The Krebs cycle (alias, the citric acid cycle, alias the tricarboxylic acid cycle), when reduced to its most fundamental purpose, generates reducing power in the form of NADH and FADH2. It does this by "dissecting off" hydrogens from two carbon fragments remaining after glucose goes through glycolysis and subsequent decarboxylation of pyryvate yielding acetyl coenzyme A. The acetyl group is fed into the cycle by attachment to oxaloacetate, yielding citrate.
Here is an overview of the molecules involved in the Krebs cycle:
What follows are molecular models of the sequential molecules
involved
in the Krebs Cycle:
 |
Oxaloacetate Acetyl CoA transfers its acetyl group to the number two carbonyl carbon via the methyl end forming citrate. |
 |
Citrate
Note that it has a tertiary alcohol which is not oxidizable. |
 |
Isocitrate
The hydroxyl has been shifted so that it is now a secondary alcohol, and can be oxidized. |
 |
Alpha ketoglutarate
When Isocitrate is oxidized, leading to the reduction of NAD+, it also is decarboxylated |
 |
Succinyl CoA
In a reaction similar to the formation of acetyl CoA, ketoglutarate is oxidized, decarboxylated and a CoA attached. (Note that the coenzyme A moiety is indicated by a turquoise group. |
 |
Succinate
The thioester bond in succinyl CoA is hydrolyzed forming fumarate, with generation of GTP linked to the process. |
 |
Fumarate
Succinate is dehydrogenated, forming trans fumarate with the concomitant reduction of FAD to FADH. (Why isn't this molecule in the cis configuration? Anyone?) |
 |
Malate
Water is added to fumarate, leading to the formation of a secondary alcohol. |
 |
Oxaloacetate
The alcohol is oxidized (similar to oxidation of isocitrate), reducing NAD+ to NADH, forming oxaloacetate. |
!