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The citric acid cycle

Dr. Meera Kaur

The citric acid cycle

• The citric acid cycle is also known as tricarboxylic acid (TCA) cycle, or the Krebs cycle (named after its discoverer).

• It takes place in the mitochondria of the eucaryotic cells.

• All the reactions of TCA cycle are catalyzed by enzymes present in the mitochondrial inner membrane or enzymes attached to the inner surface of the interior membrane.

Reactions of the citric acid cycle

The TCA cycle begins at the 12o’clock position

1. Acetyl CoA and oxaloacatate combine to form citrate.

2. Citric acid eventually loses two carbon atoms as carbon dioxide.

3. At the end of the pathway, a molecule of oxaloacetate remains, which is why the pathway is called cycle.

4. Each turn of TCA cycle yields three molecules of NADH, one of FADH2, and one of ATP.

5. Two turn of citric acid cycle are necessary to oxidize completelytwo molecule of acetyl CoA (obtained from one molecule of glucose).

6. The complete oxidation of one molecule of glucose yields 38 ATP.

The citric acid cycle is an energy-generating catalytic cycle

Tricarboxylic cycle

Citric acid cycle intermediates as biosynthetic precursors

Anaplerotic reactions of the citric acid cycle

Oxidative phosphorylation

• Oxidative phosphorylation is the 3rd stage of cellular respiration. In eucaryots, it takes place in the mitochondria.

• ATP synthesis is driven by electron transfer to oxygen.

• It involves the reduction of oxygen to water with electron donated by NADH and FADH2 and occurs equally well in light or darkness

• The conservation of free energy involves the passage of electron through a chain of membrane bound oxidation-reduction (redox) carriers and pumping of proton across the membrane, producing an electrochemical gradients, the proton-motive forces.

• This force drives the synthesis of ATP by membrane bound enzyme complexes through which protons flow back across the membrane.