The enzyme dehydrogenase also causes decarboxylation

Citric acid cycle and oxidative decarboxylation

The citric acid cycle is a central cycle of biochemical reactions in the cell metabolism, which is used for the oxidative breakdown of organic substances (fats, sugars and amino acids) for the purpose of generating energy.

In oxidative decarboxylation, the metabolic product pyruvate is processed further in addition to metabolic products from other metabolic pathways.

The so-called reduction equivalents nicotinamide adenine dinucleotide (NAD +) and flavin adenine dinucleotide (FAD +) are reduced by hydrogen and electron transfer (reduction means electron uptake). This is indicated by adding an ’H’ to NAD or FAD (also used in NADP). Strictly speaking, two electrons are transferred, the exact notation is then NADH + H + or FADH + H +. These electrons and the hydrogen are easy to detach again (releasing energy) and are needed later in the respiratory chain.

The pyruvate is first transported into the mitochondria, the ’power plants of the cell’. There it is decarboxylated, i.e. H. it becomes CO2 split off that later finds itself in the ’breath’. In doing so, electrons are transferred to NAD. The "acetyl" residue (actually acetyl group or acetic acid residue) is unstable and has to remain bound to an auxiliary enzyme, coenzyme A, like a crutch, so that it does not break down prematurely. The so-called acetyl-CoA is created, the main substrate of the citric acid cycle.

In the citric acid cycle, acetyl-CoA is bound to oxaloacetate (a C4 body), resulting in citrate, a C6 body. The coenzyme A migrates ’back’ in order to couple to the next acetyl group from the oxidative decarboxylation. Citrate is converted to isocitrate (enzyme: aconitase) by rearrangement of water, then to alpha-ketoglutarate, also called 2-oxoglutarate (enzyme: isocitrate dehydrogenase). In turn, CO2 free (so 2-oxoglutarate is a C5 body), here too NAD is reduced to NADH. Another decarboxylation (it becomes CO2 split off and a C4 body is formed) and electron transfer to NAD results in succinyl-CoA (enzyme: alpha-ketoglutarate dehydrogenase). Here, too, the resulting substrate is not viable on its own and must remain bound to coenzyme A. Succinyl-CoA synthetase produces succinate and guanosine triphosphate (GTP), analogous to ATP. In the next step, succinate is converted to fumarate by succinate dehydrogenase, and FAD is reduced to FADH. Malate (enzyme: fumarase) is finally produced from fumarate, and from this malate dehydrogenase ultimately becomes the starting material oxaloacetate. This in turn reduces NAD to NADH. This closes the circle.

The Overall balance of the citric acid cycle thus reads:

2 acetyl groups (CH3-CO-COOH) + 8 NAD + 2 FAD + 2 GDP + 2 P + 6 H2O -> 6 CO2 + 8 NADH + 2 FADH + 2 GTP

As mentioned above, reduction equivalents in the form of NADH and FADH, which are important in the following, are obtained here. The CO2 is superfluous and can be found in the breath of animals and plants.