I recenty learned the Kreb cycle, in that I saw many chemical are same.

I want to know if there is logic/threrom/technique in which I can write a chemical cycle to get a product i.e I want basic reactions and how to put it in sequence to make a cycle ?

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    $\begingroup$ The Krebs/TCA cycle is not off-topic here per se... but this is something that takes places in a biological system, and if you want to know the "logic" behind it, you'll have to ask this at the Biology.SE [ biology.stackexchange.com ]. Here, at Chem.SE, we can point at parts of cycle and tell you "Oh, this is oxidation happening here; that's is decarboxylation going on there...ooh, and this one right here is oxidative decarboxylation..." etc, etc. If you're looking for a general idea/pattern as to why biochemical processes like these occur in a given tissue, take it to Bio.SE. $\endgroup$ – paracetamol Sep 16 '17 at 9:18
  • $\begingroup$ @paracetamol- it seems like he's asking about how to work out what reactions are taking place- much of the elucidation work on these mechanisms was indeed done by chemists, isolating and characterising the intermediates in the cycle! $\endgroup$ – NotEvans. Sep 16 '17 at 9:20
  • $\begingroup$ @Not "...indeed done by chemists..." True, but unless the OP clarifies further (in a future edit, perhaps?) I'm assuming he wants to know why molecules are shuttled around, broken down, rearranged and eventually built up again in biochemical processes (such as the Krebs cycle). Besides, it would be naive to assume that a given process occurs in an identical manner in all relevant tissue, across all life-forms. It's a good question, but is probably better suited on the Bio.SE... unless the OP wants to know the thermodynamics/equilibrium side to the story. ;) $\endgroup$ – paracetamol Sep 16 '17 at 9:28
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    $\begingroup$ I must say, I really don't understand this question. The Krebs cycle, or any cycle for that matter cycle (eg Suzuki coupling), cannot be a priori worked out just by using logic or a mathematical theorem. Maybe you can use chemical intuition, but that is an entirely different matter. $\endgroup$ – orthocresol Sep 16 '17 at 9:53

tl;dr: You can't reasonably 'propose' a biochemical pathway on paper the same way you might be able to for an organic reaction.

Biochemical pathways are highly complex and interwoven. The Krebs cycle doesn't act in isolation, but rather is part of a far larger network which provides it with the components it needs (enzymes, cofactors, oxidatants/reductants etc.) and the regulation required to ensure it works correctly.

The figure below demonstrates a more elaborated Krebs cycle, complete with all of the relevant interactions with various other pathways:[1]

Kreb cycle/citric acid cycle

Given their complexity, the discovery of the true nature of a pathway takes a long time, and uncertainty often remains due to the difficulty in directly observing the reactions taking place (though with modern genomics/sequencing its becoming easier to see what might be taking place).

In the case of the Krebs cycle, the key discovery was made by Gyorgyi who won the 1937 Nobel Prize in Physiology for 'his discoveries in connection with the biological combustion processes, with special reference to vitamin C and the catalysis of fumaric acid'.

Using isolated cells from pigeon breast tissue, Gyorgyi was able to identify the importance of succinic acid and fumaric acid in the pathway, noting that the rate of respiration was increased significantly if the tissue was incubated in their presence (as measured by gas evolution, which was already known at the time to be a consequence of respiration).

Building on this discovery, Krebs published the first full account of cellular respiration in his 1937 paper Metabolism of Ketonic acids in animal tissues.[2] In this paper he outlines all of the key steps needed, but even at this stage many of the important details were missing (many of the other necessary pathways weren't known, so Krebs had no idea where the succinate was coming from in the body, for example).

[1]: Gerhard, M.; Schomburg, D. Biochemical Pathways- An Atlas of Biochemistry and Molecular Biology; Wiley: New Jersey, 2012

[2]: Krebs, H. A.; Johnson, W. A. Biochem. J. 1937, 31, 645

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