Krebs Cycle: Steps and Products

The catabolism of biomolecules like glucose, amino acids, and lipids occurs either in the presence or the absence of oxygen. Aerobic catabolism of the biomolecules is known as cellular respiration, which occurs in most eukaryotic cells and aerobic bacteria.

The process of cellular respiration consists of three stages; conversion of complex molecules to a 2-carbon compound, acetyl CoA, feeding of acetyl CoA into the Krebs cycle, and release of energy by the process of oxidative phosphorylation.

The Krebs cycle is one of the crucial cyclic breaking down acetyl CoA in the presence of oxygen. Each cycle yields a molecule of FADH₂ (flavin adenine dinucleotide+ H₂), a molecule of GTP (guanosine triphosphate), three molecules of NADH (nicotinamide adenine dinucleotide + hydrogen) , and two molecules of carbon dioxide.

The aerobic breakdown of pyruvate (final product of glycolysis), beta oxidation of fatty acids, and catabolism of amino acids results in the formation of acetyl CoA. Krebs cycle occurs inside the mitochondrial matrix of the eukaryotic cell and in the cytoplasm of prokaryotic cells.

Krebs cycle is also known as the TCA (Tricarboxylic acid) cycle or citric acid cycle because citric acid is produced in the first step which has 3 carboxyl (-COOH) groups.

Steps of Krebs Cycle

After the aerobic fate of pyruvate inside the mitochondrial matrix, the 8-step Krebs or Citric acid cycle begins.

• Firstly the formation of six-carbon compound-citrate occurs by the combination Acetyl CoA and oxaloacetate in the presence of citrate synthase enzyme.
• Secondly, there is the conversion of citrate to its isomer isocitrate in the presence of the enzyme aconitase.
• Thirdly, the isocitrate converts into ɑ-ketoglutarate, a 5-carbon compound prompting the release of a molecule of carbon dioxide. The catalyst of this step is NAD⁺ and isocitrate dehydrogenase.
• Then, the four-carbon compound succinyl CoA forms under the control of the enzyme ɑ-ketoglutarate dehydrogenase. Here, a molecule of carbon dioxide releases. Also NADH forms in this step as the enzyme is NAD⁺ dependent.
• Fifthly, this step involves the removal of CoA from succinyl CoA to form succinate. GTP (guanosine triphosphate) forms after the substrate level phosphorylation of GDP (guanosine diphosphate). The catalyst is succinyl CoA synthetase.
• Similarly, in the sixth step, the succinate converts into fumarate. The enzyme succinate dehydrogenase helps the reaction. The succinate dehydrogenase reduces FAD to FADH₂.
• Likewise, with the addition of H₂O, the fumarate changes into malate in the seventh step. The presence of fumarase in this step acts as a catalyst.
• The final step is the formation of oxaloacetate with the help of the enzyme malate dehydrogenase.

Krebs cycle is amphibolic; catabolism (breakdown) and anabolism (synthesis) co-occur. 

Overall Equation of Krebs Cycle

2Acetyl CoA+6NAD⁺+2GDP+2FAD+2Phosphate (Pi)+2H₂O→ 4CO₂+6NADH+2GTP+2FADH₂+2CoA

Regulators

The regulators of TCA are not the enzymes involved in the cycle but other factors like the production of pyruvate or conversion of pyruvate to acetyl CoA. The factors regulating the Krebs cycle are:

1. NADH inhibits almost all the enzymes of TCA, and FADH₂ inhibits succinate dehydrogenase.
2. The citrate inhibits phosphofructokinase, an essential enzyme of glycolysis. It controls the production of pyruvate.
3. Whereas calcium ions accelerate the TCA cycle.
4. The rate of conversion of pyruvate to acetyl Coenzyme A also regulates the TCA.

Products of Krebs Cycle

A molecule of acetyl CoA gives following products:

• Carbon dioxide: In addition to the third and the fourth steps produce one carbon dioxide molecule each.
• NADH: The third, fourth, and last steps of the cycle produce three NADH molecules.
• GTP: A molecule of GTP forms in the fifth step of the process.
• Hydrogen ions: Three molecules of hydrogen ions are released in the third, fourth, and last steps of the cycle.
• FADH₂: A molecule of FADH₂ is produced in the sixth step.

Since two molecules of acetyl CoA are produced after conversion, each undergo an individual TCA. So, each product must be multiplied by two.

From the process of oxidative phosphorylation a molecule FADH₂ produces 1.5 ATP and a molecule of NADH produces 2.5 ATP. So, Krebs cycle produce almost 10 ATP from a molecule of acetyl CoA.

Importance of Krebs Cycle

The citric acid cycle is a critical stage of the cellular metabolism of amino acids, fats, and carbohydrates. It is also the stage that is responsible for producing a high amount of energy in eukaryotic animals. Some importance of the Krebs cycle are:

1. Krebs cycle is the final pathway for catabolism or the breakdown of fats, carbohydrates, and amino acids.
2.  A single Krebs cycle alone produces 10 ATP molecules, considered the cell’s energy currency. Hence, the lack of the Krebs cycle in aerobic organisms can lead to a lack of energy.
3.  Lack of the Krebs cycle can lead to liver and neural damage.
4.  Krebs cycle also helps convert amino acids like ɑ-ketoglutarate into glutamine, gluconeogenesis, and lipogenesis.

Reference and Further Reading

• Nelson, D., Lehninger, A., Cox, M., & Nelson, D. (2005). Lecture notebook for Lehninger principles of biochemistry, fourth edition (pp. 601-612). W.H. Freeman.
• Rodwell, V., Bender, D., Botham, K., Kennelly, P., & Weil, P. (2015). Harper’s illustrated biochemistry (30th ed., pp. 161-167). McGraw Hill.
• (2022). Retrieved 2 May 2022, from https://teachmephysiology.com/biochemistry/atp-production/tca-cycle-2/.
• tricarboxylic acid cycle | biochemistry. Encyclopedia Britannica. (2022). Retrieved 2 May 2022, from https://www.britannica.com/science/tricarboxylic-acid-cycle.