CITRIC ACID CYCLE

  🔊Good day to you all...

Today's post is about the citric acid cycle the most important cycle and is directly related to biological oxidation and electron transport chain.

Before I actually start talking about the citric acid cycle let's quickly go through the key points of this cycle.

•  The citric acid cycle is the final common oxidative pathway this oxidizes acetyl coenzyme to carbon dioxide. It also is a direct link between catabolic and anabolic Pathways so in a way it plays an amphibolic role.
• This cycle is the source of reduced coenzyme as well as precursors for the synthesis of proteins and nucleotides, hence  an anaplerotic pathway
• The sources of acetyl coenzyme a pyruvate(glycolysis) fatty acids (beta-oxidation) and ketogenic amino acids.
• All enzymes of the cycle are located inside the mitochondria.
• A total of 3 NADH molecules are generated in the cycle and one FADH2  is formed along with one GTP.
• Both the carbon atoms of acetyl coenzyme a are removed as carbon dioxide at steps 3 and 4.
• 10 molecules of ATP are produced per turn of the TCA cycle. Hence it is the main generator of ATP among all the metabolic Pathways.
• Alpha-ketoglutarate dehydrogenase is the only step that is irreversible in this cycle.
• Oxaloacetate is the true catalyst that enters and leaves the cycle unchanged.
• Oxidation of fat (acetyl CoA) needs the help of oxaloacetate(coming from pyruvate & hence from Carbohydrates). In other words, fats are burnt in the fire of carbohydrates. It means for the burning of fats body needs carbohydrates.
• This cycle also tells us that fat cannot be converted back to glucose because pyruvate to acetyl CoA is an irreversible step.
• Cellularneed of  ATP requirement regulates TCA cycle. 
• Lastly, deficiency of pyruvate dehydrogenase enzyme causes lactic acidosis and neurological disorders in the body.

Fig1: Overall reactions of TCA

 🔎Let us Dive into Understanding the working of the TCA cycle :

Definition: 

TCA cycle is also known as the Krebs cycle or tricarboxylic acid cycle and the Central oxidative pathway; It is a metabolic pathway in which acetyl CoA obtained from various sources is finally oxidized. Intermediate of this pathway can also be used to synthesize a number of compounds. This pathway is a cyclic pathway.

Location: the pathway is present in all the cells that have mitochondria also the enzymes of this pathway are located in mitochondria.

Main features of the cycle:

the cycle begins with the condensation of acetyl CoA (2- carbon compound) with oxaloacetate (4-carbon compound) to form citrate(six-carbon compound).

The process:

citrate is converted into yellow acetate by a series of reactions while two carbon atoms are removed as carbon dioxide and a number of reducing equivalents are so also removed which are later oxidized to water in the respiratory chain.

                                      Fig. 2 Its a cyclic process

Now let's talk about sources of oxaloacetate:

first of all, in this cycle, there is no net utilization of oxaloacetate because at the end of the cycle it is regenerated.

However, if the overall rate of the reactions is to be increased, the concentration of intermediates of this cycle will have to be raised.

Reactions that lead to the net entry of intermediates into the cycle are known as anaplerotic reactions.

👉 an important anaplerotic reaction in this cycle is the synthesis of oxaloacetate from pyruvate which is catalyzed by pyruvate carboxylase and design in the presence of biotin. The energy for this covalent bond formation is provided by ATP.

Now oxaloacetate may also be formed by a transamination reaction between aspartate and Alpha-ketoglutarate however this reaction is not enough protic in nature because oxaloacetate is formed at the expense of Alpha-ketoglutarate.

Sources of acetyl CoA:

Acetyl CoAoccupies a very important role in this metabolism and can be formed from 

1. Glucose

2. Fatty acids

3. Amino acids

👉Glucose, lactate, and some amino acids can be converted into pyruvate which in turn is converted into acetyl CoA.

👉The end product of fatty acid oxidation is acetyl CoA.

👉Ketone bodies are also converted into acetyl CoA before their final oxidation.

Oxidative decarboxylation of pyruvate:

  👉oxidative decarboxylation:

pyruvate is finally converted into acetyl CoA occurs by a series of reactions in the mitochondria by a process called oxidative decarboxylation.

