We are now ready to begin going through the reactions of the citric acid cycle. The cycle begins with the reaction between acetyl-CoA and the four-carbon oxaloacetate to form six-carbon citric acid. Through the next steps of the cycle, two of the six carbons of the citric acid leave as carbon dioxide to ultimately yield the four carbon product, oxaloacetate, which is used again in the first step of the next cycle. During the eight reactions that take place, for every molecule of acetyl-CoA the cycle produces three NADH and one flavin adenine dinucleotide (FAD/FADH2), along with one molecule of ATP.
The first reaction of the citric acid cycle is catalyzed by the enzyme citrate synthase. In this step, oxaloacetate is joined with acetyl-CoA to form citric acid. Once the two molecules are joined, a water molecule attacks the acetyl leading to the release of coenzyme A from the complex.
The next reaction of the citric acid cycle is catalyzed by the enzyme acontinase. In this reaction, a water molecule is removed from the citric acid and then put back on in another location. The overall effect of this conversion is that the OH group is moved from the 3' to the 4' position on the molecule. This transformation yields the molecule isocitrate.
Two events occur in reaction 3 of the citric acid cycle. In the first reaction, we see our first generation of NADH from NAD. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the OH group at the 4' position of isocitrate to yield an intermediate which then has a carbon dioxide molecule removed from it to yield alpha-ketoglutarate.
In reaction 4 of the citric acid cycle, alpha-ketoglutarate loses a carbon dioxide molecule and coenzyme A is added in its place. The decarboxylation occurs with the help of NAD, which is converted to NADH. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase. The mechanism of this conversion is very similar to what occurs in the first few steps of pyruvate metabolism. The resulting molecule is called succinyl-CoA.
The enzyme succinyl-CoA synthetase catalyzes the fifth reaction of the citric acid cycle. In this step a molecule of guanosine triphosphate (GTP) is synthesized. GTP is a molecule that is very similar in its structure and energetic properties to ATP and can be used in cells in much the same way. GTP synthesis occurs with the addition of a free phosphate group to a GDP molecule (similar to ATP synthesis from ADP). In this reaction, a free phosphate group first attacks the succinyl-CoA molecule releasing the CoA. After the phosphate is attached to the molecule, it is transferred to the GDP to form GTP. The resulting product is the molecule succinate.
The enzyme succinate dehydrogenase catalyzes the removal of two hydrogens from succinate in the sixth reaction of the citric acid cycle. In the reaction, a molecule of FAD, a coenzyme similar to NAD, is reduced to FADH2 as it takes the hydrogens from succinate. The product of this reaction is fumarate.
FAD, like NAD, is the oxidized form while FADH2 is the reduced form. Although FAD and NAD perform the same oxidative and reductive roles in reactions, FAD and NAD work on different classes of molecules. FAD oxidizes carbon-carbon double and triple bonds while NAD oxidizes mostly carbon-oxygen bonds.
In this reaction, the enzyme fumarase catalyzes the addition of a water molecule to the fumarate in the form of an OH group to yield the molecule L- malate.
In the final reaction of the citric acid cycle, we regenerate oxaloacetate by oxidizing Lmalate with a molecule of NAD to produce NADH.
We have now concluded our discussion of the reactions that compose the citric acid cycle. It is helpful at this point to take a minute to take stock of what the citric acid cycle has generated from one acetyl-CoA molecule.