Glycolysis

What is glycolysis?

The pathway that breaks one glucose molecule into two pyruvate molecules.

Where does glycolysis occur?

In the cytosol.

Does glycolysis directly require oxygen?

No. Glycolysis can occur with or without oxygen.

How many reactions are in glycolysis?

Ten reactions.

What are the two phases of glycolysis?

• Energy-investment phase

• Energy-payoff phase

What happens during the energy-investment phase?

Two ATP are used to prepare glucose for breakdown.

What happens during the energy-payoff phase?

ATP and NADH are produced as the three-carbon molecules become pyruvate.

What molecule begins glycolysis?

One glucose molecule.

What are the major net products of glycolysis per glucose?

• 2 pyruvate

• 2 ATP

• 2 NADH

How many ATP are consumed during glycolysis?

Two ATP.

How many total ATP are produced during glycolysis?

Four ATP.

What is the net ATP yield of glycolysis?

Two ATP.

Why is the net ATP yield two if glycolysis produces four ATP?

Four ATP are produced, but two ATP were used earlier.

How many NADH are produced during glycolysis?

Two NADH per glucose.

How many pyruvate molecules are produced from one glucose?

Two pyruvate molecules.

What are the major glycolysis intermediates in order?

Glucose
→ glucose-6-phosphate
→ fructose-6-phosphate
→ fructose-1,6-bisphosphate
→ glyceraldehyde-3-phosphate
→ 1,3-bisphosphoglycerate
→ 3-phosphoglycerate
→ 2-phosphoglycerate
→ phosphoenolpyruvate
→ pyruvate

What happens when fructose-1,6-bisphosphate is split?

It forms:

• Glyceraldehyde-3-phosphate, or G3P

• Dihydroxyacetone phosphate, or DHAP

What happens to DHAP during glycolysis?

DHAP is converted into a second G3P molecule.

How many G3P molecules continue through the payoff phase per glucose?

Two G3P molecules.

Why do the payoff-phase reactions occur twice per glucose?

One glucose produces two G3P molecules.

What enzyme converts glucose into glucose-6-phosphate?

Hexokinase in most tissues or glucokinase in the liver.

What enzyme converts fructose-6-phosphate into fructose-1,6-bisphosphate?

Phosphofructokinase-1, or PFK-1.

What enzyme splits fructose-1,6-bisphosphate?

Aldolase.

What enzyme produces NADH during glycolysis?

Glyceraldehyde-3-phosphate dehydrogenase.

What enzyme produces ATP when 1,3-bisphosphoglycerate becomes 3-phosphoglycerate?

Phosphoglycerate kinase.

What enzyme converts phosphoenolpyruvate into pyruvate?

Pyruvate kinase.

At which two reactions is ATP consumed?

• Glucose → glucose-6-phosphate

• Fructose-6-phosphate → fructose-1,6-bisphosphate

At which reaction is NADH produced?

Glyceraldehyde-3-phosphate → 1,3-bisphosphoglycerate.

At which two reactions is ATP produced?

• 1,3-bisphosphoglycerate → 3-phosphoglycerate

• Phosphoenolpyruvate → pyruvate

What type of ATP production occurs during glycolysis?

Substrate-level phosphorylation.

What is substrate-level phosphorylation?

The direct transfer of a phosphate from a metabolic intermediate to ADP to make ATP.

What are the three irreversible glycolysis enzymes?

• Hexokinase or glucokinase

• PFK-1

• Pyruvate kinase

What is the rate-limiting enzyme of glycolysis?

PFK-1.

What is the committed step of glycolysis?

Fructose-6-phosphate → fructose-1,6-bisphosphate.

What molecules activate PFK-1?

• AMP

• ADP

• Fructose-2,6-bisphosphate

What molecules inhibit PFK-1?

• ATP

• Citrate

• Low pH in exercising muscle

What do high ATP and citrate levels tell the cell?

The cell already has enough energy, so glycolysis should slow down.

What does a high AMP level tell the cell?

The cell is low on energy, so glycolysis should speed up.

What happens to pyruvate under aerobic conditions?

Pyruvate enters the mitochondrion and is converted into acetyl-CoA.

What pathway does acetyl-CoA enter?

The citric acid cycle.

What happens to pyruvate under anaerobic conditions in human cells?

Pyruvate is converted into lactate.

What enzyme converts pyruvate into lactate?

Lactate dehydrogenase.

Why is pyruvate converted into lactate?

To regenerate NAD⁺ so glycolysis can continue.

Does converting pyruvate into lactate produce additional ATP?

No.

Why do red blood cells depend completely on glycolysis?

Red blood cells do not have mitochondria.

Are any carbons released as CO₂ during glycolysis?

No.

What is the carbon-count pattern of glycolysis?

One 6-carbon glucose
→ two 3-carbon molecules
→ two 3-carbon pyruvate molecules

Which glycolysis intermediate can enter the pentose phosphate pathway?

Glucose-6-phosphate.

Which glycolysis intermediate can contribute to triglyceride synthesis?

Dihydroxyacetone phosphate, or DHAP.

What reaction connects glycolysis to the citric acid cycle?

Pyruvate → acetyl-CoA.

Recite glycolysis with the major enzymes and energy changes.

1. Glucose → glucose-6-phosphate
   Hexokinase or glucokinase
   Uses ATP

2. Glucose-6-phosphate → fructose-6-phosphate

3. Fructose-6-phosphate → fructose-1,6-bisphosphate
   PFK-1
   Uses ATP

4. Fructose-1,6-bisphosphate → G3P + DHAP
   Aldolase

5. DHAP → G3P

6. G3P → 1,3-bisphosphoglycerate
   Produces NADH

7. 1,3-bisphosphoglycerate → 3-phosphoglycerate
   Produces ATP

8. 3-phosphoglycerate → 2-phosphoglycerate

9. 2-phosphoglycerate → phosphoenolpyruvate

10. Phosphoenolpyruvate → pyruvate
    Pyruvate kinase
    Produces ATP

Net per glucose:

• 2 pyruvate

• 2 ATP

• 2 NADH