3. Energy Storage - Glycogen And Fat Flashcards
Which tissues have an absolute requirement for glucose as an energy source?
Red blood cells
Neutrophils
Innermost cells of kidney medulla
Lens of the eye
How is glycogen stored?
As granules either in muscle or the liver
Why is glucose not stored?
Glucose would have great osmotic effect
How are chains of glycogen organised?
Have branches originating from a dimer of the protein glycogenin
Glucose residues liked by alpha 1-4 glycosidic bonds with alpha 1-6 glycosidic bonds forming branch points every 8-10 residues
Why is there branching in glycogen?
It allows efficient release of glucose residues as can be from many sections of glycogen
What is the first reaction in glycogenesis?
Glucose + ATP -> glucose 6-phosphate + ADP Uses hexokinase (glucokinase in liver)
What is the second reaction in glycogenesis?
Glucose 6-phosphate glucose 1-phosphate
Using phosphoglucomutase
What is the third reaction in glycogenesis?
Glucose 1-phosphate + UTP + H2O -> UDP-glucose + PPi
Using G1P uridylyltransferase
What is the fourth reaction in glycogenesis?
Glycogen(n residues) + UDP-glucose -> glycogen(n+1 residues) +UDP
Using glycogen synthase (1-4) or branching enzyme (1-6)
What is the first reaction in glycogenolysis?
Glycogen(n residues) + Pi -> glucose 1-phosphate + glycogen(n-1 residues)
Using glycogen phosphorylase or de-branching enzyme
What is the second reaction in glycogenolysis?
Glucose 1-phosphate glucose 6-phosphate
Using phosphoglucomutase
How are glycogen stores used in liver?
Glycogen -> glucose 1-phosphate -> glucose 6-phosphate -> glucose
G6P converted to glucose and exported to blood
Liver glycogen is a buffer of blood glucose levels
How are glycogen stores used in muscle?
Glycogen -> glucose 1-phosphate -> glucose 6-phosphate -> glycolysis
Muscle lacks enzyme glucose-6-phosphatase
G6P enters glycolysis for energy production
What is the rate limiting enzyme in glycogen synthesis?
Glycogen synthase
What is the rate limiting enzyme for glycogen degradation?
Glycogen phosphorylase
How are muscle glycogen stores different?
Glucagon has no effect as it has no glucagon receptors on membrane
AMP is an allosteric activator of muscle glycogen phosphorylase
Where do glycogen storage diseases arise from?
Deficiency or dysfunction of enzymes of glycogen metabolism
What happens in glycogen storage diseases?
Liver and/or muscle can be effected
Excess glycogen storage can lead to tissue damage
Diminished glycogen stores can lead to hypoglycaemia and poor exercise tolerance
Give 2 examples of glycogen storage diseases
Von Gierke’s disease - glucose 6-phosphatase deficiency
McArdle disease - muscle glycogen phosphorylase deficiency
What is gluconeogenesis?
Production of new glucose
Used after 8 hours of fasting as liver glycogen stores start to deplete
Where does gluconeogenesis occur?
Liver and to lesser extent in kidney cortex
What are the 3 major precursors of gluconeogenesis?
Lactate from anaerobic glycolysis (Cori cycle) Glycerol released from adipose tissue 9breakdown on triglycerides) Amino acids (mainly alanine)
Why is there no net synthesis of glucose from acetyl CoA?
Acetyl CoA cannot be converted into pyruvate because the pyruvate dehydrogenase reaction is irreversible
What are the 3 key enzymes in gluconeogenesis?
PEPCK
Fructose 1,6-bisphosphatase
Glucose-6-phosphatase
What does PEPCK do?
Converts oxaloacetate to phosphoenolpyruvate
What does fructose 1,6-bisphosphatase do?
Converts fructose 1,6-bisphosphate to fructose 6-phosphate
What does glucose 6-phosphatase do?
Converts glucose 6-phosphate to glucose
What are the 2 key enzymes released in response to starvation/fasting, prolonged exercise or stress?
Fructose 1,6-bisphosphatase
PEPCK
What is excess energy intake converted to?
Triacylglycerol (TAG) for storage
Why are TAGs stored in adipose tissue?
Hydrophobic so stored in anhydrous form
Highly efficient energy store
When are TAGs utilised?
In prolonged exercise, stress, starvation, during pregnancy
What control is TAG storage and mobilisation under?
Hormonal control
Where does fatty acid synthesis take place?
Mainly in liver
What happens in fatty acid synthesis?
Glucose -> pyruvate in cytoplasm
Pyruvate into mitochondria and forms acetyl CoA and OAA, which then condense to form citrate
Citrate -> cytoplasm and cleaved back to acetyl CoA and OAA
Acetyl CoA carboxylate produces malonyl-CoA from acetyl CoA
Fatty acid synthase complex builds fatty acids by sequential addition of 2 carbon units provided by malonyl CoA
What is the key regulatory enzyme in liver lipogenesis?
Acetyl CoA carboxylase
What mobilises fat (lipolysis) from adipose tissue?
Hormone sensitive lipase
