Glycolysis and Gluconeogenesis

What is glycolysis?

Catabolic pathway occurring in the cytosol which converts glucose → pyruvate

How many steps are in glycolysis?

10

What is special about the glycolysis pathway?

The only pathway that can act in an aerobic and anaerobic environments

What are the two main types of glycolysis?

• aerobic glycolysis: sufficient oxygen so final products are CO2 and H2O

• anaerobic glycolysis: insufficient oxygen so final product is lactate

What are the two main phases of glycolysis?

1) Energy-requiring phase (preparatory phase)

2) Energy-releasing phase (payoff phase)

What is gluconeogenesis?

Metabolic pathway in which glucose is synthesised from non-carbohydrate precursors (e.g. lactate, glycerol, amino acids)

What does lactate dehydrogenase do?

Converts lactate to pyruvate

What does aminotransferase (transaminase) do?

Converts alanine to pyruvate

What is the pathway for conversion of glycerol to DHAP?

1) Glycerol kinase uses 1 ATP to convert glycerol to glycerol-3-phosphate

2) Glycerol phosphate dehydrogenase uses NAD+ to convert glycerol-3-phosphate to dihydroxyacetone phosphate (DHAP)

Describe the first reaction in gluconeogenesis

Conversion of pyruvate to phosphoenolpyruvate, catalysed by pyruvate carboxylase and phosphoenolpyruvate carboxykinase

Describe the second reaction in gluconeogenesis

Fructose-1,6-bisphosphate converted to fructose-6-phosphate by fructose-6-phosphatase

Where does gluconeogenesis take place?

Different subcellular compartments for the steps 

Where does glycolysis take place?

Cytosol

Describe the terminal (third) step of gluconeogenesis

Glucose 6-phosphate is converted to glucose by glucose-6-phosphatase

What 4 factors control flux in metabolic pathways?

1) Substrate availability

2) Enzyme concentration

3) Allosteric regulation of enzymes

4) Covalent modulation of enzymes

Where are GLUT 4 transporters found and what do they do?

What is the role of GLUT2?

What 4 ways does insulin increase glycolysis?

What 3 things is gluconeogenesis regulated by?

Glycolysis is glucose → ?

Pyruvate

When is gluconeogenesis needed?

When glucose stores a depleted, lactate/glycerol/amino acids are converted to glucose (pyruvate → glucose)

Anabolic vs catabolic reactions

• Anabolic → small molecules assembled into large ones (requires energy)

• Catabolic → large molecules broken down into small ones (energy is released)

Describe step 1 of glycolysis

• GLUT facilitates entry of glucose into cells

• Glucose is phosphorylated to glucose-6-phosphate, catalysed by hexokinase, using 1 ATP and using Mg2+ as a cofactor

Why is step 1 of glucose important?

1) Produces a more reactive molecule

2) Traps glucose inside the cell

Describe step 2 of glycolysis

Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase. The process of the ring structure changing is called aldose-ketose isomerisation

Describe step 3 of glycolysis

Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate through action of phosphofructokinase (PFK) transferring phosphate group from ATP to carbon 1.

IRREVERSIBLE STEP

BOTTLENECK/COMMITTED STEP

What is the importance of step 3 of glycolysis?

1) Prevents re-formation of glucose-6-phosphate

2) Second phosphate → one in each triose in step 4

Describe step 4 of glycolysis

Fructose-1,6-bisphosphate is cleaved into two 3-carbon compounds: GAP and DHAP (isomers). This is catalysed by aldolase. Only GAP continues on glycolytic pathway

Describe step 5 of glycolysis

DHAP is converted to GAP by triose-phosphate isomerase. Results in 2 molecules of GAP (glyceraldehyde-3-phosphate) being formed

Describe step 6 of glycolysis

• GAP is oxidised to 1,3-bisphosphate by GAP dehydrogenase which generates energy by adding Pi

• - NAD+ is reduced and NADH is generated

• aerobic → NADH enters into mitochondria

• anaerobic → NADH utilised by lactate dehydrogenase

Describe step 7 of glycolysis

• 1,3-biphosphoglycerate is converted to 3-phosphoglycerate

• - only kinase in glycolysis that is reversible

• ATP is formed in substrate level phosphorylation

Describe step 8 of glycolysis

• 3-phosphoglycerate isomerised to 2-phosphoglycerate by phosphoglycerate mutase (required Mg2+)

Why does step 8 of glycolysis take place (mutase)?

The phosphate is transferred from position 3 to 2 in preparation for transfer to ADP

Describe step 9 of glycolysis

• Enolization/dehydration step in which 2-phosphoglycerate converted to phosphoenolpyruvate (PEP)

• Enolase is used and requires Mg2+

Describe step 10 of glycolysis

• PEP is dephosphorylated to pyruvate by pyruvate kinase

• second substrate level phosphorylation, generates ATP

• IRREVERSIBLE

What is the purpose of malate dehydrogenase?

Converts oxaloacetate to malate so it can be transported out of mitochondria into cytosol before being converted back to oxaloacetate (no transporter mechanism otherwise)