Citric Acid Cycle
Below is a diagram of what happens in the mitochondria.
Today we learned about the "Citric Acid Cycle," sometimes the "Krebs cycle" and, more rarely, the "tricarboxylic acid" or TCA cycle.
The key points are:
- It takes place in the mitochondrial matrix
- It takes two-carbon units carried by the cofactor "coenzyme A," or CoA. The two-carbon units are "acetyl" units. So, this is called "acetyl-CoA."
- It is a cycle, meaning it regenerates the same starting material and can be run over and over provided there is more acetyl-CoA.
- One at a time, CO2 is released as a gas. This oxidation comes with reduction of NAD+ or FAD to the reduced (electron-carrying) form. These carry electrons to the ETS.
- There is one "substrate-level phosphorylation" generating a GTP, which can be used to make an ATP
- The cycle is also central to most of the synthesis of biomolecules in you. It is in the center of the Big Scary Chart.
- TCA cycle. Pyruvate, at the end of glycolysis, has a carboxyl at the end (that thing at the right with the resonant double bond). It gets removed as CO2, a “waste product.” That’s an oxidation and the thing that gets reduced is NAD, again, yielding NADH + H+. You get one for each of the pyruvates from glycolysis. The remaining acetyl is transferred to the cofactor “Coenzyme A” or “CoA” for short. We looked at that a little. Acetyl CoA will be an important source of building blocks for other things. Here, it is used to transfer the acetyl to a 4-carbon molecule called oxaloacetate to make Citrate, a tricarboxylic acid: with a total of 6 carbons. This is going to go through a series of steps that will regenerate oxaloacetate. A CO2 molecule will come off in both step three and four. Again, this is a net oxidation of the carbon. The oxidizing agent (the thing reduced) is NAD+, yielding NADH plus a free proton. Step five is another substrate level phosphorylation producing GTP. In the book, it looks like this immediately transfers its phosphate to ADP to make ATP. It can. But GTP is useful itself in many places. Keep that in mind. GTP will be a major player in regulating enzymatic pathways. Two more steps of import: reduction of FAD (Flavin adenine dinucleotide...the flavin (top left) is not technically a nucleotide...so the name is not quite right. ADP is the bottom right half). It works much like NAD. It gets reduced (gains electrons) and can spit them back out into Oxidative Phosphorylation. Finally, there is 1 last reduction of NAD+ to NADH + H+, regenerating oxaloacetate.