Respiration in Plants · Lesson 4 of 9
Pyruvate Oxidation and the Citric Acid Cycle
“Acetyl-CoA formation, TCA intermediates, carbon losses, 3 NADH, 1 FADH₂ and 1 GTP per acetyl-CoA.”
• Trace the conversion of pyruvate into acetyl-CoA and identify its redox and carbon products. • Explain the sequence and regeneration of oxaloacetate in the TCA cycle. • Identify two oxidative decarboxylations and their carbon changes. • Count NADH, FADH₂ and GTP/ATP per cycle turn and per glucose. • Explain why carbon oxidation is coupled to later electron transport.
From three-carbon pyruvate to two-carbon acetyl-CoA
After glycolysis, each three-carbon pyruvate can enter the mitochondrion in cells carrying out oxygen-supported oxidative metabolism. In the matrix, a multi-enzyme pyruvate dehydrogenase complex removes a carboxyl carbon as CO₂, transfers electrons to NAD⁺ and attaches the remaining two-carbon acetyl group to coenzyme A. This links cytosolic glycolysis to the mitochondrial citric acid cycle.
Removal of CO₂ from a molecule accompanied by oxidation and transfer of electrons to an acceptor such as NAD⁺.
The simplified reaction is pyruvate + NAD⁺ + CoA → acetyl-CoA + CO₂ + NADH + H⁺. The source indicates a pyruvate dehydrogenase system with several coenzymes and a magnesium-related enzymatic context. In full biochemical detail the complex uses multiple cofactors including thiamine pyrophosphate, lipoamide, FAD and NAD⁺. For one starting glucose, two pyruvates enter, producing two acetyl-CoA, two CO₂ and two NADH.
Crucially, pyruvate dehydrogenase is not a reaction within the cyclic set of TCA intermediates itself. It is the linking step that prepares an acetyl group for entry. The citric acid cycle is located in the mitochondrial matrix, apart from succinate dehydrogenase, which is attached to the inner membrane as complex II.
How a two-carbon acetyl group enters a four-carbon cycle
A cyclic series of mitochondrial reactions that oxidises acetyl-derived carbon and regenerates oxaloacetate while reducing NAD⁺ and FAD and making GTP or ATP.
The two-carbon acetyl group from acetyl-CoA combines with oxaloacetate (OAA), a four-carbon acceptor, to form six-carbon citrate. The enzyme citrate synthase catalyses this reaction, using water and releasing coenzyme A. The cycle is possible because OAA is regenerated at the end. Think of OAA as a reusable carrier of the entering acetyl fragment rather than a molecule consumed permanently in each turn.
Carbon accounting explains the cycle's name: citrate has three carboxyl groups and six carbons, and two separate decarboxylation steps later release CO₂. Intermediate molecules change from six carbons to five and then four. Oxaloacetate is ultimately restored, so the pathway can accept the next acetyl group.
The eight principal transformations of the cycle
| Step | Intermediate transformation | Enzyme | Key output |
|---|---|---|---|
| 1 | Oxaloacetate (4C) + acetyl-CoA (2C) → citrate (6C) | Citrate synthase | CoA released |
| 2 | Citrate ↔ isocitrate (6C) | Aconitase | Isomer rearrangement |
| 3 | Isocitrate (6C) → α-ketoglutarate (5C) | Isocitrate dehydrogenase | CO₂ + NADH |
| 4 | α-ketoglutarate (5C) → succinyl-CoA (4C) | α-Ketoglutarate dehydrogenase complex | CO₂ + NADH |
| 5 | Succinyl-CoA → succinate (4C) | Succinyl-CoA synthetase | GTP (or ATP in some isoforms) |
| 6 | Succinate → fumarate (4C) | Succinate dehydrogenase | FADH₂-equivalent reducing power |
| 7 | Fumarate → malate (4C) | Fumarase | Water added |
| 8 | Malate → oxaloacetate (4C) | Malate dehydrogenase | NADH |
Two oxidation and CO₂-release stages
The isocitrate dehydrogenase reaction oxidises isocitrate and releases CO₂, creating the five-carbon α-ketoglutarate and NADH. The α-ketoglutarate dehydrogenase system then removes another CO₂ while producing four-carbon succinyl-CoA and a second NADH. These reactions explain why one turn releases two carbon dioxide molecules. Together with the earlier pyruvate-to-acetyl-CoA step, processing one pyruvate yields a total of three CO₂, consistent with its original three carbons.
An important nuance: the two CO₂ molecules released in the first turn are not necessarily the same two carbon atoms that just arrived as acetyl-CoA, because carbons can be retained through early turns of the cycle. The net stoichiometry is still oxidation of two-carbon acetyl units across repeated cyclic processing. This distinction prevents confusing overall carbon balance with atom-by-atom tracing.
