Krebs cycle, mnemonics, steps, regulations

The Krebs cycle is a sequence of reactions in mitochondria that occurs in all aerobic organisms to release stored energy. Krebs cycle is also known as the citric acid cycle or Tricarboxylic acid cycle.

The Krebs cycle occurs in mitochondria and all the enzymes are present in the mitochondrial matrix, either free or attached to the inner mitochondrial membrane and the cristae membrane. This process is aerobic because it requires oxygen as the final oxidant of the reduced enzymes. In addition, it has a central role in gluconeogenesis, lipogenesis, and interconversion of amino acids

HISTORY

The complete cycle was proposed by Sir Hans Krebs in 1937. Various investigators defined many of the enzymes of krebs cycle but it was ‘Hans Krebs‘ who pieced them together. Therefore, the cycle is named after him. He received a Nobel prize in 1953 for the same. Please note that the name is the Krebs cycle (there is no apostrophe).

Krebs proposed the original name as TCA (tricarboxylic acid) cycle. Later, Ogston in 1948, showed that the tri-carboxylic acid is indeed citric acid, and so the name citric acid cycle was given later.

Where does the krebs cycle take place?

The enzymes of the Krebs cycle are located in the mitochondrial matrix, in close proximity to the electron transport chain. This enables the synthesis of ATP by oxidative phosphorylation without any hindrances.

krebs cycle location

Does the krebs cycle require oxygen?

Although the Krebs cycle does not use oxygen in any of its reactions, it requires oxidative metabolism in the mitochondria for reoxidation of reduced coenzymes.

KREBS CYCLE SUMMARY

Krebs cycle basically involves the combination of a two-carbon acetyl CoA with four-carbon oxaloacetate to produce a six-carbon tricarboxylic acid, citrate. In the reactions that follow, the two carbons are oxidized to CO2 and oxaloacetate is regenerated and recycled. Oxaloacetate is considered to play a catalytic role in the citric acid cycle.

The primary location of the enzyme of the TCA cycle is in the mitochondria. Although, isozymes of some are also present in the cytosol. The substrate of the cycle is acetyl CoA. Altogether, Krebs cycle end products are two CO2, one high energy phosphate bond as GTP and four reducing equivalents i.e., three NADH and one FADH2.

Firstly, the acetyl portion of acetyl-CoA combines with oxaloacetate (4 carbons) to form citrate (six carbons), which will rearrange to form isocitrate.

In the next two oxidative decarboxylation reactions, electrons will transfer to NAD+ to form NADH, and two molecules of electron-depleted CO2 will release.

Subsequently, a high-energy phosphate bond in GTP will generate from substrate-level phosphorylation. Lastly, succinate will oxidize to oxaloacetate with the generation of one FAD(2H) and one NADH.

The net reaction of the TCA cycle shows that the two carbons of the acetyl group will oxidize to two molecules of CO2, with the conservation of energy as six molecules of NADH, two of FAD(2H), and two of ATP.

Particularly, the TCA cycle requires a large number of vitamins and minerals to function. These include niacin (NAD+), riboflavin (FAD and flavin mononucleotide [FMN]), pantothenic acid (coenzyme A), thiamin, Mg2+, Ca2+, Fe2+, and phosphate.

Functions

  1. It is the final common oxidative pathway that oxidizes acetyl CoA to CO2.
  2. Krebs cycle is the source of reduced coenzymes that provide the substrate for the respiratory chain.
  3. Basically, the citric acid cycle acts as a link between catabolic and anabolic pathways (amphibolic role).
  4. Also, it provides precursors for the synthesis of amino acids and nucleotides.
  5. Components of the Krebs cycle have a direct or indirect controlling effect on key enzymes of other pathways.
  6. A primary function of the Tricarboxylic acid cycle is reducing equivalents that will utilize to generate energy in the form of ATP.

KREBS CYCLE MNEMONIC

krebs cycle mnemonic
Krebs cycle mnemonic

Can I Ask Some Super Fantastic Memes On (Krebs cycle)

Here, (in krebs cycle mnemonic) capital letters (CIASSFMO) stand for:

C= citrate

I= isocitrate

A= alpha-ketoglutarate

S= succinyl CoA

S= succinate

F= fumarate

M=malate

O= oxaloacetate

Some more mnemonics

I have one more mnemonic in mind for Krebs cycle with just a slight modification of the above sentence and that is:

  • Can I Ask Some Super Fantastic Mnemonic On (Krebs cycle)… whatever you like, you can use.

