Carbohydrate metabolism Flashcards
How to we produce glucose? (3)
From food
Breakdown of glycogen (glycogenolysis)
From glycerol and amino acids (gluconeogenesis)
Where is glucose stored?
In the liver/skeletal muscles as glycogen
Describe the structure of glycogen?
Polysaccharide with alpha 1,4 links between adjacent glucose molecules and alpha 1,6 links between branches
What is the purpose of the branching structure of glycogen?
Allows for rapid release of glucose
What are the 3 steps in glycogenesis?
Donor formation
Elongation
Formation of branches
How is the donor formed in glycogenesis?
Glucose-6-p to glucose-1-p
Glucose-1-p reacts with UDP to form UDP-glucose (the activated donor)
How does elongation occur in glycogenesis?
UDP is released and glucose forms alpha-1,4 links with adjacent glucose using glycogen synthase
How does branching occur in glycogenesis?
When the chain is long enough a specific branching enzyme will break an alpha-1,4 link and add tot he chain by an alpha-1,6 link
Describe glycogenesis
- Glucose-6-p to glucose-1-p
- glucose-1-p to UDP-glucose
- UDP released and glucose added to the end of a chain by glycogen synthase (alpha-1,4 link)
- When the chain is long enough a specific branching enzyme will break an alpha-1,4 link and add tot he chain by an alpha-1,6 link
What activates glycogen synthase?
Glucose-6-p and ATP activate allosterically
What inhibits glycogen synthase?
Glycogen and adrenaline inhibit by phosphorylation
What is glycogenolysis?
Breakdown of glycogen to glucose
Describe glycogenolysis
- The sequential phosphorolysis of terminal alpha-1,4 links. the glycosidic bond breaks releasing 1 glucose suing glycogen phosphorylase
- Glucose-1-p converted to glucose-6-p which is the end point in the muscles
- In the liver glucose-6-p converted to glucose using glucose-6-phosphorylase.
- When there are 4 glucoses left on a branch the last 3 are broken off and added to the end of the chain and the last glucose is released as free glucose
Why is glucose-6-p the end point of glycogenolysis in the muscles?
It will fedd straight into glycolysis
Why is glucose-6-p converted to glucose in the liver?
The function of the liver is to increase blood glucose levels
What activates glycogen phosphorylase?
Glucagon (in the liver only) and adrenaline by phosphorylation
Describe the phosphorylation cascade that occurs in the presence of glucagon or adrenaline
ATP converted to cAMP
cAMP activates pKA
pKA inactivates glycogen synthase and activates protein kinase
Protein kinase activates glycogen phosphorylase
When does glycolysis occur?
When there is a limited supply of oxygen
Where does glycolysis occur?
In the cytosol
What is the net energy gain of glycolysis?
2 ATP
What are the steps of glycolysis?
Glucose to glucose-6-p using hexokinase (ATP–>ADP)
Glucose-6-p converted to fructose-6-p
Fructose-6-p converted to fructose-1,6-bisp using phosphofructokinase (ATP–>ADP)
Fructose-1,6-bisp converted to dihydroxyacetone-p or glyceraldehyde-3p.
Dihydroxyacetone-p converted to glyceraldehyde-3p forming 2 glyceraldehyde-3p
2 glyceraldhyde -3p converted to 2 1,3-bisphsophoglycerate (NAD–>NADH)
2 1,3-bisphosphoglycerate converted to 2 3-phosphoglycerate (2ADP–>2ATP substrate level phosphorylation)
2 3-phosphoglycerate converted to 2 2-phospholgycerate
2 2-phosphoglycerate converted to 2 phosphenolpyruvate
2 phosphenolpyruvate converted to 2 pyruvate using pyruvate kinase (2ADP–>2ATP substrate level phosphorylation)
How is ATP formed in glycolysis?
Substrate level phosphorylation
What inhibits hexokinase?
Glucose-6-p inhibits by feedback inhibition (allosteric)
What activates phosphofructokinase?
AMP
