ML9: Glycogenesis and gluconeogenesis Flashcards
What happens to food fuel supplies that exceed immediate needs?
Carbohydrates: stored as glycogen
Lipids: stored as TAGs
Proteins: not stored but can be converted to lipids and stored if necessary
Where is the majority of fuel stored in the human body?
Adipose tissue (lipids), muslce (protein and glycogen), and liver (glycogen)
What is the largest energy store in humans?
Lipids (triacylglycerols)
When is muscle protein broken down?
During prolonged starvation
How is glucose stored in the body? Give facts about this molecule.
As glycogen
High molecular weight polymers (107 – 108)
Highly polar so attracts H2O (this limits storage)
No specialised storage tissue (stored in granules in muscle and liver)
How is glycogen used in skeletal muscle?
Skeletal muscles are designed to contract and provide mechanical power
Glycogen is used to power contraction
It is converted to glucose-1P, then glucose-6P, then undergoes glycolysis to form ATP, lactate, and CO2
How is glycogen used in the liver?
The liver is used as a reservoir of glycogen and stores reserve glycogen for release as glucose when blood sugar is low
Glycogen is converted to glucose-1P then glucose-6P, then is broken down by glucose-6-phosphatase into glucose so that it can be released into the blood. Gluconeogenesis also raises blood glucose levels.
How is glycogen made?
Glycogen synthase (makes a straight chain) adds glucosyl residues from UDP-glucose
It catalyses α-1,4 linkages
Approx. 7 residues are transferred and joined via α-1,6 linkages by a branching enzyme
How is glycogen broken down?
Glycogen phosphorylase removes glucose-1-phosphate units
Glycogen phosporylase can only break α-1,4 linkages
Approx. 3 residues are transferred onto another branch by transferase
Glucosidase breaks α-1,6 linkages
Why are different pathways and enzymes involved in glycogen synthesis and breakdown?
With separate pathways and enzymes, it is possible to control processes separately, otherwise the cycle would just be continuous and energy would be lost
There is a ‘sliding scale’ of activity of synthesis and breakdown – not 100% on or off, but tightly regulated
Explain the reciprocal regulaton of glycogen synthesis and breakdown.
When glucagon or adrenaline is secreted, glycogen synthases decrease in quantity, whilst phosphorylases increase and inhibit
When insulin is secreted, glycogen synthases increase and amplify the signals, whilst phosphorylases decrease
Which organ is primarily affected by glycogen storage diseases?
Liver
What happens if we run out of glycogen?
There is only enough liver glycogen for 8-12 hours
Skeletal muscle glycogen cannot be used to maintain plasma glucose because G6P cannot be released from the muscle cells (it lacks glucose-6-phospatase)
New glucose must be synthesised by the body through a process called gluconeogenesis
What is gluconeogenesis? Where does it take place?
Synthesis of glucose from (mainly) non-carbohydrate sources
Occurs in the liver (and kidney cortex)
Why is gluconeogenesis necessary?
The liver only has enough glycogen for 8-12 hours so runs out
Some tissues are totally dependent on glucose, such as RBCs, WBCs, kidney medulla and the lens of eye
