Krebs Cycle Definition and Meaning
What is Krebs Cycle?
The Krebs cycle, or citric acid cycle, generates most of the electron (energy) carriers that will connect to the electron transport chain (CTE) in the latter 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 oxalacetate. Oxalacetate 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 they will produce, in what is considered to be the preparatory phase of the Krebs cycle, acetyl-CoA, necessary to obtain citrate or citric acid.
The reactions of the Krebs cycle take place 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.
Despite this, if we consider the reuse of a molecule to designate a cycle, as the regenerated molecule is four-carbon oxaloacetate, we will consider the previous phase as preparatory.
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, transforming into a two-carbon acetyl-CoA molecule and generating a NADH of NAD +.
The Krebs cycle goes through 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 catalyst enzymes for the regulation of the necessary energy balance will be detailed:
The two-carbon acetyl-CoA molecule binds to the four-carbon oxaloacetate molecule.
Release CoA group.
Produces six carbon citrate (citric acid).
Second and third step
The six-carbon citrate molecule is converted to an isocitrate isomer, first by removing one molecule of water and, in the next step, incorporating it again.
Releases water molecule.
Produces isocitrate isomer and H2O.
The six-carbon isocitrate molecule oxidizes to α-ketoglutarate.
Releases CO 2 (a carbon molecule).
Produces five-carbon α-ketoglutarate and NADH + NADH.
Relevant enzyme: isocitrate dehydrogenase.
The five-carbon α-ketoglutarate molecule is oxidized to succinyl-CoA.
Releases CO 2 (a carbon molecule).
Produces four-carbon succinyl-CoA.
Relevant enzyme: α-ketoglutarate dehydrogenase.
The four-carbon succinyl-CoA molecule replaces its CoA group with a phosphate group producing succinate.
Produces four-carbon succinate and ATP from ADP or GTP from GDP.
The four-carbon succinate molecule oxidizes to form fumarate.
Produces four-carbon fumarate and FDA FADH2.
Enzyme: Allows FADH2 to transfer its electrons directly to the electron transport chain.
The four-carbon fumarate molecule is added to the malate molecule.
Releases H 2 O.
Produces four-carbon malate.
The four-carbon malate molecule is oxidized by regenerating the oxalacetate molecule.
Produces: four-carbon oxaloacetate and NADH from NAD +.
Krebs Cycle Products
The Krebs cycle produces the vast majority of theoretical ATP generated by cellular respiration.
The Krebs cycle will be considered from the combination of the four-carbon molecule oxalacetate or oxalacetic acid with the two-carbon coenzyme acetyl-CoA to produce citric acid or six-carbon citrate.
In this sense, each cycle of Krebs produces 3 NADH of 3 NADH +, 1 ATP of 1 ADP and 1 FADH2 of 1 FAD.
Since 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 above sum gives us 24 out of the 38 theoretical ATPs that result from cellular respiration.
The remaining ATP will be obtained from glycolysis and pyruvate oxidation.