The Citric Acid Cycle

Overview

The citric acid cycle is a key energy-producing pathway in cells that connects carbohydrate, fat and protein metabolism, through the breakdown of the common intermediate substrate, acetyl-CoA. It involves a series of enzymatically regulated steps (or reactions) that occur in the mitochondria. The overall reaction is:

1 Acetyl CoA → 1 ATP (as GTP) + 2 CO₂ + 3 NADH + 1 FADH₂

Regulation of this pathway is complex, but similar to other metabolic pathways like Glycolysis it is responsive to substrate supply and also to the ATP requirements or energetic demands of the cell. The major regulatory mechanism is allosteric modulation of the key regulatory enzymes: Citrate synthase, Succinate dehydrogenase and α-Ketoglutarate dehydrogenase.

Key points:

The Citric Acid cycle, is also known as the Tricarboxylic acid (TCA) cycle or Krebs cycle
The pathway produces ATP and generates carbon dioxide (CO₂) as the end product
The NADH and FADH₂ generated by this pathway are used by many other pathways including oxidative phosphorylation (see Electron Transport Chain pathway) to generate ATP. Each cycle of the pathway will ultimately produce 11 molecules of ATP through these intermediate electron carriers.

Pyruvate

The final product of Glycolysis, Pyruvate can be transported into the mitochondria for oxidation by citric acid cycle (see The Citric Acid Cycle pathway) or converted to Lactate (in the cytosol).

Pyruvate Dehydrogenase (PDH)

Within the mitochondria, pyruvate can undergo conversion to acetyl CoA and enter the citric acid cycle. The mitochondrial pyruvate dehydrogenase (PDH) complex catalyses this reaction.

Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH

Key points:

Localised metabolites ensures that the conversion of pyruvate to acetyl CoA and subsequent use by the citric acid cycle is closely coupled to ATP demand of the cell.
Pyruvate dehydrogenase is controlled by altering the proportion of this enzyme in a less-active form and a more-active form.

Activated by:

Transformation of pyruvate dehydrogenase to the more-active form occurs in response to an increase in ADP (reflects low cell energy availability), and an increase in the availability of its substrate pyruvate (see Figure B).
In muscle, the transformation of PDH to the more-active form also occurs in response to an increase in the localised levels of calcium that occur with muscle contraction.

Inhibited by:

When energy availability in the cell is high, elevated levels of ATP, Acetyl CoA, and NADH act to feedback and lower the activity of pyruvate dehydrogenase (see Figure A) by conversion of the enzyme into the less-active form.

Acetyl CoA

Acetyl-CoA , or Acetyl coenzyme A, is the primary substrate for the Citric acid cycle. Acetyl CoA is generated by the breakdown of carbohydrates through glycolysis (see Glycolysis pathway), by the breakdown of fatty acids through β-oxidation (see Beta-Oxidation pathway) and via amino acid (protein) metabolism.

Citrate Synthase

Citrate synthase is the enzyme that regulates the first committed step in the Citric acid cycle:

Acetyl-CoA + Oxaloacetate → Citrate + CoA

Key points:

Reaction is controlled by substrate availability

Activated by:

Elevated levels of its substrate Acetyl CoA and Oxaloacetate

Inhibited by:

Elevated levels of citrate (product)

Citrate

Elevated levels of this citric acid cycle intermediate, Citrate can feedback to inhibit the key glycolytic enzyme (Phosphofructokinase, see Glycolysis pathway), as well as Pyruvate dehydrogenase, and key enzymes of the citric acid cycle including Citrate synthase, and Succinate dehydrogenase.

This level of regulation ensures that energy substrate supply is closely matched to the ATP requirements or energetic demands of the cell.

Citrate is also exported from the citric acid cycle and mitochondria, into the cytosol, where it is converted back to acetyl-CoA for the synthesis of fatty acids (see Fatty Acid Synthesis pathway) and cell signalling functions as a substrate for protein acetylation.

Isocitrate dehydrogenase

This reaction is one of the irreversible reactions in the citric acid cycle and is therefore carefully regulated:

Isocitrate + NAD⁺ → α-ketoglutarate + NADH + CO₂

Key points:

NADH generated by this pathway is used by many other pathways including oxidative phosphorylation (see Electron Transport Chain pathway) to further generate energy in the form of ATP

α-Ketogluturate dehydrogenase

This enzyme (also known as 2-oxoglutarate dehydrogenase) is the most important enzyme in the citric acid cycle as it controls this rate limiting reaction:

α-Ketoglutarate + CoA + NAD⁺ → Succinyl CoA + NADH + CO₂

Key points:

NADH generated by this pathway is used by many other pathways including oxidative phosphorylation (see Electron Transport Chain pathway) to further generate energy in the form of ATP
α-Ketoglutarate can also be produced through transamination reactions from a number of amino acids

Succinyl CoA synthetase

This enzyme catalyses the conversion of Succinyl CoA to Succinate, which also facilitates the formation of energy (ATP) in the form of GTP.

Succinyl CoA → Succinate + CoA + ATP (as GTP)

Succinate dehydrogenase

This enzyme plays an important role in regulating reactions in both the citric acid cycle and the electron transport chain.

Succinate + FAD → Fumarate + FADH₂

Key points:

FADH₂ generated by this pathway is used in oxidative phosphorylation (see Electron Transport Chain pathway) to further generate energy in the form of ATP
High levels of this enzyme in tissues correlates with high mitochondrial content and oxidative potential
Succinate dehydrogenase is also an essential component of complex II of the electron transport chain (see Electron Transport Chain pathway).

Malate dehydrogenase

The mitochondria form of this enzyme controls the reversible reaction between malate and oxaloacetate

Malate + NAD⁺ ↔ Oxaloacetate + NADH + H⁺

For the citric acid cycle to operate at a high rate (when the energy demand of the cell is high) then there must be sufficient oxaloacetate produced to accept acetyl groups from acetyl CoA in the first reaction of the citric acid cycle.

Key points:

NADH generated by this pathway is used by many other pathways including oxidative phosphorylation (see Electron Transport Chain pathway) to further generate energy in the form of ATP
When the energy demands of the cell are low, this enzyme also plays an important role through this reversible reaction to produce sufficient malate to be transported out of the mitochondria where it is converted to glucose via the gluconeogenic pathway (see Gluconeogenesis pathway)