The importance of the Krebs Cycle can not be overstated;
almost all of our ATP is produced in this cycle. We
can see two main problems as a result of glycolysis:
Remember that the body needs NAD+ molecules to act as electron carriers receiving the extra hydrogen atom and electrons from G3P to form NADH. Since the cell contains only a small supply of NAD+, there must be a way to recycle NAD+ from NADH to continue the shuttling of electrons. Sometimes the body reacts by immediately passing the hydrogen atom from NADH to an organic compound without the aid of an electron transport system. This process is called fermentation. However, fermentation occurs in an anaerobic environment and will only support the muscles for a short period of time before invoking lactic acid poisoning. Therefore, our bodies have evolved to "fix" the problems of our energy history. Unlike fermentation, the Krebs Cycle is able to gather the energy trapped in the NADH molecules and reproduce more NAD+. Also the conversion of NADH to NAD+ is coupled with the production of 24 ATPs.
This step consists of the combination of pyruvic acid and coenzyme A to form acetyl CoA. (This is described fully on the previous page)
Acetyl - CoA, consisting of two carbons, joins oxaloacetic acid, made up of four carbons, to form citric acid, a six-carbon compound.
One of citric acid's two recently acquired carbons is oxidized to form CO2. At the same time, a NADH is produced leaving ketoglutaric acid.
The last carbon remaining from pyruvic acid is removed in the form of CO2. Another NADH is formed, while the only ATP directly generated in the Krebs Cycle is produced. Left behind is succinic acid, a four-carbon molecule lacking any of pyruvic's original carbons
Two hydrogen atoms and their corresponding electrons are released from succinic acid and attach to FAD to form FADH2. The remaining molecule is malic acid.
The continuing oxidation of succinic acid releases another hydrogen atom and two electrons forming NADH. This completes one turn of the circle as oxaloacetic acid is reproduced and ready to pick up another Acetyl CoA and form citric acid.
The Krebs Cycle begins with oxaloacetate and combines with Acetyl CoA
to cycle through one complete turn. After Acetyl CoA is oxidized to CO2
and H2O, the electrons drive proton pumps that generate ATP that
is greatly needed by the cell. Remember that the NADH molecules are
important because they contain extracted electrons that ultimately reduce
NAD+. However, when the electrons do not have enough energy to
reduce NAD+, they are stored temporarily in the FADH2 molecule,
as in Step 5. Each NADH molecule is responsible for the production of three
ATP molecules while FADH2 is responsible for the production of two ATP
molecules. In the table below, the products of glycolysis, oxidation of
pyruvic acid, and the Krebs Cycle are summarized: