Citric Acid Cycle Practice 3: Videos & Practice Problems

The key intermediates of the citric acid cycle include oxaloacetate, citrate, α-ketoglutarate, succinyl CoA, fumarate, and malate. Oxaloacetate is crucial for anaplerotic reactions that replenish cycle components. Citrate is involved in fatty acid synthesis. α-Ketoglutarate is essential for amino acid production. Succinyl CoA contributes to the synthesis of porphyrins and hemes. Fumarate and malate are involved in the regeneration of oxaloacetate. These intermediates play vital roles in both anabolic and catabolic pathways, highlighting the interconnectedness of cellular metabolism.

The isocitrate dehydrogenase reaction is a crucial step in the citric acid cycle. It catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate, producing NADH and releasing CO₂. The reaction can be summarized as:

${\mathrm{isocitrate}}_{}+{\mathrm{NAD}}_{}+H_{}\to {\alpha }_{}-{\mathrm{ketoglutarate}}_{}+{\mathrm{CO}}_{}2+{\mathrm{NADH}}_{}$

This reaction is important for energy production and the regulation of the cycle, as NADH is a key electron carrier in cellular respiration.

Anaplerotic reactions are metabolic pathways that replenish intermediates of the citric acid cycle. These reactions are crucial because intermediates are often diverted for biosynthetic processes, such as amino acid and nucleotide synthesis. For example, pyruvate carboxylase converts pyruvate to oxaloacetate, ensuring a continuous supply of this key intermediate. The importance of anaplerotic reactions lies in maintaining the cycle's functionality, which is essential for energy production and various biosynthetic pathways.

Succinyl CoA is a key intermediate in the citric acid cycle that contributes to the synthesis of hemes and porphyrins. It combines with glycine to form δ-aminolevulinic acid (ALA), the first step in the biosynthesis of porphyrins. These porphyrins are then converted into hemes, which are essential components of hemoglobin, myoglobin, and various cytochromes. This pathway highlights the interconnectedness of the citric acid cycle with other vital biochemical processes.

α-Ketoglutarate is a crucial intermediate in the citric acid cycle that plays a significant role in amino acid production. It serves as a precursor for the synthesis of glutamate through the action of glutamate dehydrogenase. Glutamate can then be used to synthesize other amino acids via transamination reactions. This process is vital for protein synthesis and various metabolic functions, demonstrating the importance of α-ketoglutarate in cellular metabolism.