Meaning of the Krebs Cycle - encyclopedia - 2023



What is the Krebs Cycle:

The Krebs cycle, or citric acid cycle, generates most of the electron carriers (energy) that will be connected in the electron transport chain (CTE) in the last part of the cellular respiration of eukaryotic cells.

It is also known as the citric acid cycle because it is a chain of oxidation, reduction and transformation of citrate.

Citrate or citric acid is a six-carbon structure that completes the cycle by regenerating in oxaloacetate. Oxaloacetate is the molecule necessary to produce citric acid again.

The Krebs cycle is only possible thanks to the glucose molecule that produces the Calvin cycle or the dark phase of photosynthesis.

Glucose, through glycolysis, will generate the two pyruvates that will produce, in what is considered the preparatory phase of the Krebs cycle, acetyl-CoA, necessary to obtain citrate or citric acid.

See also Calvin Cycle.

The reactions of the Krebs cycle occur in the inner membrane of the mitochondria, in the intermembrane space that is located between the crystals and the outer membrane.

This cycle needs enzymatic catalysis to function, that is, it needs the help of enzymes so that the molecules can react with each other and it is considered a cycle because there is a reuse of the molecules.

Steps of the Krebs cycle

The beginning of the Krebs cycle is considered in some books from the transformation of glucose generated by glycolysis into two pyruvates.

In spite of this, if we consider the reuse of a molecule to designate a cycle, since the molecule is regenerated four-carbon oxaloacetate, we will consider the phase before it as preparatory.

See also Glucose.

In the preparatory phase, the glucose obtained from glycolysis will separate to create two three-carbon pyruvates, also producing one ATP and one NADH per pyruvate.

Each pyruvate will oxidize into a two-carbon acetyl-CoA molecule and generate a NADH of NAD +.

The Krebs cycle runs each cycle twice simultaneously through the two acetyl-CoA coenzymes that generate the two pyruvates mentioned above.

Each cycle is divided into nine steps where the most relevant catalytic enzymes for regulating the necessary energy balance will be detailed:

First step

The two-carbon acetyl-CoA molecule binds to the four-carbon oxaloacetate molecule.

Free group CoA.

Produces six-carbon citrate (citric acid).

Second and third step

The six-carbon citrate molecule is converted to an isocitrate isomer, first by removing a molecule of water and, in the next step, incorporating it again.

Releases water molecule.

Produces isomer isocitrate and H2O.

Fourth step

The six-carbon isocitrate molecule is oxidized to α-ketoglutarate.

LiberaCO2 (a carbon molecule).

Produces five-carbon α-ketoglutarate and NADH from NADH +.

Relevant enzyme: isocitrate dehydrogenase.

Fifth step

The five-carbon α-ketoglutarate molecule is oxidized to succinyl-CoA.

Releases CO2 (a carbon molecule).

Produces four-carbon succinyl-CoA.

Relevant enzyme: α-ketoglutarate dehydrogenase.

Sixth step

The four-carbon succinyl-CoA molecule replaces its CoA group with a phosphate group, producing succinate.

It produces four-carbon succinate and ATP from ADP or GTP from GDP.

Seventh step

The four-carbon succinate molecule is oxidized to form fumarate.

Produces four-carbon fumarate and FDA FADH2.

Enzyme: allows FADH2 to transfer its electrons directly to the electron transport chain.

Eighth step

The four-carbon fumarate molecule is added to the malate molecule.

Release H2OR.

Produces four-carbon malate.

Nineth step

The four-carbon malate molecule is oxidized, regenerating the oxaloacetate molecule.

Produces: four-carbon oxaloacetate and NADH from NAD +.

See also Molecule.

Krebs cycle products

The Krebs cycle produces the vast majority of the theoretical ATP generated by cellular respiration.

The Krebs cycle will be considered from the combination of the four-carbon molecule oxaloacetate or oxaloacetic acid with the two-carbon acetyl-CoA coenzyme to produce citric acid or six-carbon citrate.

In this sense, each Krebs cycle produces 3 NADH of 3 NADH +, 1 ATP of 1 ADP and 1 FADH2 of 1 FAD.

As the cycle occurs twice simultaneously due to the two acetyl-CoA coenzymes, product of the previous phase called pyruvate oxidation, it must be multiplied by two, which results in:

  • 6 NADH that will generate 18 ATP
  • 2 ATP
  • 2 FADH2 that will generate 4 ATP

The sum above gives us 24 of the 38 theoretical ATPs that result from cellular respiration.

The remaining ATP will be obtained from glycolysis and the oxidation of pyruvate.

See also


Types of respiration.