I searched on the internet and found some more interesting mnemonics on the Krebs cycle. I am sharing it below. Enjoy reading!

(If you have your mnemonics, then please share them with us in the comments section… Thank you)

  1. Oh! Can I keep Some Succinate For Myself? ( here, K refers to alpha-ketoglutarate )
  2. Can I Keep Selling Sex For Money Officer?
  3. Citrate Is Krebs Starting Substrate For Making Oxaloacetate

Krebs cycle Diagram

krebs cycle diagram

KREBS CYCLE STEPS

 

1. Formation of citrate

Firstly, oxaloacetate (4 carbon atom) condenses with acetyl CoA (2 carbon atom) to form 6 carbon compound-the citrate (a tricarboxylic acid). Citrate synthase catalyzes this reaction. The hydrolysis of the thioester bond in acetyl CoA drives the reaction forward (exothermic reaction). This is an irreversible step. However, the body can reverse this step by another enzyme, ATP-citrate lyase.

Because oxaloacetate will regenerate with each turn of the cycle, it is not really considered a substrate of the citric acid cycle or a source of electrons or carbon.

2. Formation of Isocitrate

Secondly, aconitase isomerizes citrate to isocitrate. This reaction is a two-step process: dehydration to cis-aconitate and rehydration to isocitrate. At first, one water molecule is removed from citrate forming cis-aconitate; a transient compound with a very short half-life. Immediately, one water molecule is added to aconitate to form isocitrate.

3. Formation of Alpha Ketoglutarate

Thirdly, isocitrate (6 carbon) undergoes dehydrogenation catalyzed by isocitrate dehydrogenase. Initially, it forms oxalosuccinate. It is an unstable compound which undergoes spontaneous decarboxylation to form alpha-ketoglutarate (5 carbon). In this reaction, one molecule of CO2 liberates.

Basically, there are three isoenzymes of isocitrate dehydrogenase. One, which uses NAD+, is present only in mitochondria. While the other two use NADP+ and are present in mitochondria and the cytosol.

4. Formation of Succinyl CoA

After that, ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA by the enzyme alpha-ketoglutarate dehydrogenase. The NADH thus generated enters into the electron transport chain (ETC) to generate ATPs. In this reaction, the second molecule of CO2 and the second reducing equivalent(i.e., NADH+H+) of the cycle will produce. This is the reversible step in the whole reaction cycle.

5. Generation of Succinate

Succinyl-CoA will convert to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). Emphatically, this is the only example in the Tricarboxylic acid cycle of substrate-level phosphorylation.

Further, a molecule of GDP will phosphorylate to GTP and succinate will form. The GTP will convert to ATP by reacting with an ADP molecule:

GTP + ADP → GDP + ATP

6. Formation of Fumarate

Succinate will dehydrogenate to fumarate, the first dehydrogenation reaction, by succinate dehydrogenase. In this reaction, the hydrogen atoms are accepted by FAD. After that, the FADH2 enters into ETC to generate ATPs. The enzyme is a flavoprotein.

7. Formation of malate

After that, Fumarase (fumarate hydratase) catalyzes the addition of water across the double bond of fumarate, yielding malate.

8. Regeneration of Oxaloacetate

Finally, malate dehydrogenase oxidizes oxaloacetate, a reaction requiring NAD+. This step generates NADH, which enters the electron transport chain during the production of ATPs.

The oxaloacetate can further condense with another acetyl CoA molecule and the krebs cycle continues. Oxaloacetate is an important junction point in metabolism. Oxaloacetate may act as a catalyst, which enters into the reaction, causes complete oxidation of acetyl CoA and comes out of it without any change.

The equation for the Krebs cycle

The overall reaction of the Krebs cycle is as follows:

Acetyl CoA + 3NAD+ + FAD + GDP +Pi + 2H2 O → 2CO2 + 3NADH+ FADH2 + GTP + 3H++ CoA

To summarize, for complete oxidation of a glucose molecule, Krebs cycle yields 4 CO2, 6NADH, 2 FADH2 and 2 ATPs.

