Metabolic pathways: Glycogen Flashcards
Define glycogenesis?
Synthesis of glycogen from glucose
Define glycogenolysis?
Breakdown of glycogen to form glucose
What is glycogen?
Main storage form of glucose
Where is glycogen present?
Liver and muscle
- same molecule
- different function
Why does liver store glycogen?
broken down between meals and released to maintain blood glucose levels for RBCs and brain
What is the use of glycogen in muscle?
Not available for maintenance of blood glucose
Provides energy via glycolysis and the TCA during bursts of physical activity
When does glycogenolysis occur?
Fluctuates dependant on meal times
What is the primary source of glucose overnight?
Gluconeogenesis when hepatic glycogen is depleted
Describe the structure of glycogen?
Polymer consisting of glucose molecules
a 1-4 and a 1-6 glycosidic links
Where are a 1-4 glycosidic links found?
Between the number 1 carbon of one glucose and the number 4 carbon of another glucose
Form long chains
Where are a 1-6 glycosidic links found?
Between the number 1 carbon of one glucose and the number 6 carbon of another glucose
Introduce branches
How does synthesis of glycogen occur?
Sticking glucose molecules to the ends of glycogen
How does breakdown of glycogen occur?
Cleaving glucose molecules from the ends of glycogen
What is found at the centre of glycogen?
Glycogenin protein
What is the function of glycogenin?
Can add glucose molecules to itself
Why is glycogenin important?
Acts ad a primer for glycogen synthase which can only add glucose to existing glycogen
What happens to glucose in order for it to be used?
phosphorylated to glucose-6-phosphate by hexokinase and ATP
What can glucose-6-phosphate be used for?
Glycolysis
or synthesis of glycogen
How can glucose-6-phosphate be used for glycogen?
Converted to glucose-1-phosphate by phosphoglucomutase
THEN
converted to UDP glucose by UDP- glucose pyrophosphorylase and UTP
What is the substrate for glycogen synthase?
Glucose part of UDP glucose
What happens do left-over UDP?
Phosphorylated to UTP by ATP
What is UDP glucose?
Uridine-diphosphate bound to glucose
What makes UDP glucose active?
bond between second phosphate and glucose.
Ester linkage releases energy on hydrolysis
What does glycogen synthase do?
Adds one glucose molecule to glycogen at a time
Rate limiting
Where do glycogen branches come from?
Transglycosylase produces a1-6 glycosidic branches into glycogen
what catalyses glycogenolysis?
Glycogen phosphorylase
What does glycogen phosphorylase do?
Cleave one glucose at a time
Rate-limiting
What happens to glucose-6-phosphate in the liver during glycogenolysis?
De-phosphorylated and the resulting glucose is released into blood stream?
What happens to glucose-6-phosphate in the skeletal muscle during glycogenolysis?
Cannot be de-phosphorylates by instead used to provide energy via glycolysis and the TCA cycle
What does debranching require?
Additional enzymes
How does free glucose enter the blood?
Via GLUT2 transporter
What is the hormone of the fed state? what does it do?
Insulin
Stimulates glycogen synthatse
Inhibits glycogen phosphorylase
What is the hormone of the fasting state? What does it do?
Glucagon
Stimulate glycogen phosphorylase
Inhibits glycogen synthase
What is the role of adrenaline and cortisol?
Signal a need for energy
Stimulate glycogen phosphorylase
