Krebs Cycle 8 Steps- A Detailed Walkthrough

What the Krebs Cycle Actually Is

The Krebs cycle—also called the citric acid cycle or TCA cycle—is the core of cellular respiration. It happens in the mitochondrial matrix. This is where acetyl-CoA from glycolysis and fatty acid breakdown gets fully oxidized, releasing stored energy.

You need to know this: one acetyl-CoA molecule generates 3 NADH, 1 FADH2, and 1 GTP per turn. Two turns occur per glucose molecule. That's it. That's the whole point.

The cycle doesn't produce ATP directly in significant amounts. It harvests high-energy electron carriers that feed the electron transport chain. Without the Krebs cycle, your cells have no way to extract most of the energy from glucose.

The 8 Steps of the Krebs Cycle (Detailed)

Each step has a specific enzyme. Each step changes one molecule into the next. Memorizing the names helps, but understanding the transformations matters more.

Step 1: Acetyl-CoA + Oxaloacetate → Citrate

Enzyme: Citrate synthase

Acetyl-CoA (2 carbons) bonds to oxaloacetate (4 carbons). The result is citrate (6 carbons). This is the condensation reaction that starts the whole cycle.

This step is essentially irreversible. The enzyme citrate synthase is regulated by ATP, NADH, and succinyl-CoA—all feedback inhibitors. When energy is high, the cycle slows down here.

Step 2: Citrate → Isocitrate

Enzyme: Aconitase

Citrate gets rearranged into isocitrate through dehydration and rehydration. The hydroxyl group moves from one carbon to the adjacent one. This prepares isocitrate for oxidation.

Aconitase contains an iron-sulfur cluster. It's also the target of fluoroacetate, a poison that gets converted into fluorocitrate and inhibits this step.

Step 3: Isocitrate → α-Ketoglutarate + CO₂ + NADH

Enzyme: Isocitrate dehydrogenase

Isocitrate loses a carboxyl group as CO₂. The remaining 5-carbon molecule is α-ketoglutarate. Simultaneously, NAD⁺ gets reduced to NADH.

This is the first decarboxylation step. NADH is the product. The reaction is irreversible and highly regulated. NAD⁺ activates the enzyme; NADH and ATP inhibit it.

Step 4: α-Ketoglutarate → Succinyl-CoA + CO₂ + NADH

Enzyme: α-Ketoglutarate dehydrogenase complex

Another decarboxylation. α-ketoglutarate loses CO₂ and gets attached to coenzyme A, forming succinyl-CoA. Another NADH is produced.

This enzyme complex is similar to pyruvate dehydrogenase. It requires five cofactors: thiamine pyrophosphate, lipoic acid, CoA-SH, FAD, and NAD⁺. A deficiency in any of these cripples the cycle.

Step 5: Succinyl-CoA → Succinate + GTP

Enzyme: Succinyl-CoA synthetase

The only step in the Krebs cycle that directly makes a high-energy phosphate bond. Succinyl-CoA releases its CoA and transfers the energy to make GTP (or ATP in some tissues).

This is a substrate-level phosphorylation. The reaction is reversible. The enzyme is a dimer with two different subunits—each catalyzes the reaction in opposite directions depending on which direction the cycle runs.

Step 6: Succinate → Fumarate + FADH₂

Enzyme: Succinate dehydrogenase

Succinate gets oxidized. Two hydrogens are removed, creating a double bond between carbons 2 and 3. FAD gets reduced to FADH₂.

This is the only membrane-bound enzyme of the Krebs cycle. It's part of Complex II in the electron transport chain. Unlike the other steps, this one happens on the inner mitochondrial membrane, not in the matrix.

Step 7: Fumarate → Malate

Enzyme: Fumarase

Fumarate gets hydrated. Water adds across the double bond, creating malate. This is a stereospecific addition—only L-malate is produced.

No energy carriers are generated here. This is a "holding" step that repositions the molecule for the final oxidation.

Step 8: Malate → Oxaloacetate + NADH

Enzyme: Malate dehydrogenase

Malate gets oxidized. The hydroxyl group becomes a carbonyl group. NAD⁺ gets reduced to NADH. The product is oxaloacetate—the starting molecule that accepts acetyl-CoA to begin the cycle again.

