Understand the Mechanisms of Citric Acid Cycle

Pyruvate Enters the Mitochonrial Matrix and is Oxidized via The Citric Acid Cycle:

Citric Acid Cycle is also known as TCA (Tricarboxylic Acid Cycle) and Kreb’s Cycle. This cycle constitutes the second stage in respiration and takes place in the mitochondrial matrix. The two molecules of pyruvate generated during glycolysis in the cytosol is transported through the impermeable inner mitochondrial membrane via a specific transport protein. Once pyruvate reaches the mitochondrial matrix, pyruvate is decarboxylated in an oxidation reaction in the presence of an enzyme called pyruvate dehydrogenase. The products are NADH, CO₂ and acetyl-CoA, in which a thioester bond links the acetic acid to a sulfur containing cofactor, coenzyme A (CoA).

Note: All enzymes of the Citric Acid Cycle are present in the mitochondrial matrix except Succinate dehydrogenase which is present on the inner mitochondrial membrane.

Citric Acid Cycle produces 3NADH, 1ATP/GTP, and 1FADH₂

Citric Acid Cycle in Plants has got some unique features:

Plants TCA cycle is somewhat different from animals. For example, the step catalyzed by Succinyl-CoA synthetase produces ATP in plants (GTP in animals). Another significant feature of plant TCA cycle is the presence of cytosolic NAD⁺ malic enzyme, which has also been found in the mitochondrial matrix catalyzes the oxidative decarboxylation of malate:

The presence of NAD⁺ malic enzyme enables plant mitochondria to operate alternative pathways for the metabolism of PEP derived from glycolysis. As already described, malate can be synthesized from PEP in the cytosol via the enzymes PEP carboxylase and malate dehy- drogenase . Malate is then transported into the mitochondrial matrix, where NAD+ malic enzyme can oxidize it to pyruvate. This reaction makes possible the complete net oxidation of citric acid cycle intermediates such as malate or citrate (Oliver and McIntosh 1995).

Alternatively, the malate produced via the PEP car- boxylase can replace citric acid cycle intermediates used in biosynthesis. Reactions that can replenish intermediates in a metabolic cycle are known as anaplerotic. For example, export of 2-oxoglutarate for nitrogen assimilation in the chloroplast will cause a shortage of malate needed in the citrate synthase reaction. This malate can be replaced through the PEP carboxylase pathway. The presence of an alternative pathway for the oxidation of malate is consistent with the observation that many plants, in addition to those that carry out CAM metabolism, store significant levels of malate in their central vacuole.