Krebs Cycle with Molecular Models

Krebs Cycle with Molecular Models

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

Krebs_Cycle

What follows are molecular models of the sequential molecules involved in the Krebs Cycle:

00_AcetylCoA_oxaloacetate_PB120039
Oxaloacetate Acetyl CoA transfers its acetyl group to the number two carbonyl carbon via the methyl end forming citrate
01_citrate_PB120030
Citrate Note that it has a tertiary alcohol which is not oxidizable.
02_Isocitrate_PB120024
Isocitrate The hydroxyl has been shifted so that it is now a secondary alcohol, and can be oxidized.
03_alpha_ketoglutarate_PB120025
Alpha ketoglutarate When Isocitrate is oxidized, leading to the reduction of NAD+, it also is decarboxylated
04_Succinyl_CoA_PB120033
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
05_Succinate_PB120034
Succinate The thioester bond in succinyl CoA is hydrolyzed forming fumarate, with generation of GTP linked to the process.
06_fumarate_PB120029
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?)
07_Malate_PB120036
Malate Water is added to fumarate, leading to the formation of a secondary alcohol.
08_Oxaloacetate_PB120021
Oxaloacetate The alcohol is oxidized (similar to oxidation of isocitrate), reducing NAD+ to NADH, forming oxaloacetate.
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