Tricarboxylic Acid (TCA) Cycle | Krebs Cycle | Citric Acid Cycle

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Introduction

The tricarboxylic acid (TCA) cycle, also known as the Krebs or citric acid cycle, occurs following glycolysis.

Under aerobic conditions, acetyl-CoA enters the mitochondria and acts as a starter molecule. Fatty acids via β-oxidation, ketone bodies, branched-chain amino acids and alcohols can also act as a source of acetyl-CoA.

The TCA cycle produces three nicotinamide adenine dinucleotide (NADH), one flavin adenine dinucleotide (FADH₂)_,and one guanosine triphosphate (GTP) per acetyl-CoA that enters. If we include the conversion of pyruvate to acetyl-CoA, which is part of the link reaction, we can add another NADH to the products.

NADH and FADH₂ act as electron carriers. They take electrons to the electron transport chain (ETC), where they are used in oxidative phosphorylation to generate adenosine triphosphate (ATP). As the name suggests, this process requires oxygen, and the TCA cycle is inhibited in its absence.¹

Overview

The whole TCA cycle can be simplified as:¹

Acetyl-CoA + 3NAD + FAD + GDP + Pi → 2CO₂ + 3NADH + FADH₂ + GTP

NAD: nicotinamide adenine dinucleotide; FAD: flavin adenine dinucleotide; GDP: guanosine diphosphate; Pi: phosphate ion.

Pyruvate

Although it is not the sole source of acetyl-CoA and is not strictly part of the TCA cycle, pyruvate is worth mentioning.

Pyruvate is produced by glycolysis and the degradation of amino acids such as alanine and serine. It serves as an essential metabolic crossroad as it can form acetyl-CoA, lactate, oxaloacetate, and alanine.

Pyruvate decarboxylation (link reaction)

The link reaction is an irreversible reaction catalysed by pyruvate dehydrogenase (PDH) that converts pyruvate to acetyl-CoA, and connects glycolysis and the TCA cycle.

Clinical relevance: thiamine deficiency

Thiamine (vitamin B1) is a vital cofactor for PDH. Thiamine deficiency causes Beri-Beri (wet and dry) and Wernicke’s encephalopathy. It is found in alcoholics, malnourished individuals, and women suffering from hyperemesis gravidarum.²

Pyruvate decarboxylation (link reaction) — **Figure 1**. Pyruvate decarboxylation. CoASH = coenzyme A

PDH is a key step in metabolic regulation. Importantly, NADH and acetyl-CoA exert negative feedback on PDH, while pyruvate and adenosine diphosphate (ADP) activate the enzyme.¹

Clinical relevance: pyruvate dehydrogenase deficiency

A metabolic disorder presenting in the young with severe neurological deficits and structural neurological abnormalities, this enzyme deficiency leads to the shuttling of pyruvate to lactate and chronic lactic acidosis.

It is essentially incurable and results in death.³

Acetyl-CoA

Acetyl-CoA has numerous fates depending on the location and the demands of the cell:

Complete oxidation in the TCA cycle
Ketone body generation in the liver
Synthesis of long-chain fatty acids and sterols in the cytoplasm

The TCA cycle

The TCA cycle is primarily a mitochondrial process, which is where most acetyl-CoA is produced, from pyruvate dehydrogenase and β-oxidation. It is also where the ETC is, which signifies the fate of NADH and FADH₂, the products of the TCA cycle.¹

NADH forms 2.5 ATPs, FADH₂forms 1.5 ATPs, and GTP is an ATP equivalent. Therefore, each acetyl-CoA has the potential to create 10 ATPs through the TCA cycle.¹

1. Acetyl-CoA + oxaloacetate → citrate

Acetyl-CoA (two-carbon molecule) and oxaloacetate (four-carbon molecule) join to form citrate (six-carbon molecule)
A condensation reaction, catalysed by citrate synthase, using one molecule of water

2. Citrate → isocitrate

Catalysed by aconitase, releasing and then using one molecule of water

Clinical relevance: rat poison

Rat poison contains fluoroacetate, which is converted to fluorocitrate. This is a potent inhibitor of aconitase. The lethal dose for an animal, 50% of the time (LD₅₀) is 0.2 mg/Kg.¹

3. Isocitrate → α-ketoglutarate

An oxidation reaction catalysed by isocitrate dehydrogenase, which is a rate-limiting step
Converts one NAD to NADH
Releases one CO₂

4. α-ketoglutarate → succinyl-CoA

An oxidation reaction catalysed by α-ketoglutarate dehydrogenase, which is almost identical to pyruvate dehydrogenase, in terms of its substrates, co-factors, products and structure
Converts one NAD to NADH
Releases one CO₂

5. Succinyl-CoA → succinate

Substrate-level phosphorylation, catalysed by succinyl-CoA synthase
Creates one GTP molecule

6. Succinate → fumarate

Catalysed by succinate dehydrogenase, a key regulatory enzyme

7. Fumarate → malate

A hydration reaction, catalysed by fumarase
Freely reversible reaction

Clinical relevance: fumarase deficiency

A rare deficiency, it is characterised by severe neurological impairment presenting soon after birth (encephlomyopathy, dystonia).

It is an autosomal recessive condition.⁴

8. Malate → oxaloacetate

Catalysed by malate dehydrogenase
Converts one NAD to NADH

**Figure 2**. Tricarboxylic acid (citric acid) cycle

TCA cycle intermediates

The TCA cycle is essential not only in energy metabolism but also in the production of biosynthetic intermediates.¹

Some intermediate molecules from the TCA cycle and their functions include:

Citrate → fatty acid and sterol synthesis
α-ketoglutarate → amino acid and neurotransmitter synthesis
Succinyl-CoA → haem synthesis
Malate → gluconeogenesis
Oxaloacetate → amino acid synthesis

Key points

The TCA cycle is a vital, mitochondrial, energy-producing step in oxidative metabolism
Acetyl-CoA, mainly from glycolysis and β-oxidation, enters the cycle to high-energy electron carriers (NADH and FADH₂) that are used in the ETC to produce ATP
Multiple, rare genetic deficiencies of the enzymes involved exist
The TCA cycle also serves as a source of important biosynthetic intermediates

Reviewer

Senior Clinical Scientist

Editor

Dr Jamie Scriven

References

Devlin TM. Textbook of Biochemistry with Clinical Correlations. 6th ed. John Wiley & Sons. 2005.
Smith TJ, Johnson CR, Koshy R, et al. Thiamine deficiency disorders: a clinical perspective. Annals of the New York Academy of Sciences. 2020. Available from: [LINK].
Patel MS, Harris RA. Mammalian α-keto acid dehydrogenase complexes: gene regulation and genetic defects. The FASEB Journal. 1995. Available from: [LINK].
Bourgeron T, Chretien D, Poggi-Bach J, et al. Mutation of the fumarase gene in two siblings with progressive encephalopathy and fumarase deficiency. The Journal of Clinical Investigation. 1994. Available from: [LINK].