The Krebs Cycle (Citric Acid Cycle)

Historical Context & Motivation

The question of how living cells extract energy from food puzzled biochemists for decades. By the early twentieth century, researchers knew that sugars could be broken down in fermentation, but the fate of the carbon atoms—and the means by which cells captured the bulk of their usable energy—remained mysterious. The discovery of the Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle) closed this gap, revealing an elegant circular pathway that links the breakdown of carbohydrates, fats, and proteins to the oxygen-dependent machinery of the mitochondrion.

The fundamental question the Krebs cycle addresses is this: once a glucose molecule has been split into two molecules of pyruvate during glycolysis, how does the cell finish extracting the remaining energy locked in those carbon bonds? The answer is a cycle of eight enzyme-catalyzed reactions that strips electrons from carbon compounds and loads them onto carrier molecules—NADH and FADH₂—that will subsequently power the electron transport chain and drive the synthesis of ATP.

Core Principles & Definitions

Before diving into the individual reactions, it is essential to understand the key principles that govern the Krebs cycle and its place in metabolism. The cycle operates in the mitochondrial matrix of eukaryotes (and the cytoplasm of prokaryotes), and its primary role is not direct ATP production but rather the generation of high-energy electron carriers that feed oxidative phosphorylation.

Visual Explanation — The Cycle at a Glance

The diagram below illustrates the eight steps of the Krebs cycle arranged as a circular pathway. Each intermediate is shown with its carbon count, and the key products—NADH, FADH₂, GTP, and CO₂—are indicated at their respective steps. Follow the arrows clockwise from the entry of acetyl-CoA at the top.

As you trace the cycle from acetyl-CoA entry at the top, notice how the six-carbon citrate is systematically dismantled. Two oxidative decarboxylation steps (steps 3 and 4) release CO₂ and reduce NAD⁺ to NADH. The four-carbon intermediates in the lower half are then reshuffled and further oxidized, producing FADH₂ and one more NADH, before the cycle regenerates oxaloacetate and is ready to accept another acetyl-CoA.

Step-by-Step Mechanism

The Krebs cycle consists of eight enzyme-catalyzed reactions. Understanding each step's chemistry—condensation, isomerization, oxidative decarboxylation, substrate-level phosphorylation, oxidation, hydration, and further oxidation—reveals how the cycle systematically extracts every possible electron from the two carbons that entered as acetyl-CoA.

Below is a summary of each of the eight steps, including the enzyme, the type of reaction, and the products formed.

Step 1 — Citrate Synthase (Condensation)

Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), releasing CoA-SH. This is an aldol condensation followed by hydrolysis of the thioester bond, making the step highly exergonic (ΔG°′ ≈ −31.4 kJ/mol) and essentially irreversible under cellular conditions. This is the first major regulatory point of the cycle.

Step 2 — Aconitase (Isomerization)

Citrate is converted to isocitrate via a dehydration–rehydration mechanism, with cis-aconitate as a transient intermediate. The enzyme contains an iron–sulfur cluster (Fe₄S₄) essential for catalysis. This step repositions the hydroxyl group so that the subsequent oxidative decarboxylation can occur on the correct carbon.

Step 3 — Isocitrate Dehydrogenase (Oxidative Decarboxylation)

Isocitrate (6C) is oxidized and decarboxylated to produce α-ketoglutarate (5C), the first molecule of CO₂, and the first NADH. This is the rate-limiting step of the cycle and its second major regulatory point: the enzyme is allosterically activated by ADP and Ca²⁺ and inhibited by ATP and NADH.

Step 4 — α-Ketoglutarate Dehydrogenase Complex (Oxidative Decarboxylation)

α-Ketoglutarate (5C) is oxidized and decarboxylated to form succinyl-CoA (4C), releasing the second CO₂ and producing the second NADH. This multi-enzyme complex requires five coenzymes: TPP, lipoamide, CoA, FAD, and NAD⁺—structurally and mechanistically analogous to the pyruvate dehydrogenase complex. It is inhibited by succinyl-CoA and NADH (the third regulatory point).

