The citric acid cycle

Citric acid cycle purpose

Oxidises acetyl-CoA to CO₂ and captures energy in NADH and FADH₂

Efficiency of CAC

High energy yield through multiple reduced cofactors feeding the electron transport chain

Citric acid cycle location

Occurs in the mitochondrial matrix

Central metabolic role

Integrates oxidation of carbohydrates, fats, and amino acids

Oxidation

Loss of electrons (often loss of hydrogen atoms)

Reduction

Gain of electrons

Decarboxylation

Removal of a carboxyl group as CO₂

Condensation

Bond formation between molecules with release of a small molecule

Dehydration

Removal of water forming a double bond

Hydration

Addition of water across a double bond

Substrate-level phosphorylation

Direct formation of ATP or GTP from a metabolic intermediate

ΔG°′

Standard free energy change under biochemical conditions

Electron carriers in CAC

NAD⁺ and FAD reduced to NADH and FADH₂

Two-carbon oxidation

Acetyl-CoA enters cycle and is fully oxidised

Citrate synthase

Catalyses condensation of acetyl-CoA and oxaloacetate

Citryl-CoA intermediate

Transient intermediate in citrate formation

Thioester hydrolysis

Drives citrate formation energetically

Ordered binding

Oxaloacetate binds first, then acetyl-CoA

Citrate synthase regulation

Prevents premature hydrolysis of acetyl-CoA

Aconitase

Enzyme converting citrate to isocitrate

Aconitase mechanism

Dehydration followed by hydration

Iron-sulfur protein

Contains non-haem iron bound to sulphur atoms

Citrate binding

Occurs at iron-sulfur cluster

Isocitrate dehydrogenase

Catalyses oxidative decarboxylation of isocitrate

Oxalosuccinate

Unstable intermediate in reaction

α-ketoglutarate formation

Product after CO₂ release

NADH production

Generated during oxidation

α-ketoglutarate dehydrogenase

Converts α-ketoglutarate to succinyl-CoA

Second oxidative decarboxylation

Produces NADH and releases CO₂

Succinyl-CoA synthetase

Converts succinyl-CoA to succinate

Substrate-level phosphorylation in CAC

Produces ATP or GTP

Reversibility of reaction

Reaction can proceed in both directions

Isozymes of succinyl-CoA synthetase

Different forms use GDP or ADP

GDP-linked enzyme

Predominates in anabolic tissues (e.g. liver)

ADP-linked enzyme

Predominates in energy-demanding tissues (e.g. muscle)

Oxaloacetate regeneration

Final steps restore starting molecule for next cycle

Succinate dehydrogenase

Oxidises succinate to fumarate

FAD role

Accepts electrons when NAD⁺ reduction is unfavourable

Succinate dehydrogenase location

Embedded in inner mitochondrial membrane

Complex II

Part of electron transport chain

Fumarase

Converts fumarate to L-malate

Hydration reaction

Adds H⁺ and OH⁻ stereospecifically

L-malate

Only isomer produced

Malate dehydrogenase

Converts malate to oxaloacetate

Positive ΔG°′ reaction

Driven forward by downstream reactions

Coupling

Reaction proceeds due to use of products in other processes

ATP yield per acetyl-CoA

Approximately 10 ATP equivalents

NADH ATP yield

~2.5 ATP per NADH

FADH₂ ATP yield

~1.5 ATP per FADH₂

Anaplerotic reactions

Replenish TCA cycle intermediates

Need for replenishment

CAC intermediates are continuously withdrawn for biosynthesis

Glutamine anaplerosis

Glutamine replenishes cycle via α-ketoglutarate

Reductive carboxylation

Reverse flux of TCA reactions under hypoxia or ETC defects

Irreversible entry step

Conversion of pyruvate to acetyl-CoA cannot be reversed

Pyruvate dehydrogenase complex

Controls entry into CAC

Allosteric regulation

Activity adjusted by cellular energy state

Isocitrate dehydrogenase regulation

Activated by ADP, inhibited by ATP and NADH

α-ketoglutarate dehydrogenase regulation

Inhibited by NADH, succinyl-CoA, and ATP

Inherited metabolic disorders

Caused by mutations in metabolic enzymes

Fumarate hydratase (FH) mutation

Disrupts TCA cycle function

Fumaric aciduria

Disorder caused by loss of FH activity

Fumarate accumulation

Leads to toxicity and neurological impairment

Hereditary leiomyomatosis and renal cell cancer (HLRCC)

Cancer syndrome linked to FH mutation

Loss of heterozygosity (LOH)

Loss of normal gene copy after inherited mutation

FH dysfunction effects

Alters mitochondrial function and metabolism

CAC biosynthetic role

Provides intermediates for synthesis of biomolecules

Reduced cofactors role

Supply electrons to electron transport chain

CAC summary

Produces NADH, FADH₂, ATP/GTP and supports metabolism and biosynthesis