Glucose Metabolism: Glycogen and Glycolysis

Question 1

1) Brief summary of glucose

Answer 1

Monosaccharide
Osmotically active
Immediate energy source

Question 2

2) Brief summary of glycogen

Answer 2

Polysaccharide
Low osmolarity
Medium term fuel source

Question 3

3) Explain the importance of liver glycogen in the maintenance of blood glucose concentration

Answer 3

Glucose homeostasis: liver is sensitive to blood glucose concentration and acts to maintain blood glucose under the control of insulin and glucagon

Question 4

4) Explain the importance of skeletal muscle glycogen as a metabolic fuel

Answer 4

Muscle is sensitive to local tissue needs as well as hormones (adrenaline), calcium, AMP, ATP which are all local regulators

Question 5

5) Outline the first step of glycogen synthesis (activation of glucose)

Answer 5

Glucose –> Glucose 6-phosphate –> glucose 1-phosphate –> UDP glucose

Question 6

6) Outline the second step of glycogen synthesis (glycogen synthase reaction)

Answer 6

UDP glucose ( +glycogen primer molecule) –> Glycogen molecule (+UDP released) [branching enzyme needed for alpha 1,6 bond formation]

Question 7

7) Outline the steps of glycogen breakdown (phosphorolysis)

Answer 7

Glycogen chain (+Pi) –> glucose 1-phosphate + glycogen chain [debranching enzyme needed]
Glucose 1-phosphate –> glucose 6-phosphate –> glucose (+Pi)
[G6Pase in last reaction is only in liver and kidney, not muscles as they utilise G6P directly]

Question 8

8) State 3 sources of glucose for glycolysis

Answer 8

• Sugars and starch from diet
• Breakdown of stored glycogen from liver
• Recycled glucose (from LA, AA, glycerol)

Question 9

9) Outline the first 3 activation steps of glycolysis

Answer 9

1. D-Glucose –> glucose 6-phosphate (hexakinase in muscles, glucokinase in liver) [IRREVERSIBLE]
   ATP –> ADP
2. G6P –> Fructose 6-phosphate (phosphohexose isomerase switches C=O group from 1st to 2nd Carbon)
3. F6P –> Fructose 1,6-bisphosphate (phosphofructokinase) ATP–>ADP [IRREVERSIBLE]

Question 10

10) Outline the next 2 steps of glycolysis (splitting of 6C to 3C)

Answer 10

1. F1,6BP –> Glyceraldehyde 3-phosphate and Dihydroxyacetone phosphate (Aldolase)
2. Interconversion between Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (triose phosphate isomerase)

Question 11

11) Outline the oxidation step of glycolysis

Answer 11

Glyceraldehyde 3-phosphate –> 1,3 -bisphosphoglycerate (glyceraldehyde 3-phosphate dehydrogenase)
NAD + Pi –> NADH + H+

Question 12

12) Outline the 4 ATP synthesis stages of glycolysis

Answer 12

1. 1,3-BPG –> 3-phosphoglycerate (3-phosphoglycerate kinase) ADP –> ATP [substrate level phosphorylation]
2. 3-phosphoglycerate –> 2-phosphoglycerate (phosphoglycerate mutase)
3. 2-phosphoglycerate –> phosphoenolpyruvate (enolase)
   produces H2O
4. Phosphoenolpyruvate –> Pyruvate (pyruvate kinase)
   ADP –> ATP [IRREVERSIBLE]

Question 13

13) Define substrate level phosphorylation

Answer 13

A phosphorylated compound donates its phosphate group to ADP, for the phosphorylation to ATP

Question 14

14) Describe processes for the regeneration of NAD+ from NADH under aerobic and anaerobic conditions

Answer 14

Anaerobic: limited O2 supply causes conversion of pyruvate to lactate, in order to convert the cofactor NADH –> NAD+
Aerobic: NAD regenerated via oxidative phosphorylation, donation of e- and H+ to pyruvate/ derivatives

Question 15

15) Describe the role of lactate dehydrogenase in muscle

Answer 15

In the reaction: Pyruvate –> L-Lactate

NADH + H+ –> NAD+ (reduction in muscles, reverse occurs in the liver)

Question 16

16) Give two control mechanisms in the regulation of glycogen and glycolysis

Answer 16

1. Via glucagon/adrenaline
   - hormone binds to cell surface receptor and activates internal signalling pathway, activating a protein kinase which phosphorylates (activates/deactivates)
2. Feedback inhibition by allosteric regulation
   - e.g. phosphofructokinase –> ATP and citrate are inactivators while AMP is an activator, 2 binding sites [ATP may bind to either regulatory or active site]

Question 17

17) State the role of glycolysis in skeletal muscles

Answer 17

ATP production during intense exercise

Question 18

18) State the role of glycolysis in red blood cells

Answer 18

This is the only pathway for ATP production due to absence of mitochondria

Question 19

19) State the role of glycolysis in the brain

Answer 19

Major ATP source, as fats cannot be used as fuels