TCA Cycle or Krebs cycle

 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 P_i    →   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.