8. Energy Storage - Glycogen and Fat Flashcards
Where is glycogen stored and in what form?
In the muscle, as intramyofibrillar and intermyofibrillar glycogen granules. In the liver as granules within hepatocytes.
What is glycogen made of?
It’s a polymer of glucose residues joined by a-1-4-glycosidic bonds in straight lines and a-1-6-glycosidic bonds at branch points (every 8-10 glucose residues).
Describe the reactions involved in glycogenesis.
Glucose + ATP –> glucose 6-phosphate + ADP
Hexokinase (or glucokinase in liver)
Glucose 6-phosphate glucose 1-phosphate
Phosphoglucomutase
Glucose 1-phosphate + UTP + H2O –> UDP-glucose + 2Pi
Uridyl transferase
Glycogen (n res) + UDP-glucose –> glycogen (n+1 residues) + UDP
Glycogen synthase (a-1-4) or branching enzyme (a-1-6)
Describe the reactions involved in glycogenolysis.
Glycogen(n res)+Pi–>glucose 1-phosphate+glycogen (n-1 res)
Glycogen phosphorylase or de-branching enzyme
Glucose 1-phosphate glucose 6-phosphate
Phosphoglucomutase
Glucose 6-phosphate –> muscle –> glycolysis
Or –> liver –> glucose.
What happens to glycogen stores in the liver?
Glucose 6-phosphate is converted to glucose and exported into the blood.
What happens to glycogen stores in the muscle?
Glucose 6-phosphate enters glycolysis for energy production.
What are the rate limiting enzymes for glycogen synthesis and degradation and what are they stimulated and inhibited by?
Glycogen synthesis is regulated by the enzyme glycogen synthase. It is stimulated by high insulin:glucagon ratios.
Glycogen degradation is regulated by the enzyme glycogen phosphylase. It is stimulated by low insulin:glucagon ratios.
What causes glycogen storage diseases?
Deficiency or dysfunction of enzymes of glycogen metabolism.
What are the results of glycogen storage disease?
Excess or diminished glycogen stores. Excess stores lead to tissue damage. Diminished stores lead to hypoglycaemia and poor exercise intolerance.
What is a disease causing glucose 6-phosphate deficiency?
von Gierke’s disease.
What is McArdle disease?
It causes muscle glycogen phosphorylase deficiency.
What happens after 8 hours of fasting when glycogen stores start to deplete?
Gluconeogenesis in the liver and a little bit in the kidney cortex.
What are the three major precursors for gluconeogenesis?
Lactate, glycerol and amino acids.
What are the three key enzymes involved in gluconeogenesis?
Phosphoenolpyruvate carboxykinase (PEPCK), fructose 1,6-biphosphotase and glucose 6-phosphatase.
How is gluconeogenesis regulated?
A low insulin:glucagon ratio stimulates PEPCK and fructose 1,6-bisphosphotase and therefore stimulates gluconeogenesis.
Why are TAGs stored in adipose tissue?
TAGs are hydrophobic so need to be stored in anhydrous conditions, like adipose tissue.
When are lipid stores utilised for energy?
In prolonged exercise, stress, starvation or pregnancy.
What is the difference between hyperplasia and hypertrophy?
Hyperplasia is an increase in the number of cells and hypertrophy is an increase in the size of cells.
Where does lipogenesis take place?
Mainly in the liver.
Outline the steps in lipogenesis.
Glucose –> pyruvate in the cytoplasm
Pyruvate –> acetyl-CoA + OAA –> citrate in the mitochondria
Citrate –> acetyl-CoA + OAA
Acetyl-CoA –> malonyl-CoA using acetyl-CoA carboxylase
What is the key regulatory enzyme of lipogenesis?
Acetyl-CoA carboxylase.
What increases the activity of lipogenesis?
A high insulin:glucagon ration, high ATP:AMP ratio. Also citrate increases activity.
What stimulates fat mobilisation?
Low insulin:glucagon ratio. And adrenaline.
What cells have an absolute requirement for glucose as their energy source?
Erythrocytes & leukocytes, testes, kidney medulla and lens & cornea of the eye.