This reaction is highly unfavorable thermodynamically (ΔG°′ = +29.7 kJ/mol). The cycle keeps going because citrate synthase pulls oxaloacetate out of solution by immediately reacting it with acetyl-CoA.

Quick Comparison Table

What the Cycle Actually Produces

Per one turn of the cycle:

• 2 CO₂ molecules released (both decarboxylation steps)
• 3 NADH molecules (steps 3, 4, and 8)
• 1 FADH₂ molecule (step 6)
• 1 GTP molecule (step 5)

Per glucose molecule (2 turns):

• 4 CO₂
• 6 NADH
• 2 FADH₂
• 2 GTP

The NADH and FADH₂ feed the electron transport chain. Each NADH yields roughly 2.5 ATP via oxidative phosphorylation. Each FADH₂ yields roughly 1.5 ATP. The GTP is already equivalent to ATP.

Total ATP from one glucose through the Krebs cycle and oxidative phosphorylation: approximately 30-32 ATP. Glycolysis makes 2 ATP directly. The rest comes from the electron carriers produced by the Krebs cycle.

Regulation of the Krebs Cycle

The cycle runs when there's acetyl-CoA to feed it and NAD⁺ available to accept electrons. It stops when energy is abundant.

Key regulatory points:

• Citrate synthase (step 1): Inhibited by ATP, NADH, succinyl-CoA, and citrate. Activated by ADP.
• Isocitrate dehydrogenase (step 3): Inhibited by ATP and NADH. Activated by ADP and NAD⁺.
• α-Ketoglutarate dehydrogenase (step 4): Inhibited by NADH and succinyl-CoA. Similar regulation to pyruvate dehydrogenase.

The cycle is also tied to the availability of oxaloacetate. If oxaloacetate gets diverted elsewhere (for gluconeogenesis, for example), the cycle slows down.

Connections to Other Pathways

The Krebs cycle isn't isolated. It feeds into and pulls from multiple other metabolic pathways:

• Glycolysis: Produces pyruvate, which becomes acetyl-CoA via pyruvate dehydrogenase.
• Fatty acid oxidation: Produces acetyl-CoA directly.
• Amino acid metabolism: Several amino acids get converted into Krebs cycle intermediates. α-ketoglutarate, succinate, fumarate, and malate all have amino acid counterparts.
• Gluconeogenesis: Oxaloacetate and malate can leave the cycle to make glucose in the liver.
• Electron transport chain: NADH and FADH₂ donate electrons here.

Anaplerotic reactions feed raw materials into the cycle when intermediates get drained. Cataplerotic reactions remove them. The cycle maintains balance through these inputs and outputs.

How to Actually Remember the Steps

Forget mnemonics that try to fit all 8 steps into one phrase. They don't work because the steps aren't equally important.

Focus on the three energy-producing steps:

1. Isocitrate → α-ketoglutarate (NADH + CO₂)
2. α-ketoglutarate → succinyl-CoA (NADH + CO₂)
3. Malate → oxaloacetate (NADH)

Add succinate → fumarate (FADH₂) and succinyl-CoA → succinate (GTP). That's five steps that matter. The other three (citrate synthesis, isomerization, and hydration) are structural.

Know the carbon count at each stage:

• Acetyl-CoA enters: 2 carbons
• Citrate: 6 carbons
• Isocitrate to α-ketoglutarate: loses 1 carbon as CO₂
• α-ketoglutarate to succinyl-CoA: loses 1 carbon as CO₂
• Succinyl-CoA to oxaloacetate: 4 carbons throughout

Clinical Relevance

Defects in Krebs cycle enzymes cause severe metabolic disorders. Deficiencies in α-ketoglutarate dehydrogenase or succinate dehydrogenase are linked to neurological problems and cancer.

Some cancers rewire their metabolism to feed the Krebs cycle differently—a phenomenon called the Warburg effect. They use glutamine as an anaplerotic fuel instead of glucose-derived acetyl-CoA.

Thiamine (vitamin B1) deficiency impairs α-ketoglutarate dehydrogenase and pyruvate dehydrogenase. This leads to neurological damage because the brain depends heavily on these reactions.

Bottom Line

The Krebs cycle is eight enzyme-catalyzed steps that extract energy from acetyl-CoA. Three steps produce NADH. One produces FADH₂. One produces GTP. Two release CO₂. The cycle regenerates oxaloacetate to keep itself running.

That's the entire process. Everything else is details.