Glycogen Metabolism Flashcards
Glycogen Structure
- glycogen is a branched chain homopolysaccharide made up of a-D-glucose
- primary glycosidic bond is an a(1,4) linkage
- after 8-10 glucoses, there is a branch formed by an a(1,6) linkage
- single glycogen molecule may be very large
Glycogenin protein core
1st building block of glycogen
Glycogen Metabolism
- constant blood glucose supply is essential for life
- glucose is the preferred source of energy for brain or in cells w/ lacking mitochondria or in excercising muscle where it is the substrate for anaerobic glycolysis
branching of glycogen
greater amount of glucose that can be stored in a small space
What sources are blood glucose obtained from?
- Diet- may be sporadic, not always reliable
- Gluconeogenesis- slow in responding to falling glucose levels
- Glycogen breakdown- glucose stored in a rapidly mobilizable form
What do depletion of glycogen stores result in?
results in glucose synthesis by gluconeogenesis using a.a from body’s proteins as substrates
What is the function of liver glycogen?
functions to maintain blood glucose levels, primarily during the initial stages of fasting
Where is glycogen stored?
stored in liver and skeletal muscle
What is the function of muscle glycogen?
functions as a fuel reserve for ATP synthesis during muscle contraction
Glycogen degradation requires 2 enzymes
- Glycogen phosphorylase
- Debranching enzyme
Glycogen phosphorylase
- cleaves a(1-4) linkages
- produces glucose 1-phosphate
Debranching enzyme part A
A) First glycosyl (4,4) transferase removes the outer 3 of 4 glucose molecules from the branch to the straight chain forming a new a(1-4) bond
Debranching enzyme part B
Amylo (1,6) glucosidase removes the remaining glucose
Products of glycogen degradation
Glucose 1-P and glucose
Glycogen synthesis
substrate of glycogen synthesis is UDP-glucose (activated form of glucose)
Glycogen synthesis requires 2 enzymes
- Glycogen Synthase
- Branching enzyme forms the branches
What is Glycogen synthase
catalyzes transfer of glucose from UDP-glucose to growing chain
What is Branching enzyme
introduces the a(1,6) linkages
Reciprocal regulation of glycogen synthesis and degradation
- synthesis stimulated by high levels of glucose and ATP (well-fed state)
- breakdown stimulated by low glucose and energy levels (fasted state)
Regulation of glycogen synthase and phosphorylase at 2 levels
- Allosteric
- Hormonal
- Allosteric
Glycogen synthase is activated by elevated levels of glucose 6-P ATP while elevated G6P, ATP and glucose inhibit glycogen phosphorylase
- Hormonal
- Elevated insulin levels increase glycogen synthesis
- Elevated glucagon increase degradation