GTP, FADH₂ and the return of oxaloacetate
Succinyl-CoA contains an energy-rich thioester linkage. During its conversion to succinate, substrate-level phosphorylation yields one GTP (or ATP in certain biochemical systems). GTP can transfer phosphate to ADP through a coupled nucleotide-diphosphate kinase reaction, making ATP. Succinate dehydrogenase oxidises succinate to fumarate and passes electrons through enzyme-bound FAD into the respiratory chain, often described as the FADH₂ contribution.
Fumarase adds water to fumarate, creating malate, and malate dehydrogenase oxidises malate to regenerate oxaloacetate while reducing another NAD⁺ to NADH. The restored OAA can combine with the next acetyl group, completing the cycle. In total, each acetyl-CoA yields three NADH, one FADH₂-equivalent contribution, one GTP/ATP and two CO₂.
What the cycle produces per glucose
One glucose yields two pyruvates and therefore two acetyl-CoA molecules, so the TCA cycle turns twice. The two turns together make six NADH, two FADH₂ equivalents, two GTP/ATP and four CO₂. Adding the link reaction gives eight NADH from pyruvate processing and TCA combined and six CO₂ overall (two from pyruvate oxidation, four from the two cycle turns). Glycolysis additionally supplied two NADH and a net of two ATP.
| Stage | NADH per glucose | FADH₂ per glucose | ATP/GTP directly | CO₂ |
|---|---|---|---|---|
| Glycolysis | 2 | 0 | 2 net ATP | 0 |
| Pyruvate oxidation (2 pyruvates) | 2 | 0 | 0 | 2 |
| TCA (2 acetyl-CoA) | 6 | 2 | 2 GTP/ATP | 4 |
| Combined before oxidative phosphorylation | 10 | 2 | 4 ATP/GTP equivalents | 6 |
Why oxygen is still required despite not appearing in the cycle's carbon equations
The source notes that oxygen is not directly used by most reactions of pyruvate oxidation and the TCA cycle. Nevertheless, sustained aerobic flux depends on the respiratory electron transport system using oxygen to accept electrons. Without reoxidation of NADH and FADH₂, supplies of NAD⁺ and FAD-linked oxidised states become limited. This slows or stops reactions requiring those acceptors. Thus oxygen is essential indirectly for continued TCA activity even when not listed as a reactant in every cycle step.
The immediate output of most TCA redox steps is stored reducing power rather than a large amount of ATP. Those electron carriers feed the mitochondrial inner-membrane system described next. The small GTP yield illustrates substrate-level phosphorylation within the cycle.
A cycle that is also a source of building blocks
OAA, α-ketoglutarate and other intermediate carbon skeletons can be withdrawn for amino-acid and other biosynthetic pathways. If withdrawals are extensive, the cell must replenish cycle intermediates through anaplerotic reactions, or the cycle would slow. This point provides a bridge to the chapter's later account of amphibolic metabolism: TCA serves energy extraction and supplies precursors for synthesis.
Problem
Each acetyl-CoA traversing the TCA cycle produces 3 NADH, 1 FADH₂ and 1 GTP. If one glucose makes two acetyl-CoA molecules, how many of each product result from the cycle alone?
- 1.Two acetyl groups mean two turns of the cycle.
- 2.Multiply each per-turn yield by two: 6 NADH, 2 FADH₂ and 2 GTP.
- 3.The cycle also releases 4 CO₂ across those turns; the prior pyruvate oxidation releases another 2 CO₂.
Quick Quiz
Quiz
What happens to pyruvate during the link reaction?
What is the four-carbon acceptor regenerated by the TCA cycle?
What six-carbon product forms when acetyl-CoA enters the TCA cycle?
How many NADH molecules are generated in one TCA turn per acetyl-CoA?
Which cycle reaction produces FADH₂-equivalent reducing power?
Which cycle step makes GTP through substrate-level phosphorylation?
How many CO₂ molecules are released during two TCA turns?
Why does the TCA cycle depend on oxygen-supported electron transport indirectly?
Practice Problems
- Write the link reaction and per-glucose products.
- Trace the main TCA intermediates in order.
- Account for products of one TCA cycle turn and of two turns.
- Compare pyruvate oxidation with the TCA cycle.
- Why does a mitochondrion need a continuous supply of oxidised NAD⁺ and FAD-linked acceptor states?
Pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH + H⁺. Two pyruvates per glucose yield 2 acetyl-CoA, 2 CO₂ and 2 NADH.
Key Takeaways
• Pyruvate dehydrogenase converts each 3C pyruvate to 2C acetyl-CoA, CO₂ and NADH. • Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). • Each TCA turn releases two CO₂ and conserves energy as 3 NADH, 1 FADH₂ and 1 GTP/ATP. • One glucose gives two acetyl-CoA and two TCA turns: 6 NADH, 2 FADH₂, 2 GTP and 4 CO₂ from TCA. • Pyruvate oxidation adds 2 NADH and 2 CO₂ per glucose, separate from TCA. • Regeneration of OAA, NAD⁺ and oxidised flavin-linked carriers sustains continuing cycle flux. • TCA intermediates also support biosynthesis, linking respiration with anabolic pathways.