TCA Cycle
The
substrate for the tricarboxylic acid (TCA) cycle, citric acid cycle, or Krebs
cycle is acetyl-CoA. This cycle is
termed the citric acid cycle as the first metabolic intermediate formed in the
cycle is citric acid. The citric acid
cycle is the final common pathway for the oxidation of all biomolecules;
proteins, fatty acids, carbohydrates. Molecules from other cycles and pathways
enter this cycle through Acetyl CoA. The citric acid cycle in eukaryotes takes
place in the mitochondria while in prokaryotes, it takes place in the
cytoplasm.
The
multienzyme system called the pyruvate dehydrogenase complex 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 high energy thiol ester bond. Acetyl-CoA arises from the catabolism of many
carbohydrates, lipids, and amino acids.
Lehninger
Principles of Biochemistry
The
overall reaction/ equation of the citric acid cycle is:
Acetyl
CoA + 3 NAD+ + 1 FAD + 1 ADP + 1 Pi
→ 2 CO2 + 3 NADH + 3 H+ + 1 FADH2 +
1 ATP
In
the first reaction acetyl-CoA is condensed with a four-carbon intermediate,
oxaloacetate, to form citrate and to begin the six-carbon stage. Citrate is
rearranged to give isocitrate, a more readily oxidized secondary alcohol. Isocitrate is subsequently oxidized and
decarboxylated twice to yield α ketoglutarate, then succinyl-CoA. At this point
two NADHs are formed and two carbons are lost from the cycle as CO2.
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.
1.
Condensation
of acetyl CoA with oxaloacetate - The
citric acid cycle begins with acetyl CoA condensing with oxaloacetate (OAA), a
four-carbon compound. This reaction, catalyzed by citrate synthase, forms the
six-carbon compound citric acid and releases coenzyme A.
2.
Isomerization
of citrate into isocitrate - Citrate
is reversibly converted into its isomer, isocitrate, a reaction catalyzed by
the enzyme aconitase. This isomerization occurs in two steps: a dehydration of
citrate to cis-aconitate, followed by a rehydration to isocitrate.
3. Oxidative decarboxylations of isocitrate - In the first oxidation-reduction step,
isocitrate is irreversibly oxidized and decarboxylated by isocitrate
dehydrogenase to form the five-carbon compound, α-ketoglutarate. This two-step
process yields a molecule of NADH and releases CO2.
4. Oxidative decarboxylation of α-ketoglutarate -
α-ketoglutarate undergoes a second
irreversible oxidative decarboxylation, similar to that of pyruvate, to form
the four-carbon compound succinyl-CoA and CO2. The enzyme complex α-ketoglutarate
dehydrogenase catalyzes this reaction, reducing NAD+ to NADH.
5. Conversion of succinyl-CoA into succinate - Succinyl-CoA is cleaved to form succinate in
an energy-conserving reaction catalyzed by succinyl-CoA synthase. The energy
released is used to phosphorylate GDP to GTP, which readily transfers its
phosphate to ADP to form ATP.
6. Dehydration of succinate to fumarate - Succinate is dehydrogenated to fumarate by the
enzyme succinate dehydrogenase, which is found in the intramitochondrial space.
This unique oxidation step uses FAD as the electron acceptor, forming FADH2 ,
which then enters the electron transport chain.
7. Hydration of fumarate to malate - Fumarate is reversibly hydrated to form
L-malate by the enzyme fumarate hydratase. Conversely, the reverse reaction,
dehydration, forms fumarate from L-malate.
8.
Dehydrogenation
of L-malate to oxaloacetate - The
final step is the oxidation of L-malate to oxaloacetate by L-malate
dehydrogenase. This reaction is reversible and produces the third molecule of
NADH, completing the cycle for another round of acetyl CoA metabolism.
Reactions
of the citric acid cycle. Image Source: Lehninger Principles of Biochemistry.
Importance
of the Krebs Cycle
The
cycle serves two major, interconnected roles:
- Energy Production (Catabolism):
The cycle's main catabolic role is generating the majority of the reduced
coenzymes (NADH and FADH2) used for subsequent ATP synthesis. For each
turn, it produces three NADH, one FADH2, and one GTP (which is quickly
converted to ATP), effectively capturing the chemical energy to power the electron
transport chain (oxidative phosphorylation), which yields the bulk of the
cellular ATP.
- Biosynthesis (Anabolism):
The cycle is an amphibolic (both catabolic and anabolic) pathway, as its
intermediates are crucial precursors for synthesizing other vital cellular
components. For instance, α-ketoglutarate and oxaloacetate are used to
make several amino acids, while citrate is a precursor for fatty acid and cholesterol
synthesis.
TCA
cycle generates two CO2, three NADH, one FADH2, and one GTP for each
acetyl-CoA molecule oxidized.
The
cycle's operation is strictly dependent on the presence of oxygen, making it an
essential pathway for all aerobic organisms to sustain life and perform
cellular functions. 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.
