(6) 4-C Succinyl Co-A is hydrolyzed to succinic
acid (4-C) in the next step. One molecule of H2O is used
and Co-A is regenerated. The reaction is exergonic. Energy released
is used for the formation of GTP (guanosine triphosphate) from GDP
and inorganic phosphate.
Subsequently, ATP is formed when GTP reacts
with ADP
GTP
+ ADP ® ATP + GDP
(Thus, there is the direct formation of one
ATP when 5-C acid is converted to 4-C acid.)
(7) In the next step, 4-C Succinic acid is
oxidized (by removal of hydrogen) to 4-C fumaric acid in the presence
of co-enzyme FAD (flavin adenine dinucleotide). A reduced FADH2
is formed.
(8) Fumaric acid (4-C) is converted to another
4-C acid, malic acid, by the addition of H2O.
(9) In the final step of the TCA cycle, 4-C
Malic acid is oxidized (by removal of hydrogen) to 4-C Oxaloacetic
acid. NADH2 is formed in the process.
Thus, OAA is regenerated in the last step. It can
combine withanother 2-c Acetyl Co-A to form Citric acid and participate
in the TCA cycle again.
During various steps of the Kreb’s cycle, oxidation
of substrate takes place by the removal of an electron hydrogen
ion from substrate. It is accepted at each step by a suitable co-enzyme
such as NAD or FAD to form a reduced co-enzyme molecule, NADH2
or FADH2, respectively.
The chart of the Kreb’s cycle indicates the result
of the participation of only one molecule of pyruvic acid. It should
be noted that 2 pyruvic acid molecules are formed at the end of
glycolysis. Therefore, the final analysis of the Kreb’s cycle involving
2 pyruvic acids may be summarized as follows.
(i) Total number of reduced co-enzyme molecules
formed = 8#NADH2 and 2#FADH2
(ii) Total number of ATP formed directly (when
5 - C acid is converted to 4 - C acid) = 2#ATP
(iii) Total number of CO2
molecules released = 6#CO2
(iv) Total number of H2O
molecules utilized = 6#H2O
