Home › Biochemistry › Citric Acid Cycle (Krebs Cycle)

Citric Acid Cycle

Q: Which enzyme of the tricarboxylic acid cycle is embedded in the inner mitochondrial membrane?

Succinate dehydrogenase (SDH) is embedded in the inner mitochondrial membrane. It also functions as Complex II of the electron transport chain (ETC).

circus acidi citrici

For medical students2 min readUpdated 2026-10-10

The citric acid cycle (CAC, tricarboxylic acid cycle, or Krebs cycle) is a closed sequence of eight biochemical reactions that ensures the complete oxidation of acetyl residues to carbon dioxide. The process serves as the primary source of hydrogen donors for the electron transport chain and is central to cellular energy metabolism.

Cycle substrateAcetyl-CoA, produced by the pyruvate dehydrogenase complex.

ProductsTwo molecules of CO₂, three NADH, one FADH₂, and one high-energy molecule (GTP/ATP).

EnergeticsFor every complete turn of the cycle, the cell yields 12 ATP equivalents.

Primary roleSupplying reduced coenzymes to the electron transport chain (ETC).

Biological Significance and the Role of Oxaloacetate

Direct oxidation of the acetyl group within the cell is difficult due to the high chemical stability of carbon-carbon (C-C) bonds. To overcome this obstacle, the acetyl residue is first incorporated into a larger molecule, rearranged, and only then oxidized in the CAC.

Oxaloacetate plays a key role in this process, functioning as a unique biological catalyst for the cycle:

It is consumed in the very first step by condensing with the acetyl group.
It passes through a cascade of transformations, facilitating the oxidation of numerous acetyl residues.
It is completely regenerated at the final step to enter a new turn of the cycle.

Sequence of Reactions

The citric acid cycle is a strict sequence of eight steps during which the two-carbon fragment is fully oxidized:

Citrate formation (condensation). Acetyl-CoA condenses with oxaloacetate and water in a reaction catalyzed by citrate synthase. Citric acid (citrate) is formed and a large amount of energy is released, making this step irreversible and driving the entire cycle forward.
Isomerization. The enzyme aconitase relocates the hydroxyl group in the citrate molecule, converting it into isocitrate to prepare it for subsequent oxidation.
First oxidative decarboxylation. Isocitrate dehydrogenase cleaves carbon dioxide from the substrate and reduces $NAD^+$ to $NADH$.
Second oxidative decarboxylation. $\alpha$-ketoglutarate is converted into succinyl-CoA by the $\alpha$-ketoglutarate dehydrogenase complex. Another molecule of $CO_2$ is released, and a second $NADH$ is generated.
Substrate-level phosphorylation. The enzyme succinate-CoA ligase (succinate thiokinase) cleaves the high-energy thioester bond in succinyl-CoA. The released energy drives the conversion of GDP to $GTP$ (an energetic equivalent of ATP).
Succinate dehydrogenation. Succinate dehydrogenase oxidizes the resulting succinate to fumarate. During this process, hydrogen is transferred to FAD, forming $FADH_2$.
Hydration. Water is added across the double bond of fumarate by fumarase (fumarate hydratase), yielding malate.
Oxaloacetate regeneration. Malate dehydrogenase oxidizes malate, completing the cycle. This regenerates the initial oxaloacetate and produces a third $NADH$.

Energetic Balance of the CAC

ATP synthesis during the complete oxidation of one molecule of acetyl-CoA occurs via two distinct pathways, which together yield 12 ATP molecules:

Oxidative phosphorylation (via the ETC). Three $NADH$ molecules (generated in steps 3, 4, and 8) enter the electron transport chain, where each drives the synthesis of 3 ATP (totaling 9 ATP). One $FADH_2$ molecule from step 6 yields another 2 ATP. The total yield at this stage is 11 ATP molecules.
Substrate-level phosphorylation. Occurs directly within the cycle at step 5. The substrate itself (succinyl-CoA) serves as the energy donor for high-energy bond formation, producing one molecule of $GTP$. Subsequently, nucleoside diphosphate kinase transfers the phosphate group from GTP to ADP, converting it into ATP.

