Krebs Cycle Explained: Steps, Products, and Role in Cellular Respiration

The Krebs cycle is a series of enzyme-catalyzed chemical reactions that form a central part of aerobic metabolism in nearly all living organisms. Named after Sir Hans Krebs, who first described the pathway in 1937, the Krebs cycle takes place in the mitochondrial matrix of eukaryotic cells. It is also widely known as the citric acid cycle because its first intermediate product is citrate, a form of citric acid. In many textbooks, you will also see it abbreviated as the CAC cycle or referred to as the citrate cycle, reflecting the pivotal role that citrate plays in the opening reaction.

The primary purpose of the Krebs cycle is to oxidize acetyl-CoA, a two-carbon molecule derived from the breakdown of carbohydrates, fats, and proteins. Through a series of eight sequential reactions, the cycle strips high-energy electrons from carbon-based substrates and transfers them to the electron carriers NAD+ and FAD. These reduced coenzymes, NADH and FADH2, then shuttle the electrons to the electron transport chain, where the majority of ATP is ultimately generated.

Understanding the Krebs cycle is foundational for students studying biochemistry, cell biology, and medical sciences. It represents a metabolic hub where catabolic and anabolic pathways converge. Intermediates of the citric acid cycle serve as precursors for amino acid synthesis, gluconeogenesis, and fatty acid production. This dual nature makes the Krebs cycle indispensable not only for energy extraction but also for biosynthetic processes throughout the cell.

The Krebs cycle consists of eight main enzymatic steps that convert acetyl-CoA into carbon dioxide while capturing energy in the form of reduced coenzymes and one molecule of GTP (or ATP, depending on the organism). Below is a detailed walkthrough of each step in the citric acid cycle.

Step 1: Citrate Synthesis. The cycle begins when acetyl-CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons). This irreversible reaction is catalyzed by citrate synthase and is the committed step of the citrate cycle. Step 2: Isomerization. Citrate is converted to isocitrate by the enzyme aconitase through a dehydration-rehydration sequence. Step 3: Oxidative Decarboxylation I. Isocitrate dehydrogenase oxidizes isocitrate to alpha-ketoglutarate, releasing the first molecule of CO2 and generating the first NADH. This is a key regulatory step. Step 4: Oxidative Decarboxylation II. The alpha-ketoglutarate dehydrogenase complex converts alpha-ketoglutarate to succinyl-CoA, releasing a second CO2 and producing another NADH. This enzyme complex is structurally similar to pyruvate dehydrogenase.

Step 5: Substrate-Level Phosphorylation. Succinyl-CoA synthetase cleaves the high-energy thioester bond in succinyl-CoA, coupling the reaction to the phosphorylation of GDP to GTP (equivalent to one ATP). Step 6: Oxidation of Succinate. Succinate dehydrogenase, the only Krebs cycle enzyme embedded in the inner mitochondrial membrane, oxidizes succinate to fumarate and reduces FAD to FADH2. Step 7: Hydration. Fumarase catalyzes the addition of water across the double bond of fumarate, producing malate. Step 8: Regeneration of Oxaloacetate. Malate dehydrogenase oxidizes malate back to oxaloacetate, generating the third and final NADH of the cycle. Oxaloacetate is now ready to accept another acetyl-CoA, and the CAC cycle begins again.

To fully appreciate the Krebs cycle, it is essential to understand its position within the broader framework of cellular respiration. Cellular respiration is the multi-step process by which cells break down glucose and other organic molecules to produce ATP, the universal energy currency. The relationship between cellular respiration and the Krebs cycle is one of sequential dependency: the cycle cannot operate without the upstream products of glycolysis and pyruvate oxidation, and the downstream electron transport chain cannot function without the NADH and FADH2 generated by the cycle.

Cellular respiration proceeds in four major stages. First, glycolysis splits a six-carbon glucose molecule into two three-carbon pyruvate molecules in the cytoplasm. Second, pyruvate is transported into the mitochondrial matrix and decarboxylated by the pyruvate dehydrogenase complex to form acetyl-CoA. Third, the Krebs cycle oxidizes acetyl-CoA through the eight reactions described above. Fourth, the electron transport chain and oxidative phosphorylation use the electrons carried by NADH and FADH2 to establish a proton gradient and drive ATP synthase.

The integration of cellular respiration and Krebs cycle activity means that the cycle acts as a metabolic crossroads. Amino acids can be converted into Krebs cycle intermediates through transamination and deamination, allowing proteins to be used as fuel. Similarly, fatty acids undergo beta-oxidation to produce acetyl-CoA, which enters the citric acid cycle directly. This metabolic flexibility underscores why the Krebs cycle is often called the final common pathway of fuel oxidation. Without a functioning citric acid cycle, aerobic organisms cannot efficiently extract energy from food, and cellular respiration grinds to a halt.

