Tricarboxylic acid cycle reactions

It’s an eight-step procedure. Under aerobic conditions, the Krebs cycle or TCA cycle occurs in the matrix of mitochondria.

Step 1. Citrate formation 

• Coenzyme A is released in the first step, which is the condensation of acetyl CoA with the 4-carbon compound oxaloacetate to form 6C citrate. Citrate synthase catalyzes the reaction.
• Oxaloacetate catalyzes the citric acid cycle and is regenerated at the end of the process.

Step 2. Isomerization of citrate to Isocitrate 

• In the second step, citrate is converted to isocitrate, a citrate isomer. Citrate loses a water molecule and then gains one in this reaction to form isocitrate, which occurs in two steps.
• Aconitase is an enzyme that catalyzes the isomerization of citrate to isocitrate via the intermediate cis-aconitate. This reaction is reversible.

Step 3. αlpha-ketoglutarate Formation 

• In the second step, citrate is converted to isocitrate, a citrate isomer. Citrate loses a water molecule and then gains one in this reaction to form isocitrate, which occurs in two steps.
• Aconitase is an enzyme that catalyzes the isomerization of citrate to isocitrate via the intermediate cis-aconitate. This reaction is reversible.

Step 4. Succinyl-CoA Formation 

• alpha-ketoglutarate is oxidized here, reducing NAD+ to NADH and releasing a carbon dioxide molecule.
• The remaining four-carbon molecules pick up CoA, forming the unstable compound succinyl CoA. The entire process is catalyzed by a-ketoglutarate dehydrogenase.

Step 5. Succinate Formation 

• Succinate is formed by succinyl CoA. The reaction is catalyzed by the succinyl CoA synthase enzyme. Following this, GDP is phosphorylated at the substrate level to produce GTP. GTP phosphates ADP, which leads to the formation of ATP. In this step, a molecule of CO2 is released.

Step 6.Fumarate Formation 

• Succinate is converted to fumarate through oxidation. FADH2 is created by transferring two hydrogen atoms to FAD.
• Because the enzyme responsible for the reaction is embedded in the inner membrane of mitochondria, FADH2 transfers electrons directly to the electron transport chain.
• The reaction is reversible.

Step 7. Malate formation 

• The addition of one H2O converts fumarate to malate. Fumarase is the enzyme that catalyzes this reaction. This is a reversible hydration reaction.

Step 8. Formation and regeneration of oxaloacetate –

• Malate is dehydrogenated to produce oxaloacetate, which combines with another acetyl CoA molecule to initiate the new cycle. The hydrogens that are removed are transferred to NAD+, where they form NADH.
• The reaction is catalyzed by malate dehydrogenase.
• This step regenerates oxaloacetate, which then combines with acetyl CoA to complete the cycle.

Significance of tricarboxylic acid cycle 

• The tricarboxylic acid cycle is the final oxidation pathway for glucose, fats, and amino acids.
• Amino acids (protein metabolic products) are deaminated and converted to pyruvate and other Krebs cycle intermediates. On deamination, they enter the cycle and are metabolized, for example, alanine is converted to pyruvate, glutamate to -ketoglutarate, and aspartate to oxaloacetate.
• Fatty acids are -oxidized to produce acetyl CoA, which enters the Krebs cycle.
• Many animals rely on nutrients other than glucose as a source of energy.
• It is necessary for amino acid interconversion, gluconeogenesis, and lipogenesis.
• It is the most important source of ATP production in cells. After complete nutrient oxidation, a large amount of energy is produced.

Plants respire throughout their lives because the plant cell requires energy to survive; however, plants breathe in a unique way known as cellular respiration. Photosynthesis is the process by which plants generate glucose molecules by capturing and converting sunlight energy. Several live experiments demonstrate plant respiration. All plants respire in order to provide energy to their cells, allowing them to be active or alive. 

Plants require oxygen to respire, and the process emits carbon dioxide. However, plants do have stomata (found in leaves) and lenticels (found in stems) that are actively involved in gas exchange. Plants lack specialized structures for gas exchange, in contrast to people and other creatures. Plant leaves, stems, and roots respire at a slower rate than other parts of the plant.