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+.