- 1 decade agoFavorite Answer
The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. In aerobic organisms, the citric acid cycle is part of a metabolic pathway involved in the chemical conversion of carbohydrates, fats and proteins into carbon dioxide and water to generate a form of usable energy. It is the second of three metabolic pathways that are involved in fuel molecule catabolism and ATP production, the other two being glycolysis and oxidative phosphorylation.
The citric acid cycle also provides precursors for many compounds such as certain amino acids, and some of its reactions are therefore important even in cells performing fermentation.
1.1 A simplified view of the process:
3 Major metabolic pathways converging on the TCA cycle
4 See also
6 External links
Molecule Enzyme Reaction type Reactants/
I. Citrate 1. Aconitase Dehydration H2O
II. cis-Aconitate 2. Aconitase Hydration H2O
III. Isocitrate 3. Isocitrate dehydrogenase Oxidation NAD+ NADH + H+
IV. Oxalosuccinate 4. Isocitrate dehydrogenase Decarboxylation
V. α-Ketoglutarate 5. α-Ketoglutarate
decarboxylation NAD+ +
CoA-SH NADH + H+
VI. Succinyl-CoA 6. Succinyl-CoA synthetase Hydrolysis GDP
+ Pi GTP +
VII. Succinate 7. Succinate dehydrogenase Oxidation FAD FADH2
VIII. Fumarate 8. Fumarase Addition (H2O) H2O
IX. L-Malate 9. Malate dehydrogenase Oxidation NAD+ NADH + H+
X. Oxaloacetate 10. Citrate synthase Condensation
The sum of all reactions in the citric acid cycle is:
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 3 H2O →
CoA-SH + 3 NADH + H+ + FADH2 + GTP + 2 CO2 + 3 H+
Two carbons are oxidized to CO2, and the energy from these reactions is stored in GTP , NADH and FADH2. NADH and FADH2 are coenzymes (molecules that enable or enhance enzymes) that store energy and are utilized in oxidative phosphorylation.
A simplified view of the process:
The process begins with the oxidation of pyruvate, producing one CO2, and one acetyl-CoA.
Acetyl-CoA reacts with the four carbon carboxylic acid, oxaloacetate--to form the six carbon carboxylic acid, citrate.
Through a series of reactions citrate is converted back to oxaloacetate. This cycle produces 2 CO2 and consumes 3 NAD+, producing 3 NADH and 3 H+.
It consumes 3 H2O and consumes one FAD, producing one FADH+.
1st turn end= 1 ATP, 3 NADH, 1 FADH2, 2 CO2
Since there are two molecules of Pyruvic acid to deal with, the cycle turns once more.
The complete end result= 2 ATP, 6 NADH, 2 FADH2, 4 CO2
Many of the enzymes in the TCA cycle are regulated by negative feedback from ATP when the energy charge of the cell is high. Such enzymes include the pyruvate dehydrogenase complex that synthesises the acetyl-CoA needed for the first reaction of the TCA cycle. Also the enzymes citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase, that regulate the first three steps of the TCA cycle, are inhibited by high concentrations of ATP. This regulation ensures that the TCA cycle will not oxidise excessive amounts of pyruvate and acetyl-CoA when ATP in the cell is plentiful. This type of negative regulation by ATP is by an allosteric mechanism.
Several enzymes are also negatively regulated when the level of reducing equivalents in a cell are high (high ratio of NADH/NAD+). This mechanism for regulation is due to substrate inhibition by NADH of the enzymes that use NAD+ as a substrate. This includes both the entry point enzymes pyruvate dehydrogenase and citrate synthase.
Major metabolic pathways converging on the TCA cycle
Most of the body's catabolic pathways converge on the TCA cycle, as the diagram shows. Reactions that form intermediates of the cycle are called anaplerotic reactions.
The citric acid cycle is the second step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA and enters the citric acid cycle.
In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. These amino acids are brought into the cells and can be a source of energy by being funnelled into the citric acid cycle.
In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart tissue, fatty acids are broken down through a process known as beta oxidation which results in acetyl-CoA which can be used in the citric acid cycle. Sometimes beta oxidation can yield propionyl CoA which can result in further glucose production by gluconeogenesis in liver.
The citric acid cycle is always followed by oxidative phosphorylation. This process extracts the energy from NADH and FADH2, recreating NAD+ and FAD, so that the cycle can continue. The citric acid cycle itself does not use oxygen, but oxidative phosphorylation does.
The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the citric acid cycle and oxidative phosphorylation equals about 36 ATP molecules. The citric acid cycle is called an amphibolic pathway because it participates in both catabolism and anabolism.Source(s): wiki
- AllasseLv 51 decade ago
Don't you just hate it when folk just cut and paste from Wiki? The Krebs cycle is the series of reactions that go round and round releasing electrons which will be used to phosphorylate ADP to produce ATP. It takes place in the mitochondrian - specifically in the fluid filled interior called the matrix. It is fuelled mostly by glucose metabolism outside the organelle, but fatty acids can also feed into it.
The best way to learn the Krebs is to borrow a really good text book and copy out the diagram onto a big bit of paper. Carefully read the text for each bit of the reaction and it will eventually come together. It will take a while as there is an awful lot of information to juggle as you read through it. Best of luck!
- EllieLv 41 decade ago
OK why do people cut and paste from Wikipedia? The Krebs cycle as they say is also called the TCA cycle or citric acid cycle. It is part of carbohydrate metabolism.
Basically glucose is metabolised into pyruvate by glycolysis it then enters the mitochondria where it enters the TCA cycle. ATP, NADH and FADH2 are produced along with CO2. The next step along the pathway is the electron transport chain where the NADH and FADH2 are converted to ATP.
I suggest that you look at your textbook or Wikipedia so you can see a picture of all the steps in the Krebs cycle.Source(s): PhD in Biochemistry.
- 1 decade ago
The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration
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- 1 decade ago
hmm.Krebs's cycle is not an ordinary thing to explain just by typing.you wont understand..i will give u one site OK..u go there and read it .its very simple.
actually Krebs's cycle is a series of reactions which help in the production of ATPs.it s a continuation of the oxidative pathway of glycolysis. it takes place in the mitochondria.
it is also known as TCA cycle or citric acid cycle
- Fabien Tempest™Lv 51 decade ago
The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration.
- Curious KateLv 41 decade ago
Eco-friendly transport belonging to the ex-chair of the Food Standards Agency.
- starla_o0Lv 41 decade ago
the bane of my existanceSource(s): Microbiologist