I have diagram of oxidative decarboxylation of pyruvate pathway. The pathway is between glycolisis and Krebs cycle.

How to read this diagram?

This excerpt from Harper's Illustrated Biochemistry, 30E, page 173.

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    $\begingroup$ The circles are like gears, where the points of contact are chemical reactions. $\endgroup$
    – jzx
    Commented Aug 23, 2015 at 20:10
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    $\begingroup$ To me, it is unclear exactly what you are asking about, and the question also reads as unresearched homework. $\endgroup$ Commented Aug 24, 2015 at 9:34

2 Answers 2


Texts usually have a verbal explanation of these kind of diagrams, so look in the text where it references Figure 17.5 and read the section before and section after.

That being said, if you start at the top with pyruvate, it enters the cycle where the curved circles touch on pyruvate dehydrogenase, which catalyzes its decarboxylation. That product then enters into the next cycle where the disulphide bond is broken and it is added to one of the sulfurs. Co-enzyme A then reacts with that and you get Acetyl CoA as a byproduct and dihydrolipoamide. The cycle continues and you get one hydride (I think) and one proton transfer to FAD and the reformation of the disulfide bond. You then have a hydride transfer to NAD+ and a deprotonation to regenerate FAD.

So each place a curved arrow touches another is what goes into the reaction and then following the split of the arrows is the product that comes out of the reaction. Where there are transfers to different circles means that there are different enzymes catalyzing the reactions.


Take a look on reaction catalyzed by dihydrolipoyl dehydrogenase. It acts upon $NAD^+$ according to following equation: $NAD^+ + FADH_2\rightarrow NADH+H^++FAD$

Same way you can write other reactions, e.g. by pyruvate dehydrogenase: $Pyruvate+TDP\rightarrow CO_2+TDP-Hydroxyethyl$


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