TCA Cycle - DMLT Final year

                                              It is also known as TriCarboxylic Acid (TCA) cycle. In prokaryotic cells, the citric acid cycle occurs in the cytoplasm; in eukaryotic cells, the citric acid cycle takes place in the matrix of the mitochondria. 

STEP 1: Formation of Citrate

The first reaction of the cycle is the condensation of acetyl-CoA with oxaloacetate to form citrate, catalysed by citrate synthase. 

STEP 2: Formation of Isocitrate

The citrate is rearranged to form an isomeric form, isocitrate by an enzyme acontinase.

In this reaction, a water molecule is removed from the citric acid and then put back on in another location.

STEP 3: Oxidation of Isocitrate to α-Ketoglutarate

In this step, isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to form α-ketoglutarate.

In the reaction, generation of NADH from NAD is seen.

STEP 4: Oxidation of α-Ketoglutarate to Succinyl-CoA

Alpha-ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added to form the 4-carbon compound succinyl-CoA.

During this oxidation, NAD+ is reduced to NADH + H+. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase.

STEP 5: Conversion of Succinyl-CoA to Succinate

CoA is removed from succinyl-CoA to produce succinate.

The energy released is used to make guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi by substrate-level phosphorylation.

GTP can then be used to make ATP. The enzyme succinyl-CoA synthase catalyzes this reaction of the citric acid cycle.

STEP 6: Oxidation of Succinate to Fumarate

Succinate is oxidized to fumarate.

During this oxidation, FAD is reduced to FADH2. The enzyme succinate dehydrogenase catalyzes the removal of two hydrogens from succinate.

STEP 7: Hydration of Fumarate to Malate

The reversible hydration of fumarate to L-malate is catalyzed by fumarase (fumarate hydratase).

Fumarase continues the rearrangement process by adding Hydrogen and Oxygenback into the substrate that had been previously removed.

STEP 8: Oxidation of Malate to Oxaloacetate

Malate is oxidized to produce oxaloacetate, the starting compound of the citric acid cycle by malate dehydrogenase. During this oxidation, NAD+ is reduced to NADH + H+.