycogen phosphorylase default state

a (active form)

phosphorylase kinase

homotetramer protein that phosphorylates glycogen phosphorylase, the activity of this enzyme is stimulated by PKA signaling and intracellular calcium ions that reflect cellular needs

epinephrine effect on glycogen degredation

released during exercise to stimulate glycogen breakdown mainly in muscle cells

glucagon effect on glycogen degradation

secreted when blood glucose is low, stimulates liver to breakdown glycogen