Although some energy is obtained from the breakdown of glucose
to pyruvate by the pathways previously described, much
more is released when pyruvate is degraded aerobically to CO2 in
stage three of catabolism. The multienzyme system called the
pyruvate dehydrogenase complex first oxidizes pyruvate to form
CO2 and acetyl coenzyme A (acetyl-CoA), an energy-rich molecule
composed of coenzyme A and acetic acid joined by a highenergy
thiol ester bond . Acetyl-CoA arises from the
catabolism of many carbohydrates, lipids, and amino acids .
 It can be further degraded in the tricarboxylic acid cycle.
The substrate for the tricarboxylic acid (TCA) cycle, citric
acid cycle, or Krebs cycle is acetyl-CoA . The traditional way to think about the cycle is in terms of
its intermediates and products, and the chemistry involved in each
step. In the first reaction acetyl-CoA is condensed with a fourcarbon
intermediate, oxaloacetate, to form citrate and to begin the
six-carbon stage. Citrate (a tertiary alcohol) is rearranged to give
isocitrate, a more readily oxidized secondary alcohol. Isocitrate is subsequently oxidized and decarboxylated twice to yield -
ketoglutarate, then succinyl-CoA. At this point two NADHs are
formed and two carbons are lost from the cycle as CO2. Because
two carbons were added as acetyl-CoA at the start, balance is
maintained and no net carbon is lost. The cycle now enters the
four-carbon stage during which two oxidation steps yield one
FADH2 and one NADH per acetyl-CoA. In addition, GTP (a
high-energy molecule equivalent to ATP) is produced from
succinyl-CoA by substrate-level phosphorylation. Eventually oxaloacetate
is reformed and ready to join with another acetyl-CoA.
Inspection of figure 9.12 shows that the TCA cycle generates two
CO2s, three NADHs, one FADH2, and one GTP for each acetyl-
CoA molecule oxidized.
Another way to think of the TCA cycle is in terms of its function
as a pathway that oxidizes acetyl-CoA to CO2. From this perspective,
the first step is the attachment of an acetyl group to the
acetyl carrier, oxaloacetate, to form citrate. The second stage begins
with citrate and ends in the formation of succinyl-CoA.
Here, the acetyl carrier portion of citrate loses two carbons when it is oxidized to give two CO2s. The third and last stage converts succinyl-CoA back to oxaloacetate, the acetyl carrier, so that it
can pick up another acetyl group.
TCA cycle enzymes are widely distributed among microorganisms.
The complete cycle appears to be functional in many
aerobic bacteria, free-living protozoa, and most algae and fungi.
This is not surprising because the cycle is such an important
source of energy. However, the facultative anaerobe E. coli does
not use the full TCA cycle under anaerobic conditions or when
the glucose concentration is high but does at other times. Even
those microorganisms that lack the complete TCA cycle usually
have most of the cycle enzymes, because one of TCA cycle’s major

Fig : Kreb's Cycle

functions is to provide carbon skeletons for use in biosynthesis.


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