Glycogen Metabolism Flashcards
What is a primary reason for converting glucose to glycogen?
linking many glucose residues together reduces osmotic pressure in the cell
residues liked by a-1,4 and a-1,6
Where is glycogen primarily found?
liver and muscle
the glucose residue at the reducing end of glycogen is bound to what?
a protein called glucogenin
What advantage does the branching nature of glycogen offer?
Multiple rxns can be occurring at different points of the glycogen at the same time
What is the primary role of glycogenolysis in the liver?
glycogen is broken down to maintain BG levels
What is the primary role of glycogenolysis in skeletal muscle?
glycogen is broken down for ATP generation as needed (i.e. doesn’t respond to the feed-fast cycle)- the only fate is glycolysis
What allows the liver to move broken down glycogen out of the cell to maintain BG?
the liver has glu-6-phosphatase to change glu-6-p into glucose (BG lowers about 2 hrs after a meal)
After 24 hrs, how do you get glucose?
glycogen stores are depleted, so the body uses gluconeogenesis
What are the intermediates from glucose to glycogen?
glucose -> glu-6-p -> glu-1-p -> glycogen
What converts glu-6-p to glu-1-p?
phosphoglucomutase
What is the first step from glu-1-p to glycogen?
UTP changes glu-1-p into uridine diphosphate glucose (UDP-glucose)
What are the steps of adding UDP-glucose to a growing glycogen chain?
1) glycogen synthase adds a glucose residue from the UDP-glucose (i.e. it takes glucose, leaving UDP) and adds it to the chain by a 1,4 linkage
2) when the branches of the glycogen get too big, Amylo-4,6-transferase (branching enzyme) moves a portion of a 1,4 branch and re-attaches it to another spot on the glycogen polymer by a 1,6 linkage (cleaves 1,4- generates 1,6)
What does glycogen synthase need to synthesize glycogen?
a primer
Where does glycogen synthase get a primer from?
glycogenin
How does glycogenin make a primer for de novo glycogen synthesis (using glycogen synthase)?
Glycogenin catalyzes the addition of the gluosyl residue from UDP-glucose to a tyrosine residue within the protein. Then glycogenic can extend the chain until its long enough to act as a primer
What is the first step in glucogenolysis?
glycogen phosphorylase successively removes glucosyl residues from the glycogen chain (one at a time) and releases them as glu-1-phosphates- break 1,4 linkages
this step is regulated
What does debranching enzyme do?
two actions/’activities’: (note that it has both of the following activities- these are not alternative names of the enzyme)
1) 4:4 transferase activity:
can act as a 4:4 transferase (i.e. moves a 1,4 linked residue to another spot on the chain in a 1,4 manner
2) a-1,6-glucosidase activity:
completely breaks 1,6 bonds on terminal glucosyl resides on a branch and releases it as GLUCOSE not glu-1-p (important distinction!!)- remember, glycogen phosphorylase will remove sucessive glucosyl resides as GLU-1-P
Does glycogenesis and glycogenolysis occur at the same time ever within a cell?
No. the cell ensures that only one or the other is occurring
How long do BG levels stay elevated after a meal?
2 hrs. After this is reservoir is gone, liver will breakdown glycogen for energy (will muscle start to break down glycogen for use in the bloodstream?). What happens after the glycogen is gone?
Assuming that both liver and muscle have deb ranching enzymes with a-1,6-glucosidase activity, what stops glucose residues from leaving muscle cells once cleaved from glycogen?
Hexokinase has an extremely low Km (~0.1mM), so any ‘free’ glucose will be rapidly phosphorylated in the cell before intracellular glucose concentrations have a chance to rise enough to be transported out of the cell by facilitated movement (using GLUT)
