ML5: Stage II CHO catabolism

Question 1

What is glycolysis? Give the overall reaction.

Answer 1

• The central pathway of sugar metabolism
• Present in all cell types
• Takes place in the cytoplasm

glucose + 2 Pi + 2 ADP + 2 NAD⁺ →
2 pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

Question 2

Outline the key steps of glycolysis.

Answer 2

Stage 1

• Input of some energy (ATP)
• ATP used to phosphorylate intermediates (i.e. gets reactants ready by trapping and making them more reactive)
• glucose → fructose-1,6-bisphosphate

Stage 2 (substrate-level phosphorylation)

• C6 converted to 2 x C3
• 4 x ATP produced
• NADH produced

Question 3

What are the different enzymes involved in the preparatory phase of glycolysis and what are their roles?

Answer 3

• Hexokinase (HK)
  
  • Phosphorylates glucose
  • glucose → glucose-6-phosphate
• Phosphohexose isomerase
  
  • ​Changes ring structure
  • glucose-6-phosphate → fructose-6-phosphate
• Phosphodructokinase (PFK)
  
  • ​Phosphorylates fructose-6-phosphate
  • ATP consumed
  • Irreversible
  • fructose-6-phosphate → fructose-1,6-bisphosphate
• Aldolase
  
  • Converts doubly phosphorylated C6 sugar into two monophosphorylated 3C sugars
  • fructose-1,6-bisphosphate → dihydroxyacetone phosphate + glyceraldehyde-3-phosphate

Question 4

What are the enzymes involved in the payoff phase of glycolysis and what are their roles?

Answer 4

• Glyceraldehyde-3-phosphate dehydrogenase
  
  • ​Generation of NADH
  • glyceraldehde-3-phosphate + Pi → 1,3-bisphosphoglycerate
• Phosphoglycerate kinase
  
  • ​Generation of ATP
  • Substrate-level phosphorylation
  • 1,3-bisphosphoglycerate → 3-phosphoglycerate
• Phosphoglycerate mutase
  
  • ​3-phosphoglycerate → 2-phosphoglycerate
• Enolase
  
  • ​2-phosphoglycerase → phosphoenolpyruvate
• Pyruvate kinase
  
  • ​Irreversible conversion of phosphoenolpyruvate to pyruvate
  • Substrate-level phosphorylation
  • phosphoenolpyruvate → pyruvate

Question 5

What happens to pyruvate under aerobic conditions?

Answer 5

• It is further oxidised in citric acid cycle
• NADH used to form ATP on oxidative phosphorylation
• NAD⁺ regenerated to allow glycolysis to continue

Question 6

What happens after pyruvate is made under anaerobic conditions?

Answer 6

• Stage III of catabolism does not occur
• NADH would not be oxidised
• Glycolysis stops due to lack of NAD⁺
• No ATP produced

Question 7

What is the equation for glycolysis under anaerobic conditions in yeast and some microorganisms?

Answer 7

glucose → F-1,6-BP → pyruvate → acetaldehye → ethanol

Question 8

Why is glycolysis under anaerobic conditions in yeast useful to humans?

Answer 8

The end-product (ethanol) can be utilised for fermentation

Question 9

What is the equation for glycolysis under anaerobic conditions in mammals? What enzyme is used as a catalyst? How does it work?

Answer 9

pyruvate → lactate

using lactate dehydrogenase, which converts pyruvate to lactate by reduction

Question 10

When does glycolysis under anaerobic conditions occur in mammals?

Answer 10

When the supply of oxygen is inadequate (e.g. skeletal muscle during rigorous exercise)

In cells without mitochondria (e.g. RBCs)

Question 11

Why is lactate referred to as a ‘metabolic dead-end’?

Answer 11

It is taken to the liver and converted to pyruvate using the same enzyme

Question 12

Why does metabolism need to be regulated and how is this achieved?

Answer 12

• Must be regulated in response to the demands of the cell, i.e. when active, energy is lost; when eating, energy is replenished; when resting, there is very little energy change
• In metabolic pathways, enzymes catalysing essentially irreversible steps are potential sites of control
• Some enzymes can be regulated by:
  
  • Allostery/allosterism: activator/inhibitor binds at another site
  • Covalent modification: signals from outside the cell, e.g. hormones, causing phosphorylation/dephosphorylation (switching on/off)

Question 13

Which enzymes regulate glycolysis? Why is regulation of these enzymes required?

Answer 13

• Hexokinase
• Phosphofructokinase (key regulatory stage; AMP stimulates PFK)
• Pyruvate kinase

All steps involving these enzymes are irreversible so regulation is required to control activity

Question 14

What disease can be caused by incorrect metabolism of galactose?

Answer 14

Galactosaemia

• Cataract formation (non-classical) caused by a lack of galactokinase, so galactose builds up
• Abnormal mental development (classical) caused by a build up of galactose and galactose-1-phosphate due to product build-up slowing down the enzyme

Question 15

Give an overview of the pentose phosphate pathway (PPP).

Answer 15

• Cytosolic
• Important in liver, RBC, adipose tissue
• Phase I: Oxidative
  
  • Irreversible
  • Produces NADPH, CO₂ and 5-carbon sugars
• Phase II: Non-oxidative
  
  • Reversible
  • Produces 3-carbon sugars

Question 16

Why is the pentose phosphate pathway important?

Answer 16

• NADPH production
  
  • Reductive biosynthesis (e.g. FA biosynthesis)
  • Removal of hydrogen peroxide (free radicals)
  • Reduction of abnormally formed disulphide bonds which leads to sickle cell anaemia
  • Cytochrome P450 enzyme function (e.g. paracetamol break-down over time)
  • Phagocytosis by white blood cells
  • Synthesis of nitric oxide
• 5-carbon sugar production
  
  • Nucleic acid biosynthesis

Question 17

What results from a genetic deficiency of the G6PDH?

Answer 17

Anaemia

Heinz bodies (aggregates of haemoglobin) form, resulting in anaemia