The citric acid cycle is a closed loop; the
last part of the pathway reforms the molecule used in the first step. In the first step of
the citric acid cycle, acetyl CoA joins with a four-carbon molecule, oxaloacetate,
releasing the CoA group and forming a six-carbon molecule called citrate.
Enzyme citrate synthase helps in proceeding of this step. Citrate further
converted to isocitrate with the help of enzyme aconitase. In this step first
there is removal of water molecule followed by addition of water molecule. isocitrate
is oxidized and releases a molecule of carbon dioxide, leaving behind a
five-carbon molecule—?-ketoglutarate. Also NAD+ is reduced to form NADH. The enzyme catalyzing this is isocitrate dehydrogenase, is important in regulating the speed of the citric acid
cycle. ?-ketoglutarate that’s oxidized, reducing NAD+ to NADH and releasing a molecule of carbon dioxide in the
process. The remaining four-carbon molecule picks up Coenzyme A, forming the
unstable compound succinyl CoA. The enzyme catalyzing this step, ?-ketoglutarate dehydrogenase, is also important
in regulation of the citric acid cycle. the CoA
succinyl CoA is
replaced by a phosphate group, which is then transferred to ADP to
make ATP.
The four-carbon molecule produced in this step is called succinate.
Enzyme succinate thiokinase play important role in this part of the reaction by
making it easy for succinyl CoA to release CoA. Succinate is oxidized, forming
another four-carbon molecule called fumarate. In this reaction, two hydrogen
atoms—with their electrons—are transferred to FAD
producing FADH2.
The enzyme succinate dehydrogenase that carries out this step is embedded in
the inner membrane of the mitochondrion, so FADH2 can
transfer its electrons directly into the electron transport chain. Water
is added to the fumarate molecule in the presence of fumrase, converting it
into another four-carbon molecule called malate. In the last step of the
citric acid cycle, oxaloacetate the starting four-carbon compound is
regenerated by oxidation of malate with the help of maltase hydrogenase enzyme.
Another molecule of NAD+ is
reduced to NADH
in the process.


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