KREBS’ cycle: Definition, Steps, and Importance for Class 10th, 11TH, and NEET


The Krebs’ cycle, also known as the Citric acid cycle, oxidizes acetyl-CoA produced from pyruvate. This cycle occurs in the mitochondrial matrix, which is a clever set of eight processes that oxidize acetyl-CoA to create two molecules of CO2 while storing the freed free energy in the reduced chemicals NADH and FADH2. This cycle is the primary source of NADH and FADH2 production from acetyl-CoA and oxaloacetate in aerobic respiration. In the next phase of the electron transport system, the reduced cofactors NADH and FADH2 create ATP.

The cycle produces many organic acids with three COOH groups as intermediate molecules, hence the name Tricarboxylic Acid Cycle. The cycle is also known as the Citric Acid cycle because the first intermediate chemical generated by the condensation of acetyl-CoA and oxaloacetate is Citric acid. The reactions in the citric acid cycle are as follows:

Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O—CoA+2CO2+3NADH+FADH2 + GTP + 3H+

The following are the various stages of the cycle:

1.Enz.citrate synthase catalyzes the formation of citrate from acetyl-CoA and oxaloacetate.

H2O + Oxaloacetate + Acetyl-CoA ——(CS) ——Citrate + CoA

2. The enzyme Aconitase converts citrate to isocitrate.

Citrate —–aconitase——–Cis-Aconitate+H2O  aconitase———-Isocitrate+H2O

3. Isocitrate is converted to – Ketoglutarate via oxidative decarboxylation by the enzyme Isocitrate dehydrogenase. After NAD+ oxidizes isocitrate, an intermediate molecule called Oxalosuccinate is formed, which is then decarboxylated to create – ketoglutarate.

Isocitrate + NAD+ ———-Oxalosuccinate+ NADH + H+ —– CO2 + α- ketoglutarate

4. Succinyl-CoA, NADH, and CO2 are produced by Enz.- ketoglutarate dehydrogenase catalyzing the oxidative decarboxylation of α -ketoglutarate.

α- Ketoglutarate +NAD+ + CoA –(α- Ketoglutarate dehydrogenase)-Succinyl- CoA + CO2+ NADH+H+

 5. The enz. Succinate thiokinase catalyzes the following step, which produces one GTP equal to the high-energy compounds ATP and Succinate.

Succinyl -CoA + Pi——–Succinyl phosphate + CoA

Succinyl phosphate + GDP——–Succinate + GTP

6. The enzyme Succinate dehydrogenase (SD) reacts with succinate and transforms it to fumarate. Succinate +FAD——-(SD)—-Fumarate+FADH2

7. The enzyme Fumarase hydrates fumarate to generate malate.

Fumarate +H2O——Fumarase——-Malate

8. The enz. Malate dehydrogenase (MD) oxidizes (dehydrogenates) malate to create Oxaloacetate, the cycle’s last step. It’s a cycle because the reaction comes to a conclusion where it started (Oxaloacetate)

Malate + NAD+——-MD——–Oxaloacetate + NADH+ H +

Energy balance sheet:

Reaction step                                Reduced Co-factors            ATP molecules produced S Pyruvate —

Acetyl- CoA–                               2 molecules NADH             2 2×3=6

Citric acid cycle energy output:-

 2-Isocitrate——oxalosuccinate—        2 mol. NADH2        2×3=6

3.α- Ketoglutarate—-Succinyl -CoA—   2 mol. NADH2      2×3=6

4-Succinate————Fumarate —-       2mol. FADH2       2×2=4

5-Malate———— Oxaloacetate—–    2mol.-NADH2      2×3=6

 GTP produced– Succinyl-CoA—-Succinate–     1 ATP       2×1=2


Pyruvate– acetyl- CoA ——6

Krebs’ cycle——————–24



 Krebs’ cycle has various functions in cellular metabolism. It occupies a pivotal position in the metabolic process.

1. The majority of the energy in breathing comes from glucose molecules. Out of 38 molecules of ATP produced from one mol. of glucose, 24 ATP molecules are contributed by this cycle. Aside from that, several other chemicals essential for amino acid synthesis are Krebs’ cycle intermediates.

2. Amino acids are utilized to make nucleic acids and proteins. Many of the organic acids produced throughout this cycle serve as precursors of amino acids such as:

α ketoglutarate —Glutamate


Transamination processes are used to make a variety of amino acids from these preexisting amino acids.

3. It has a direct link to nitrogen metabolism. The initial acceptor molecule of NH3 is ketoglutarate, which forms the amino acid glutamatic acid. Because amino acids are the building blocks of proteins, the citric acid cycle is intimately linked to biological proteins and enzymes.

4. Nitrogen fixation in N2 fixing plants, glutamic acid is the first produced amino acid from ketoglutarate, which is the point of entrance of nitrogen in the form of NH3 into the metabolic pool.

5. The precursor of chlorophyll, cytochromes, and phytochromes is succinyl-CoA, which is generated during the cycle.

6. The synthesis of anthocyanin and phenols is also closely linked to Krebs’ cycle. acetyl-CoA, which is a component of the Krebs cycle, is also required for the production of anthocyanin and phenols.

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