Each turn of the Krebs cycle produces a precise set of energy-rich products. For every molecule of acetyl-CoA that enters the citric acid cycle, the following outputs are generated: three molecules of NADH, one molecule of FADH2, one molecule of GTP (or ATP), and two molecules of CO2. Since one glucose molecule yields two molecules of acetyl-CoA (via glycolysis and pyruvate oxidation), the total output from two turns of the Krebs cycle per glucose is six NADH, two FADH2, two GTP, and four CO2.

The real energy payoff of the citric acid cycle, however, is realized downstream at the electron transport chain. Each NADH donates its electrons to Complex I and ultimately drives the production of approximately 2.5 ATP molecules through oxidative phosphorylation. Each FADH2 feeds into Complex II and yields roughly 1.5 ATP. Therefore, the six NADH from two turns of the Krebs cycle produce about 15 ATP, and the two FADH2 produce about 3 ATP. Combined with the two GTP generated by substrate-level phosphorylation, the citric acid cycle accounts for roughly 20 ATP per glucose when its contributions to the electron transport chain are included.

When tallied across the entire process of cellular respiration, the theoretical maximum yield from one glucose molecule is approximately 30 to 32 ATP. Glycolysis contributes a net of 2 ATP and 2 NADH, pyruvate oxidation contributes 2 NADH, and the Krebs cycle contributes the remainder. The CO2 released during the citric acid cycle is ultimately exhaled by the organism, representing the carbon atoms originally present in glucose. This elegant accounting demonstrates why the Krebs cycle is regarded as the engine room of aerobic energy metabolism.

The Krebs cycle is tightly regulated to ensure that energy production matches the cell's metabolic demands. Regulation occurs primarily at three irreversible steps, each catalyzed by a different enzyme: citrate synthase (step 1), isocitrate dehydrogenase (step 3), and alpha-ketoglutarate dehydrogenase (step 4). These enzymes serve as control points where the flux through the citric acid cycle can be increased or decreased in response to the cell's energy status.

Citrate synthase is inhibited by its product citrate, as well as by ATP, NADH, and succinyl-CoA. When cellular energy levels are high, these molecules accumulate and slow down the entry of acetyl-CoA into the cycle. Isocitrate dehydrogenase is allosterically activated by ADP and NAD+ (signals of low energy) and inhibited by ATP and NADH (signals of high energy). This makes step 3 one of the most sensitive regulatory nodes in the CAC cycle. Alpha-ketoglutarate dehydrogenase is inhibited by its products, succinyl-CoA and NADH, as well as by ATP. It is activated by calcium ions, which is particularly important in muscle cells during contraction.

Beyond allosteric regulation, the availability of substrates plays a critical role. The Krebs cycle cannot proceed if oxaloacetate is depleted, a situation that can occur when intermediates are siphoned off for biosynthetic purposes. Anaplerotic reactions, such as the carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase, replenish cycle intermediates and keep the citric acid cycle running. This balance between cataplerosis (removal of intermediates) and anaplerosis (replenishment) is vital for maintaining the cycle's function in both energy production and biosynthesis.

The Krebs cycle is one of the most frequently tested biochemistry topics on exams such as the MCAT, USMLE Step 1, and AP Biology. Mastering it requires a structured approach that goes beyond rote memorization. Here are evidence-based strategies for learning the citric acid cycle effectively.

First, learn the cycle as a story rather than a list. Start with the entry of acetyl-CoA and follow the carbon count through each reaction: 2C joins 4C to make 6C (citrate), then two carbons are lost as CO2, reducing the molecule back to 4C (oxaloacetate). This narrative framework helps you predict products without memorizing each step in isolation. Second, use visual aids. Drawing the Krebs cycle by hand, labeling enzymes and products at each step, is one of the most effective study techniques. A glycolysis steps diagram alongside a Krebs cycle diagram helps you see how the two pathways connect within cellular respiration.

Third, focus on regulation. Exam questions frequently ask which enzymes are regulated, what molecules serve as activators and inhibitors, and why regulation matters. Create a table listing the three regulated enzymes, their activators, and their inhibitors. Fourth, practice integration. The best students understand how the citric acid cycle connects to glycolysis upstream and the electron transport chain downstream. Ask yourself: what happens to each NADH and FADH2 produced? Where does the CO2 go? Why does the cycle stop under anaerobic conditions?

Finally, use active recall and spaced repetition to reinforce your knowledge. Platforms like LectureScribe can generate flashcards, slide decks, and practice questions directly from your lecture notes, helping you test yourself on the CAC cycle and related metabolic pathways consistently over time.