Step 5 — Succinyl-CoA Synthetase (Substrate-Level Phosphorylation)

The high-energy thioester bond in succinyl-CoA is cleaved, and the released energy drives the phosphorylation of GDP to GTP (which is readily converted to ATP by nucleoside diphosphate kinase). The products are succinate (4C) and CoA-SH. This is the only step that produces a high-energy phosphate bond directly.

Step 6 — Succinate Dehydrogenase (Oxidation)

Succinate is oxidized to fumarate by the removal of two hydrogen atoms, reducing FAD to FADH₂. This enzyme is unique: it is embedded in the inner mitochondrial membrane as Complex II of the electron transport chain, directly feeding electrons to ubiquinone. Malonate, a competitive inhibitor of this enzyme, was historically used to probe the cycle.

Step 7 — Fumarase (Hydration)

Fumarate is hydrated (water is added across the double bond) to produce L-malate. This is a simple, reversible, stereospecific hydration that generates only the L-isomer of malate.

Step 8 — Malate Dehydrogenase (Oxidation)

Malate is oxidized to regenerate oxaloacetate, producing the third NADH. Although the equilibrium strongly favors malate (ΔG°′ ≈ +29.7 kJ/mol), the reaction is pulled forward by the highly exergonic citrate synthase step that immediately consumes OAA—a beautiful example of how coupling reactions drives thermodynamically unfavorable steps.

Detailed Breakdown — Intermediates & Regulation

The eight intermediates of the Krebs cycle differ in carbon number, functional group chemistry, and energy content. The table below summarizes each intermediate, its carbon count, the enzyme that produces it, and notable regulatory or anapleurotic connections.

The regulation diagram above emphasizes a crucial point: the Krebs cycle is not a constitutively active furnace. It is a finely tuned engine that responds to the cell's energy state. High concentrations of ATP and NADH signal that energy reserves are full, slowing the cycle by inhibiting its three irreversible enzymes. Conversely, rising ADP (signaling energy depletion) and calcium ions (signaling active muscle contraction, for instance) accelerate the cycle to meet demand. This feedback ensures that the cell neither wastes resources nor runs out of ATP.

Worked Example

Let us trace the complete oxidation of one molecule of glucose through aerobic respiration, focusing on the contribution of the Krebs cycle, and calculate the total ATP yield.

Strengths, Limitations & Comparisons

The Krebs cycle is remarkably efficient and versatile, but it has constraints. Understanding its strengths and limitations clarifies why organisms also rely on alternative metabolic strategies under certain conditions.

One critical limitation of the Krebs cycle is its dependence on a continuous supply of oxaloacetate. If OAA is siphoned off for gluconeogenesis (as happens during fasting), the cycle slows unless replenished by anaplerotic reactions—most notably the carboxylation of pyruvate to OAA by pyruvate carboxylase. Another limitation is that the cycle cannot run when the NAD⁺/NADH ratio is too low (i.e., when the ETC is inhibited or oxygen is absent), because the dehydrogenase steps require oxidized NAD⁺ as an electron acceptor.

Connection to Advanced Theory

The Krebs cycle does not exist in isolation; it is deeply integrated with virtually every other metabolic pathway in the cell. As students advance, several connections become central to understanding physiology, disease, and modern biochemistry.

Perhaps the most exciting frontier is the role of Krebs cycle metabolites as signaling molecules. Succinate, for example, can act as an extracellular signal via the succinate receptor GPR91, linking metabolism to inflammation and blood-pressure regulation. Citrate exported from mitochondria provides the acetyl groups for histone acetylation—an epigenetic modification that controls gene expression. These discoveries reveal that the "simple" metabolic cycle uncovered by Krebs in 1937 is, in fact, a master regulator of cellular identity and function.

Practice Problems

Lesson Summary