All these reactions are catalyzed by the pyruvate dehydrogenase(PDH) complex*. This Complex consists of three enzymes and needs five enzymes.

Pyruvate dehydrogenase (PDH) complex:

Fig:3 Components of PDH complex

The fate of acetyl CoA:

most of the acetyl CoA is oxidized in the citric acid cycle which produces a large amount of energy in the form of ATP.

👉When the supply of energy is abundant acetyl CoA  is used to synthesize fatty acids along with this a small amount of acetyl co a is used for the synthesis of cholesterol steroid hormones Vitamin D and Ketone bodies and for various acetylation reactions

Reactions of the citric acid cycle:

I will only highlight the main important points of the cycle and the details are explained in the figure below which is elaborately labeled.

                                                            Fig:4 Details of TCA 

The two reactions where two carbon atoms are removed are the 5th and 6th steps in the cycle  which are as follow:

The fifth reaction where oxalosuccinate is decarboxylated to alpha-ketoglutarate by the action of the enzyme isocitrate dehydrogenase in the presence of Manganese; one carbon atom is removed as carbon dioxide.

In the sixth reaction alpha-ketoglutarate, it undergoes oxidative decarboxylation to form succinyl CoA.

This reaction is catalyzed by the Alpha-ketoglutarate dehydrogenase complex*.

Alpha-ketoglutarate dehydrogenase complex  is made up of:

Alpha-ketoglutarate dehydrogenase

Dihydrolipoyl  acetyltransferase

Dihydrolipoyl dehydrogenase

the coenzymes needed are:

Thiamin pyrophosphate

Lipoic acid

CoA

FAD

NAD

👉The sixth reaction is also an example of a substrate linked oxidative phosphorylation*

the energy released during this is used to form high-energy thioester Bond.

The energy released in the seventh step (which split succinyl CoA to succinate and CoA); is used to phosphorylate GDP to GTP which can transfer a high energy phosphate to form ATP.

After this in the 10th reaction oxaloacetate is regenerated and acetyl CoA is oxidized.

👉The only reaction which is not reversible is the conversion of Alpha-ketoglutarate into succinyl CoA.

Inhibitors of TCA:

The following are toxic or poisonous (non-physiological) agents which inhibit the reactions.

                                         Fig 5: Inhibitors of TCA

Regulation of TCA cycle:

several factors may control the regulation of the TCA cycle including

1. Substrate level

2. enzyme level 

3. respiratory control 

4. Accessibility of cycle intermediates

 5. ketosis and finally 

6. control of enzyme activity. 

All six factors  have direct or indirect control over the regulation of the cycle 

In the diagram below the red cross in the circle indicate blocking and green triangles mean the increased activity( or positive flow).

Fig.6:Regulation of TCA cycle

Energy generation in TCA:

The various energy generation steps are 3rd 4th 5th 6th and 8th WhereNADH, GTP, FADH2 are formed and hence ATP is produced. 

In total

 9 NADH+2 FADH2+ 1 GTP 

are produced forming 12 ATP(old calculation) or 10 ATP(new calculation).

👉 only about 33% of the energy liberated is trapped as ATP the rest is used to keep the body temperature at a higher level than the environment

Metabolic defects related to the TCA cycle:

Disorders are extremely rare but some enzyme deficiencies may cause defects or may affect the operation of the TCA cycle

1. Pyruvate dehydrogenase deficiency may cause abnormal it is like lactic acidosis and neurological disorders.

2. Acyl-CoA dehydrogenase deficiency may cause organic acid urea, glutaric aciduria, acidosis, hypoglycemia, and electron flow from FAD to Co Q may be affected.

3. Pyruvate carboxylase deficiency may cause oxaloacetate deficiency, lactic acidosis, hyperammonemia, and hyperalaninemia.

🔚With this, I come to the end of this post. stay safe and keep following ...

🔜See you in the next post where I will explain how to answer the following

1.  why the TCA cycle is called the final common oxidative pathway
2. what is the significance of the TCA cycle
3. functions of the TCA cycle
4. why is this is called the amphibolic pathway
5. why there is no net synthesis of Carbohydrates from fat
6. Anaplerotic role of the TCA cycle