Carbohydrate metabolism

Question 1

How to we produce glucose? (3)

Answer 1

From food
Breakdown of glycogen (glycogenolysis)
From glycerol and amino acids (gluconeogenesis)

Question 2

Where is glucose stored?

Answer 2

In the liver/skeletal muscles as glycogen

Question 3

Describe the structure of glycogen?

Answer 3

Polysaccharide with alpha 1,4 links between adjacent glucose molecules and alpha 1,6 links between branches

Question 4

What is the purpose of the branching structure of glycogen?

Answer 4

Allows for rapid release of glucose

Question 5

What are the 3 steps in glycogenesis?

Answer 5

Donor formation
Elongation
Formation of branches

Question 6

How is the donor formed in glycogenesis?

Answer 6

Glucose-6-p to glucose-1-p

Glucose-1-p reacts with UDP to form UDP-glucose (the activated donor)

Question 7

How does elongation occur in glycogenesis?

Answer 7

UDP is released and glucose forms alpha-1,4 links with adjacent glucose using glycogen synthase

Question 8

How does branching occur in glycogenesis?

Answer 8

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

Question 9

Describe glycogenesis

Answer 9

1. Glucose-6-p to glucose-1-p
2. glucose-1-p to UDP-glucose
3. UDP released and glucose added to the end of a chain by glycogen synthase (alpha-1,4 link)
4. 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

Question 10

What activates glycogen synthase?

Answer 10

Glucose-6-p and ATP activate allosterically

Question 11

What inhibits glycogen synthase?

Answer 11

Glycogen and adrenaline inhibit by phosphorylation

Question 12

What is glycogenolysis?

Answer 12

Breakdown of glycogen to glucose

Question 13

Describe glycogenolysis

Answer 13

1. The sequential phosphorolysis of terminal alpha-1,4 links. the glycosidic bond breaks releasing 1 glucose suing glycogen phosphorylase
2. Glucose-1-p converted to glucose-6-p which is the end point in the muscles
3. In the liver glucose-6-p converted to glucose using glucose-6-phosphorylase.
4. 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

Question 14

Why is glucose-6-p the end point of glycogenolysis in the muscles?

Answer 14

It will fedd straight into glycolysis

Question 15

Why is glucose-6-p converted to glucose in the liver?

Answer 15

The function of the liver is to increase blood glucose levels

Question 16

What activates glycogen phosphorylase?

Answer 16

Glucagon (in the liver only) and adrenaline by phosphorylation

Question 17

Describe the phosphorylation cascade that occurs in the presence of glucagon or adrenaline

Answer 17

ATP converted to cAMP
cAMP activates pKA
pKA inactivates glycogen synthase and activates protein kinase
Protein kinase activates glycogen phosphorylase

Question 18

When does glycolysis occur?

Answer 18

When there is a limited supply of oxygen

Question 19

Where does glycolysis occur?

Answer 19

In the cytosol

Question 20

What is the net energy gain of glycolysis?

Answer 20

2 ATP

Question 21

What are the steps of glycolysis?

Answer 21

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)

Question 22

How is ATP formed in glycolysis?

Answer 22

Substrate level phosphorylation

Question 23

What inhibits hexokinase?

Answer 23

Glucose-6-p inhibits by feedback inhibition (allosteric)

Question 24

What activates phosphofructokinase?

Answer 24

AMP

Question 25

What inhibits phosphfructokinase?

Answer 25

ATP and citric acid

Question 26

What activates pyruvate kinase?

Answer 26

Fructose-1,6-bisp

Question 27

What inhibits pyruvate kinase?

Answer 27

ATP