PinkMonkey Online Study Guide-Biology
(i) Formation of 2-C Acetyl Co-A
This is the initial step in Phase
II of aerobic respiration. The reaction involves oxidation and decarboxylation
of 3-C pyruvic acid to form 2-C acetyl Co-A. It in the cell cytoplasm
and requires the presence of co-enzyme-A (Co-A) and NAD. It is catalyzed
by complex enzymes. In this process, carbon dioxide is released and the
reduced co-enzyme NADH2
The 2-C acetyl Co-A formed in the cytoplasm then
enters the mitochondria and takes part in the Krebís cycle reactions
(the TCA cycle).
Thus, acetyl Co-A is the connecting link between
glycolysis (in the cytoplasm) and Krebís cycle (in mitochondria).
(ii) T.C.A. cycle (Krebís cycle)
The Krebís cycle reactions occur in the matrix
of mitochondria. All enzymes required for these reactions are present
in the matrix. Each step is controlled by a specific enzyme and
The various steps in the T.C.A. Cycle are as follows:
(1) Each molecule of 2-C acetyl Co-A that enters
the T.C.A. cycle first combines with 4-C oxalo acetic acid (OAA),
and a 6-C Citric acid is formed. One molecule of H2O
is used in the reaction.
Thus, the first product in Krebís cycle is
citric acid, hence it is also called the citric acid cycle.
As citric acid is a tricarboxylic acid (with 3-COOH groups),
it is also called tricarboxylic acid (TCA) cycle.
(2) In the next step, 6-C citric acid is first
converted into 6-C -aconitic acid (deletion of H2O) and
then into 6-C isocitric acid (addition of H2O).
(3) The next reaction involves the oxidation
of isocitric acid (by removal of hydrogen) to form 6-C oxalo succinic
acid. NADH2 is formed in the process.
(4) Decarboxylation of 6-c
Oxalo succinic acid results in the formation of 5-c a
-Ketoglutaric acid with the liberation of carbon dioxide.
acid (5-C) then undergoes oxidation (by removal of hydrogen) and decarboxylation
to form 4-C succinyl Co-A. The reaction is highly complicated and takes
place in the presence of Co-A and NAD. NADH2 is formed and
carbon dioxide is released.