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Citric Acid Cycle
Key Steps, Regulation & Energy Yield

Discover how the Krebs cycle oxidizes acetyl-CoA to produce electron carriers, GTP, and \(\text{CO}_2\) in the mitochondrial matrix.

Biochemistry

The Big Picture of Cellular Respiration

Cells harvest energy from glucose in three connected stages. The citric acid cycle is the second stage, linking glycolysis to the electron transport chain.

Stage 1
Glycolysis: \( \text{glucose} \rightarrow 2~\text{pyruvate} \) in the cytoplasm
Stage 2
Citric Acid Cycle: \( \text{acetyl-CoA} \rightarrow \text{CO}_2 + \text{electron carriers} \)
Stage 3
Electron Transport Chain: \( \text{NADH}/\text{FADH}_2 \rightarrow \text{ATP} \) via oxidative phosphorylation

From Pyruvate to Acetyl-CoA

Before the cycle begins, pyruvate from glycolysis enters the mitochondrial matrix and is converted by pyruvate dehydrogenase complex (PDC):

Input
Pyruvate (\(3\text{C}\)), \(\text{NAD}^+\), CoA
Output
Acetyl-CoA (\(2\text{C}\)), \(\text{NADH}\), \(\text{CO}_2\)

The Citric Acid Cycle at a Glance

Citrate (6C) Isocitrate (6C) α-Ketoglut. (5C) Succinyl-CoA (4C) Succinate (4C) Fumarate (4C) Malate (4C) OAA (4C) Acetyl-CoA (2C)
\(\text{CO}_2\)
\(\text{CO}_2\)
\(\text{NADH}\)
\(\text{NADH}\)
\(\text{NADH}\)
\(\text{FADH}_2\)
\(\text{GTP}\)

Eight Reactions: Steps 1 through 4

1
Citrate synthase: \( \text{Acetyl-CoA} + \text{OAA} \rightarrow \text{Citrate (6C)} \). Irreversible condensation: the "entry" step.
2
Aconitase: \( \text{Citrate} \rightarrow \text{Isocitrate} \) via dehydration/rehydration (isomerization).
3
Isocitrate dehydrogenase: \( \text{Isocitrate} \rightarrow \alpha\text{-ketoglutarate} \). First oxidative decarboxylation: produces \(\text{NADH}\) + \(\text{CO}_2\).
4
\(\alpha\)-ketoglutarate dehydrogenase: \( \alpha\text{-KG} \rightarrow \text{Succinyl-CoA} \). Second oxidative decarboxylation: produces \(\text{NADH}\) + \(\text{CO}_2\).

Eight Reactions: Steps 5 through 8

5
Succinyl-CoA synthetase: \( \text{Succinyl-CoA} \rightarrow \text{Succinate} \). Substrate-level phosphorylation produces \(\text{GTP}\) (≈ 1 ATP).
6
Succinate dehydrogenase: \( \text{Succinate} \rightarrow \text{Fumarate} \). Produces \(\text{FADH}_2\). This enzyme is embedded in the inner mitochondrial membrane (Complex II).
7
Fumarase: \( \text{Fumarate} \rightarrow \text{Malate} \). A simple hydration reaction.
8
Malate dehydrogenase: \( \text{Malate} \rightarrow \text{OAA} \). Produces the third \(\text{NADH}\). OAA is regenerated to restart the cycle.

Energy Yield Per Turn of the Cycle

Product Per Turn Per Glucose (x2 turns) ATP Equivalent
\(\text{NADH}\) 3 6 \(6 \times 2.5 = 15\)
\(\text{FADH}_2\) 1 2 \(2 \times 1.5 = 3\)
\(\text{GTP}\) 1 2 2
\(\text{CO}_2\) 2 4 -
Total ATP equiv. 10 20 ATP per glucose from the cycle alone

Regulation of the Citric Acid Cycle

The cycle is regulated at three irreversible steps by allosteric effectors that sense the cell's energy charge: the ratio of ATP to ADP and the ratio of \(\text{NADH}\) to \(\text{NAD}^+\).

Citrate Synthase
Inhibited by: ATP, NADH, citrate, succinyl-CoA. Activated by: ADP.
Isocitrate DH
Inhibited by: ATP, NADH. Activated by: ADP, \(\text{Ca}^{2+}\). This is the primary regulated step.
\(\alpha\)-KG DH
Inhibited by: NADH, succinyl-CoA. Activated by: \(\text{Ca}^{2+}\), AMP.

