🔎Hello again,

A lot has been going on, so I missed out on posting yesterday. So without any delay let me tell you a few more points about the TCA cycle; that I raised in our previous post in the form of questions.

Q1. why the TCA cycle is called the final common oxidative pathway?

Ans: The TCA cycle is considered the final common oxidative pathway for all the foodstuffs because all the major ingredients of foodstuffs are finally oxidized through the cycle. Almost all the biochemical processes use ATP for meeting energy needs- like muscle contraction active transport biosynthetic reactions, etc.  All the metabolic Pathways converge   into this pathway  as follows:

Integration of major metabolic Pathways: 

  •  Carbohydrates are metabolized through glycol lighting pathway to pyruvate then converted to acetyl CoA a which enters the citric acid cycle.
  •  Fatty acids through beta-oxidation are broken down to acetyl CoA and then enter this TCA cycle
  •  Glucogenic amino acids after transamination and at some or other points in this cycle. Ketogenic amino acids are converted into acetyl CoA.
  •  The integration of metabolism is achieved at the junction point by key metabolites. Several Pathways can converge at this point with the result that carbon atoms from one source can be used for the synthesis of another. Important intermediates are pyruvate acetyl CoA and oxaloacetate.
Q2.what is the significance of the TCA cycle?


Ans: the main significance of the TCA cycle are :

  1. complete oxidation of acetyl CoA.
  2. 1. ATP generation
  3. 2. the final common oxidative pathway
  4. 3. integration of major metabolic Pathways
  5. 4. fat is burned on the wick of carbohydrates
  6. 5. excess carbohydrates are converted as neutral fats
  7. 6. no net synthesis of Carbohydrates from fats
  8. 7. carbon skeletons of amino acids finally enter the citric acid cycle
  9. 8. amphibolic pathway
  10. 9. anaplerotic role.

Q3.What are the functions of the TCA cycle?


Ans:  The TCA cycle   has the following functions:

  1. The final common oxidative pathway that oxidizes acetyl CoA to carbon dioxide
  2. .The source of reduced coenzymes that provide the substrate for the respiratory chain 
  3. The link between catabolic and anabolic pathways (Amphibolic role).
  4. Precursors for the synthesis of amino acids and nucleotides.
  5.  Components of the cycle have direct or indirect controlling effects on key enzymes of other pathways.

Q4.why is this called the amphibolic pathway?

Ans: The TCA cycle is truly amphibolic in nature because it is both catabolic and anabolic. There is a continuous influx and a continuous efflux of four carbon units from the TCA cycle. Since various compounds enter into or leave from TCA cycle it is sometimes called a metabolic traffic circle. 
Important anabolic reactions related to the citric acid cycle are:

a) oxaloacetate is the precursor of aspartate
b) alpha-ketoglutarate it can be made into glutamate
c)Succinyl CoA a is used for the synthesis of heme
d)mitochondrial citrate is transported to the cytoplasm where it is cleaved into acetyl CoA, which then is the starting point of fatty acid synthesis.

Q5.why there is no net synthesis of Carbohydrates from fat?

Ans:  Acetyl CoA entering the cycle is completely oxidized to CO2 by the time the cycle reaches succinyl  CoA. So acetyl-CoA is completely broken down in the cycle. Thus acetyl CoA can't be used for gluconeogenesis. Therefore there is no net synthesis of Carbohydrates from fat.

Q6. Anaplerotic role of the TCA cycle.

Ans: The citric acid cycle acts as a source of precursors of biosynthetic pathways for example heme is synthesized from succinyl CoA and aspartate from oxaloacetate. To counterbalance such losses and to keep the concentrations of the four-carbon units in the cell, anaplerotic reactions are essential.  

This is called the anaplerotic role of the TCA cycle. Anaplerotic means filling up reactions or influx reactions or replenishing reactions that supply four-carbon units to the TCA cycle.

The important anaplerotic reactions are:
  1. 1. pyruvate to oxaloacetate by pyruvate carboxylase enzyme
  2. 2. glutamate is transanimated to Alpha-ketoglutarate and aspartate to Acetate. 3. Other important amino acids entering the TCA cycle are shown in figure 1.
  3. 4. pyruvate can be carboxylated to malate by NADP+ dependant malic enzyme.



Fig`1: Influx of TCA cycle intermediates.

🔊  If you need to ask or know anything more about the cycle,  leave your queries in the comment section or email. 


Have a great day!