The Citric Acid Cycle involves eight chemical reactions, here we will look at the last four of these reactions. Clicking on each of the
thumbnail images will bring up a larger, labeled version of the
see the Flash movie for the following sequence of images,
In the fifth stage of the citric acid cycle, an enzyme called succinyl-CoA synthetase catalyzes a reaction between GDP, Pi, and succinyl-CoA.
This reaction results in the production of CoA, succinate, and one molecule of GTP. The GTP will later be converted into ATP.
The sixth stage of the cycle is catalyzed by a succinate dehydrogenase enzyme and requires succinate and FADH as substrates. This ezyme is embedded within the inner membrane of the mitochondrion.
The resulting products of this oxidation reaction are fumarate and one molecule of FADH2. The FADH2 can be used later to create more ATP for the cell.
In the seventh stage of the cycle, a fumarate hydratase enzyme catalyzes a reaction between water and fumarate.
This reaction results in the formation of malate.
In the eigth and final stage of the citric acid cycle, a malate dehydrogenase enzyme oxidizes NAD+ and malate.
This reaction regenerates the original oxaloacetate molecule from the first stage of the cycle, along with one NADH, and one H+ ion.
For every one molecule of glucose that enters glycolysis, two acetyl CoA molecules will be produced. These acetyl CoA molecules then enter the citric acid cycle.
Therefore, for each molecule of glucose that enters glycolysis, two turns of the citric acid cycle will occur. This means that the products of the citric acid cycle must be doubled.
In addition to sugars like glucose, proteins and fats can also enter the citric acid cycle at various stages. Proteins can be broken down into intermediates that can enter the cycle, and fatty acids can be broken down into acetyl CoA, which begins the citric acid cycle.