Regulation of the Main Catabolic Pathway

The rate of CAC reactions is strictly controlled by the cellular energy status. The fundamental regulatory principle is that energy deficiency activates the process, while energy excess inhibits it.

The primary rate-limiting enzymes are citrate synthase, isocitrate dehydrogenase, and $\alpha$-ketoglutarate dehydrogenase.

High-energy signals (inhibitors): High concentrations of ATP and $NADH$ inhibit substrate oxidation. Enzymes may also be subject to product inhibition by citrate or succinyl-CoA.
Low-energy signals (activators): Accumulation of ADP, $NAD^+$, and calcium ions ($Ca^{2+}$, signaling muscle contraction) leads to a rapid acceleration of reactions to replenish cellular ATP reserves.

Frequently asked questions

What vitamins and coenzymes are components of the alpha-ketoglutarate dehydrogenase complex?

While the complete set of vitamins and coenzymes specific to the $\alpha$-ketoglutarate dehydrogenase complex is not explicitly detailed in the source, its direct coenzymes are noted as NAD⁺ and HS-CoA.

In general, dehydrogenase complexes (such as the PDG complex and $\alpha$-KGDH) share a dependence on B-group vitamins acting as coenzyme precursors:

Vitamin B₁ (thiamine) — coenzyme TPP (thiamine pyrophosphate).
Vitamin B₂ (riboflavin) — coenzyme FAD.
Vitamin PP (niacin) — coenzyme NAD.
Vitamin B₅ (pantothenic acid) — coenzyme HS-CoA (Coenzyme A).
Vitamin (lipoic acid) — coenzyme Lipoamide.

In which part of the cell and organelle do the reactions of the Krebs cycle take place?

The reactions of the tricarboxylic acid cycle (Krebs cycle) take place within the mitochondria.

Which enzyme of the tricarboxylic acid cycle is embedded in the inner mitochondrial membrane?

Succinate dehydrogenase (SDH) is embedded in the inner mitochondrial membrane. It also functions as Complex II of the electron transport chain (ETC).

What is the amphibolic (anabolic) function of the Krebs cycle?

The amphibolic (anabolic) function of the Krebs cycle lies in the fact that its intermediates serve as precursors (substrates) for the synthesis of various macromolecules and compounds.

Key biosynthetic pathways include:

Amino acid synthesis — utilizing oxaloacetate, $\alpha$-ketoglutarate, succinate, and malate.
Gluconeogenesis (glucose synthesis) — utilizing oxaloacetate and malate.
Fatty acid synthesis — utilizing citrate.
Heme synthesis — utilizing succinyl-CoA.
Pyrimidine nucleotide synthesis — utilizing oxaloacetate.

What is substrate-level phosphorylation, and at which step of the CAC does it occur?

It is the formation of a high-energy molecule (ATP or GTP) driven by the energy of the substrate itself, independent of the electron transport chain. In the citric acid cycle, it occurs during the fifth reaction, where succinate-CoA ligase converts succinyl-CoA into succinate, generating one molecule of GTP.

How does malonic acid affect cellular respiration?

Malonate is a structural analog of succinate and acts as a competitive inhibitor of the enzyme succinate dehydrogenase. It binds to the active site and blocks the reaction, halting the cycle, terminating electron transfer to the respiratory chain, and stopping cellular oxygen consumption.

Which enzyme catalyzes the rate-limiting step of the CAC?

The rate-limiting step is considered to be the oxidative decarboxylation of isocitrate, catalyzed by NAD⁺-dependent isocitrate dehydrogenase. This enzyme is allosterically activated by ADP and calcium ions, and inhibited by excess NADH.

Go deeper

Detailed mechanism and energetics of the citrate synthase reaction
Structure and coenzymes of the $\alpha$-ketoglutarate dehydrogenase complex
Mechanism of action of nucleoside diphosphate kinase
Reversal of malonate inhibition by the addition of fumarate
Role of calcium ions in the regulation of muscle energy metabolism