The Citric Acid Cycle

Introduction

Acetyl-CoA delivers a two-carbon unit to the citric acid cycle. The cycle combines that unit with four-carbon oxaloacetate, rearranges and oxidises the resulting carbon skeleton, releases two CO₂ and restores oxaloacetate. Its main immediate energy output is reduced electron carriers, not a large pile of directly formed ATP. Counting all eight reactions once per acetyl group prevents confusion between a cycle turn and complete glucose oxidation.

Core explanation

Citrate synthase joins acetyl-CoA and oxaloacetate, producing six-carbon citrate and free CoA-SH. Hydrolysis of the acetyl-CoA thioester helps drive the condensation. Aconitase rearranges citrate to isocitrate through dehydration and rehydration, moving a hydroxyl group to prepare oxidation. No carbon is lost in these first two steps; the cycle has six-carbon intermediates.

Isocitrate dehydrogenase oxidises isocitrate and releases CO₂, producing five-carbon alpha-ketoglutarate and NADH. Alpha-ketoglutarate dehydrogenase performs another oxidative decarboxylation, forming four-carbon succinyl-CoA, a second CO₂ and another NADH. The latter complex uses cofactor chemistry related to pyruvate dehydrogenase. Two carbons have left as CO₂ by this point, but isotopic tracing shows that the carbon atoms entering as the acetyl group do not necessarily leave as those two CO₂ in the very first turn; carbon positions are mixed across later cycles.

Succinyl-CoA synthetase converts succinyl-CoA to succinate while conserving thioester energy in substrate-level phosphorylation, commonly forming GTP or ATP depending on the enzyme and organism. GTP can be converted to ATP by nucleotide-diphosphate kinase, but the immediate nucleotide product should be stated for the specific system. Succinate dehydrogenase then oxidises succinate to fumarate while reducing bound FAD to FADH₂. This enzyme is also part of the mitochondrial electron-transport chain as Complex II.

Fumarase hydrates fumarate to malate. Malate dehydrogenase oxidises malate to oxaloacetate and reduces NAD⁺ to NADH. The regenerated oxaloacetate can accept another acetyl group. Although the isolated malate-dehydrogenase reaction can be unfavourable under standard conditions, consumption of oxaloacetate by citrate synthase can keep the actual pathway moving under cellular conditions.

The net yield per acetyl-CoA entering the cycle is two CO₂, three NADH, one FADH₂ and one GTP or ATP equivalent, with oxaloacetate regenerated. Per glucose, two acetyl-CoA usually enter, doubling these cycle outputs. This accounting does not include the two NADH produced by pyruvate dehydrogenase from two pyruvate or the two glycolytic NADH; those belong to separate stages.

Step-by-step reasoning

Trace carbon counts 4+2→6→6→5→4→4→4→4→4 and mark where the two CO₂ leave. Mark NADH at isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase and malate dehydrogenase; FADH₂ at succinate dehydrogenase; and GTP or ATP at succinyl-CoA synthetase. Confirm oxaloacetate appears at both start and finish and therefore cancels from a net turn equation.

Visual explanation

Draw an eight-arrow circle with citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate and oxaloacetate. Put acetyl-CoA entering at the oxaloacetate-to-citrate arrow. Use distinct symbols for three NADH, one FADH₂, one GTP/ATP and two CO₂, with carbon counts beside each intermediate.

Real-world analogy

A reusable carrier can accept a load, process it through several stations and return empty for another load. Oxaloacetate resembles that reusable carrier because it accepts an acetyl group and is regenerated. The analogy does not mean oxaloacetate molecules are permanently conserved in a cell; intermediates can be withdrawn and replenished by other pathways.

Real-world example

In a respiring mitochondrion, NADH and FADH₂ from the cycle donate electrons to the respiratory chain, which helps regenerate NAD⁺ and FAD for continued oxidation. If electron transfer is blocked and oxidised carriers become scarce, cycle dehydrogenase steps slow even though oxygen is not a direct substrate of the cycle's eight reactions.

Why?

Why does the cycle need oxaloacetate but not consume a net molecule of it per turn? Oxaloacetate combines with acetyl-CoA at the start and is regenerated from malate at the end. Withdrawal of oxaloacetate for biosynthesis still needs replenishment, or cycle flux can decline.

Common misconception

“Each acetyl carbon becomes CO₂ during the very next cycle turn.” The net cycle releases two CO₂ per acetyl entry, but carbon atoms from the incoming acetyl group are not necessarily the two released immediately. Isotopic labelling distinguishes net stoichiometry from atom-by-atom fate.

Worked example

Three acetyl-CoA molecules complete three cycle turns under conditions where the nucleotide step produces GTP. Net outputs are six CO₂, nine NADH, three FADH₂ and three GTP, while three CoA-SH are released from acetyl groups and oxaloacetate is regenerated. The numerical yield does not say which particular incoming carbon labels appear in the first six CO₂ molecules.

Quick check

1. Which cycle step makes FADH₂ rather than NADH? Answer: Succinate dehydrogenase oxidises succinate to fumarate using bound FAD, transferring electrons onward through its associated carrier pathway.

Exam focus

Know the sequence and four-, five- and six-carbon counts. Separate cycle yields per acetyl-CoA from yields per glucose and from pyruvate-dehydrogenase outputs. State GTP or ATP according to the system and avoid claiming the first released CO₂ must come from the incoming acetyl carbons.

Advanced insight

The cycle is amphibolic: citrate, alpha-ketoglutarate, succinyl-CoA and oxaloacetate can supply biosynthetic precursors. Their withdrawal must be balanced by anaplerotic reactions if oxidative flux is to continue. Thus the cycle's apparent “catalytic” oxaloacetate pool is dynamically maintained, not sealed from the rest of metabolism.

Summary

The eight-step citric acid cycle oxidises an acetyl unit while regenerating oxaloacetate. Per turn it gives two CO₂, three NADH, one FADH₂ and one ATP/GTP equivalent. Carbon accounting, electron-carrier regeneration and intermediate replenishment all matter for sustained flux.

Practice questions

1. How many cycle-derived NADH and FADH₂ arise from the two acetyl-CoA produced from one glucose? Answer: Two turns yield six NADH and two FADH₂ from the cycle itself. This excludes NADH from glycolysis and pyruvate dehydrogenase. 2. If oxaloacetate is withdrawn faster than it is replenished, what happens to citrate formation? Answer: Citrate synthase has less oxaloacetate acceptor available, so citrate formation and subsequent cycle flux tend to fall even if acetyl-CoA remains abundant. 3. Which reaction produces a nucleotide triphosphate directly, and why is it called substrate-level phosphorylation? Answer: Succinyl-CoA synthetase couples thioester conversion to GTP or ATP formation. Energy from the substrate intermediate directly supports phosphate transfer to a nucleoside diphosphate rather than using a membrane gradient.