Citric Acid Cycle
The citric acid cycle (sometimes called the Krebs cycle) occurs in the mitochondrial matrix and is the third stage in the aerobic breakdown of glucose. The first, of course, is glycolysis, which creates pyruvate, NADH, and ATP. The second—which isn’t long enough to get its own post—is the linking reaction in which pyruvate is converted to Acetyl CoA. This is a coenzyme that the citric acid cycle breaks down to use later in energy production. Basically, the purpose of the linking reaction is to make pyruvate into something the cycle can use.
The main goal of the citric acid cycle is to convert bond energy (in the form of Acetyl CoA) into its reducing equivalents: i.e., to make some more NADH and FADH2, which are electron carriers. These then go through the electron transport chain and use their electron energy to create ATP. Remember, to reduce a compound is to add electrons to it—think of the mnemonic OILRIG.
So, how does the citric acid cycle do this?
Some diagrams get pretty complicated, especially when you include the enzymes responsible and the carbon compounds formed at every stage, but I’m going to break it into relatively simple steps.
- An enzyme joins acetyl-CoA to oxaloacetate in order to form citric acid, which is where the cycle gets its name. Then, a water molecule “attacks” the acetyl, and CoA is ejected from the cycle.
- Next, water is ejected and then put back in to help facilitate the reduction of NAD+ into NADH. For every turn of the cycle, 3 NADH molecules are created, and 2 molecules of CO2 are released.
- ADP plus a free phosphate group (denoted as “Pi”) is put into the cycle, and these are smushed together to form an ATP.
- Finally, FAD+ is reduced to FADH2. (FAD and NAD are both very similar coenzymes, performing the same oxidative and reductive roles in a reaction, but they’re different because they work on different classes of molecules: FAD oxidises carbon-carbon bonds, and NAD oxidises carbon-oxygen bonds)
A diagram might make it a little clearer:
So, let’s do a quick round-up of what’s happened:
- Acetyl-CoA has been released as two CO2 molecules
- 3 NAD+ were reduced to 3 NADH
- 1 FAD+ was reduced to 1 FADH2
- 1 ADP+Pi formed 1 ATP molecule
This isn’t the end—the main goal of citric acid cycle is to prepare the electron carriers NADH and FADH2 for the electron transport chain, where much more ATP will be made.
Onwards to the ETC!
Further resources: Khan Academy: Krebs Cycle