Further, each molecule of NADH can form 2-3 ATPs and each FADH2 gives 2 ATPs on oxidation in the electron transport chain.

SIGNIFICANCE OF CITRIC ACID CYCLE

Evidently, the Krebs cycle or Citric acid cycle is the final pathway of oxidation of glucose, fats and amino acids. Although the krebs cycle generates a considerable amount of energy, its major function is to provide precursors, not only for the strong forms of fuel but also for the building blocks of many other molecules. Amino acids (metabolic product of proteins) enter the citric acid cycle and get metabolised e.g. alanine is converted to pyruvate, glutamate to 𝝰-ketoglutarate, aspartate to oxaloacetate on deamination

Also, it plays an important role in gluconeogenesis and lipogenesis and interconversion of amino acids. For example, most of the carbon atoms in oorphyrins (the organic components of heme) come from succinyl-CoA. Thus the TCA cycle is an amphibolic cycle, which means that it functions not only in catabolism (breakdown) but also in anabolic (synthesis) reactions in the cell.

REGULATION OF CITRIC ACID CYCLE CYCLE

Regulation of citrate synthase

Citrate synthase, which is the first enzyme of the citric acid cycle, is a simple enzyme that has no allosteric regulators. Its rate is controlled principally by the concentration of oxaloacetate, its substrate, and the concentration of citrate. It is inhibited by ATP, NADH, acetyl-CoA, and succinyl CoA.

Allosteric Regulation of Isocitrate Dehydrogenase

Isocitrate dehydrogenase, which consists of eight subunits, is one of the rate-limiting steps of the citric acid cycle. It is allosterically activated by ADP and inhibited by NADH and ATP.

Regulation of alpha-ketoglutarate

The α-ketoglutarate dehydrogenase complex, although not an allosteric enzyme, is product-inhibited by NADH and succinyl-CoA and may also be inhibited by GTP.

Regulation of Krebs Cycle Intermediates

Regulation of the krebs cycle serves two functions:

  • It ensures the fast generation of NADH to maintain ATP homeostasis, and
  • it regulates the concentration of Krebs cycle intermediates.

For example, in the liver, a decreased rate of isocitrate dehydrogenase increases citrate concentration, which stimulates citrate efflux to the cytosol.

Several regulatory interactions occur in the Krebs cycle that controls the levels of the krebs intermediates and their flux into pathways that adjoin the Krebs cycle.

Regulation by calcium

Calcium is also used as a regulator in the citric acid cycle. Calcium levels in the mitochondrial matrix can reach up to the tens of micromolar levels during cellular activation. Further, it activates pyruvate dehydrogenase phosphatase which in turn activates the pyruvate dehydrogenase complex. Moreover, calcium activates isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. This increases the reaction rate of many of the steps in the cycle, and therefore increases flux throughout the pathway.

AMPHIBOLIC NATURE OF TCA CYCLE

Krebs cycle is catabolic and anabolic at the same time. Hence, it is amphibolic in nature. Moreover, the Krebs cycle actively participates in Gluconeogenesis, transamination and deamination reactions. The most important synthetic reactions connected with Krebs cycle are:

  1. Oxaloacetate and alpha-ketoglutarate: serve as the precursors for the synthesis of aspartate and glutamate which, in turn, required for the synthesis of non-essential amino acids, purines and pyrimidine.
  2. Succinyl CoA: used for the synthesis of porphyrins and heme.
  3. Mitochondrial citrate: transported to the cytosol where it cleaves to give acetyl CoA for the biosynthesis of fatty acids, sterols, etc.

REFERENCES

Textbook of Biochemistry for medical students, sixth edition by DM Vasudevan, Sreekumari S, Kannan Vaidyanathan; chapter no. 18: citric acid cycle

Harper’s illustrated Biochemistry, 28 edition by Robert K. Murray, David A Bender, Kathleen M. Botham , Peter J. Kennelly, Victor W. Rodwell , P. Anthony Weil, chapter no. 17: citric acid cycle-The catabolism of acetyl CoA

Textbook of Biochemistry with clinical correlation, Fourth edition, Wiley-LissT6.4: The Tricarboxylic Acid Cycle

Lastly, Mark’s Basic Medical Biochemistry: A clinical Approach by Michael Lieberman and Alisa Peet , 5th edition by chapter no. 23: The Tricarboxylic Acid Cycle

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