Krebs Cycle: Key Concepts, Enzymes, and Products in Cellular Respiration

Krebs Cycle

The Citric Acid Cycle or Tricarboxylic Acid (TCA) Cycle.

Location of Krebs Cycle

In the mitochondrial matrix of the cell.

Molecule entering Krebs Cycle

Acetyl-CoA.

Main purpose of Krebs Cycle

To generate high-energy molecules (NADH, FADH₂, and ATP) that power the electron transport chain.

Start and end of Krebs Cycle

Oxaloacetate (OAA) — it's regenerated each cycle.

Step 1 product

Citrate.

Step 1 enzyme

Citrate synthase.

Step 2 product

Isocitrate.

Step 2 enzyme

Aconitase.

Step 3 conversion

Isocitrate is converted to α-ketoglutarate, releasing CO₂ and NADH.

Step 3 enzyme

Isocitrate dehydrogenase.

Step 4 release

α-Ketoglutarate → Succinyl-CoA releases CO₂ and NADH.

Step 4 enzyme

α-Ketoglutarate dehydrogenase.

Step 5 production

Succinyl-CoA → Succinate produces ATP (or GTP).

Step 5 enzyme

Succinyl-CoA synthetase.

Step 6 production

Succinate → Fumarate produces FADH₂.

Step 6 enzyme

Succinate dehydrogenase.

Step 7 enzyme

Fumarate → Malate requires Fumarase.

Step 8 production

Malate → Oxaloacetate produces NADH.

Step 8 enzyme

Malate dehydrogenase.

Total products of one turn

3 NADH, 1 FADH₂, 1 ATP (or GTP), and 2 CO₂.

Krebs Cycle turns per glucose

Two turns (because each glucose gives 2 acetyl-CoA).

Role of NADH and FADH₂

Carry high-energy electrons to the electron transport chain to make more ATP.

Krebs Cycle aerobic or anaerobic

Aerobic, because it requires oxygen indirectly (via the electron transport chain).

The product of glycolosis is pyruvate, that pyruvate enters the

mitochondria

An enzyme complex called blank converts pyruvate into acetyl-CoA.

Pyruvate dehydrogenase

Pyruvate dehydrogenase requires coenzyme

T = Thiamine (TPP)

L = Lipoic acid

C = Coenzyme A

F = FAD

N = NAD

Q: What is the conversion of pyruvate to acetyl-CoA?

A key step linking glycolysis to the citric acid (Krebs) cycle in cellular respiration.

Where does this process occur in prokaryotes?

n the cytoplasm.

What enzyme complex catalyzes the conversion of pyruvate to acetyl-CoA?

The pyruvate dehydrogenase complex (PDC).

What happens to pyruvate during this process?

It is oxidized, releasing CO₂ and forming an acetyl group.

What molecule does the acetyl group attach to?

Coenzyme A (CoA), forming acetyl-CoA.

What is the final product of this reaction?

Acetyl-CoA, which enters the Krebs cycle.

Is ATP produced directly during the conversion of pyruvate to acetyl-CoA?

No, ATP is not directly produced in this step.

Why is this conversion vital?

It allows glucose-derived carbon to enter the citric acid cycle for efficient ATP production.

What are the main by-products of this conversion?

CO₂ (waste gas) and NADH (energy carrier).

At each turn of the cycle

3 NADH,

• 1 FADH2

• 1 GTP (or ATP),

•2 CO2