Citric Acid Cycle vs Glycolysis

Feature Citric Acid Cycle Glycolysis
LocationMitochondrial matrixCytoplasm
Oxygen required?Yes (aerobic)No (anaerobic capable)
InputAcetyl-CoA (2C)Glucose (6C)
Direct ATP1 GTP per turn2 ATP per glucose
NADH produced3 per turn2 per glucose
\(\text{CO}_2\) released2 per turn0
Pathway typeCyclicLinear

How to Trace the Cycle

1
Start with OAA (4C). It condenses with acetyl-CoA (2C) to form citrate (6C).
2
Track carbon count. Two decarboxylations drop the carbon count: \( 6\text{C} \rightarrow 5\text{C} \rightarrow 4\text{C} \). Each releases \(\text{CO}_2\) and generates \(\text{NADH}\).
3
Count the energy carriers. Tally \(\text{NADH}\) (×3), \(\text{FADH}_2\) (×1), and \(\text{GTP}\) (×1) as you go.
4
End at OAA. Malate is oxidized back to OAA, regenerating the cycle's starting substrate.
5
Double for glucose. Since 1 glucose yields 2 acetyl-CoA, multiply all products by 2.

Example 1: Counting Products per Glucose

Problem: How many total \(\text{NADH}\) molecules are produced from one glucose by the citric acid cycle alone (not including pyruvate dehydrogenase)?

Example 2: Total ATP from Glucose

Problem: Estimate the maximum ATP yield from complete oxidation of one glucose molecule through all stages of aerobic respiration.

Stage \(\text{NADH}\) \(\text{FADH}_2\) ATP/GTP ATP equiv.
Glycolysis202\(2 + 5 = 7\)
Pyruvate DH (×2)2005
CAC (×2 turns)622\(15 + 3 + 2 = 20\)
Total1024\( \approx 30\text{-}32~\text{ATP} \)

Example 3: Regulation in Action

Scenario: A cell's ATP concentration rises sharply after a meal. What happens to the rate of the citric acid cycle?

1
High [ATP] allosterically inhibits citrate synthase (step 1) and isocitrate dehydrogenase (step 3).
2
Inhibition of step 3 causes isocitrate to accumulate, which in turn causes citrate to accumulate.
3
Accumulated citrate inhibits phosphofructokinase-1 in glycolysis: a cross-pathway feedback that slows glucose breakdown.
4
Result: The cycle slows down, conserving fuel until ATP levels drop and ADP levels rise again.

The Cycle Is Amphibolic

The citric acid cycle is both catabolic (breaking down acetyl-CoA for energy) and anabolic (supplying precursors for biosynthesis). This dual nature is called amphibolic.

Citrate
Exported to cytoplasm for fatty acid synthesis
\(\alpha\)-Ketoglutarate
Precursor for glutamate and other amino acids
Succinyl-CoA
Used in heme (porphyrin) synthesis
OAA
Converted to aspartate for amino acid and nucleotide synthesis

Common Mistakes to Avoid

✕ Forgetting to Double
One glucose produces 2 acetyl-CoA, so cycle products must be multiplied by 2 for per-glucose totals.
✕ Confusing GTP with ATP
The cycle directly produces \(\text{GTP}\) (via substrate-level phosphorylation), not ATP. \(\text{GTP}\) is readily converted to ATP, but the distinction matters on exams.
✕ Including PDH in the Cycle
Pyruvate dehydrogenase is a linking reaction, not part of the 8-step cycle itself. \(\text{NADH}\) from PDH is separate.

Question 1 of 5

How many molecules of \(\text{CO}_2\) are released per single turn of the citric acid cycle?

Question 2 of 5

Which enzyme catalyzes the only substrate-level phosphorylation in the citric acid cycle?

Question 3 of 5

A cell has a high \[\(\text{NADH}\)\]/\[\(\text{NAD}^+\)\] ratio. Which of the following is the most likely effect on the citric acid cycle?

Question 4 of 5

Which citric acid cycle enzyme is also a component of the electron transport chain (Complex II)?

Question 5 of 5

The term "amphibolic" describes the citric acid cycle because it: