Citric Acid Cycle: Structure, Energetics, and Carbon Fate

CITRIC ACID CYCLE

Overview of the Citric Acid Cycle

The Citric Acid Cycle (also known as the Krebs Cycle or TCA Cycle) is a central metabolic pathway that oxidizes acetyl-CoA to CO2, generating high-energy electron carriers and GTP/ATP. It is essential for energy production in aerobic organisms.

• 1 acetyl-CoA generates 3 NADH, 1 FADH2, and 1 GTP/ATP per cycle.

• The ΔG of steps 1, 3, and 4 is considerably negative, making these reactions the main drivers of the cycle.

• Steps 2, 4, 6, 7, and 8 are close to equilibrium, so these reactions are readily reversible.

• Each turn of the cycle releases two CO2 molecules, but only the labeled carbons are lost as CO2 due to the randomization of succinate’s orientation.

Major Steps and Intermediates

The cycle consists of a series of enzymatic reactions, each transforming the substrate and producing key metabolic products.

• Step 1: Citrate Synthase – Acetyl-CoA combines with oxaloacetate to form citrate. $\text{Acetyl-CoA} + \text{Oxaloacetate} \rightarrow \text{Citrate}$

• Step 2: Aconitase – Citrate is isomerized to isocitrate.

• Step 3: Isocitrate Dehydrogenase – Isocitrate is oxidized to α-ketoglutarate, producing NADH and releasing CO2. $\text{Isocitrate} + \text{NAD}^+ \rightarrow \alpha\text{-Ketoglutarate} + \text{CO}_2 + \text{NADH}$

• Step 4: α-Ketoglutarate Dehydrogenase – α-Ketoglutarate is converted to succinyl-CoA, producing NADH and releasing CO2. $\alpha\text{-Ketoglutarate} + \text{NAD}^+ \rightarrow \text{Succinyl-CoA} + \text{CO}_2 + \text{NADH}$

• Step 5: Succinyl-CoA Synthetase – Succinyl-CoA is converted to succinate, generating GTP (or ATP). $\text{Succinyl-CoA} + \text{GDP} + \text{P}_i \rightarrow \text{Succinate} + \text{GTP} + \text{CoA}$

• Step 6: Succinate Dehydrogenase – Succinate is oxidized to fumarate, producing FADH2. $\text{Succinate} + \text{FAD} \rightarrow \text{Fumarate} + \text{FADH}_2$

• Step 7: Fumarase – Fumarate is hydrated to malate.

• Step 8: Malate Dehydrogenase – Malate is oxidized to oxaloacetate, producing NADH. $\text{Malate} + \text{NAD}^+ \rightarrow \text{Oxaloacetate} + \text{NADH}$

Energetics and Carbon Fate

The cycle is highly regulated and energetically favorable at key steps, ensuring efficient energy extraction from acetyl-CoA.

• Irreversible Steps: Steps 1, 3, and 4 have large negative ΔG values and are tightly regulated.

• Reversible Steps: Steps 2, 4, 6, 7, and 8 are near equilibrium and can proceed in either direction depending on cellular conditions.

• Carbon Tracking: Only the labeled carbons from acetyl-CoA are released as CO2 in each cycle, but due to succinate’s orientation, carbon loss is randomized in subsequent cycles.

Summary Table: Citric Acid Cycle Products per Acetyl-CoA

Step	Enzyme	Main Product	Energy Carrier Produced
1	Citrate Synthase	Citrate	-
3	Isocitrate Dehydrogenase	α-Ketoglutarate	NADH, CO2
4	α-Ketoglutarate Dehydrogenase	Succinyl-CoA	NADH, CO2
5	Succinyl-CoA Synthetase	Succinate	GTP (or ATP)
6	Succinate Dehydrogenase	Fumarate	FADH2
8	Malate Dehydrogenase	Oxaloacetate	NADH

Example: Cellular Respiration

In aerobic cells, the citric acid cycle is the main source of NADH and FADH2 for the electron transport chain, which ultimately produces ATP.

Additional info: The diagram in the original file visually tracks the fate of carbons through the cycle, highlighting the loss of CO2 and the production of energy carriers at each step.