Question

Which of the following relation shows substrate level phosphorylation? (a) Citric acid \(\quad \rightarrow \alpha\) -ketoglutaric acid (b) Malic acid \(\rightarrow\) oxaloacetic acid (c) \(\alpha\) -ketoglutaric acid \(\rightarrow\) Succinyl-CoA (d) Succinyl-CoA \(\rightarrow\) Succinic acid

Short answer

The correct answer is (d) Succinyl-CoA \(\rightarrow\) Succinic acid. This reaction shows substrate-level phosphorylation.

Step-by-step solution

Identifying the substrate-level phosphorylation reactions

Substrate level phosphorylation occurs in the glycolysis and citric acid cycle phase of cellular respiration. The reactions where it occurs involve the direct transfer of a phosphoryl group on to ADP or GDP to form ATP or GTP.

Analyzing the given reactions

In option (a), citric acid does not phosphorylate ATP. Instead, it actually undergoes a number of transformations and releases two carbon atoms, being converted into α-ketoglutaric acid. As such, it does not represent a case of substrate level phosphorylation.\nIn option (b), malic acid is oxidized to yield oxaloacetic acid, again, no ATP is produced here.\nIn option (c), α-ketoglutaric acid is decarboxylated to become succinyl-CoA and in the process, one molecule of NAD+ is reduced to NADH. Though energy is released in this process, it isn't the direct cause for ATP synthesis, and therefore, doesn't represent substrate level phosphorylation.\nIn option (d), succinyl-CoA donates its CoA group to a molecule of GDP to form GTP (which can be readily converted to ATP) and becomes succinic acid. This is indeed a case of substrate level phosphorylation as a high energy bond is directly transferring its energy to form ATP.

Select the correct answer

From the analysis, only the reaction in option (d), Succinyl-CoA \(\rightarrow\) Succinic acid is a substrate-level phosphorylation as it directly involves the formation of ATP.

Key concepts

citric acid cycle

The citric acid cycle, also known as the Krebs cycle, is a crucial part of cellular respiration. It occurs in the mitochondria of cells and is involved in energy production. During this cycle, acetyl-CoA is introduced and combines with oxaloacetate to form citric acid. This initiates a series of reactions that release energy stored in the chemical bonds.

• Citric acid is broken down into \( \alpha \)-ketoglutarate, releasing carbon dioxide.
• Further reactions transform \( \alpha \)-ketoglutarate into succinyl-CoA.
• The process regenerates oxaloacetate, allowing the cycle to continue.

Throughout the cycle, important intermediates are generated, and reducing equivalents like NADH and FADH extsubscript{2} are produced, which are critical for the electron transport chain.
An interesting aspect of the citric acid cycle is the occurrence of substrate-level phosphorylation. Although it doesn't happen frequently in this cycle, it enables direct production of ATP (or GTP), turning chemical energy into a usable form for cellular functions.

cellular respiration

Cellular respiration is the process by which cells convert glucose and oxygen into energy, in the form of ATP, and release carbon dioxide and water as byproducts. This involves multiple stages, each crucial for efficient energy production.

• Glycolysis: The breakdown of glucose into pyruvate, producing a small amount of ATP and NADH.
• Citric Acid Cycle: Also known as the Krebs cycle, where acetyl-CoA is oxidized for further ATP production.
• Electron Transport Chain: Uses electrons from NADH and FADH extsubscript{2} to generate a large amount of ATP.

The electron transport chain also relies on the reduction and oxidation of molecules to facilitate a flow of electrons, which powers ATP synthase. This enzyme helps synthesize ATP from ADP and inorganic phosphate, showcasing the efficiency of cellular respiration in energy conversion.

ATP synthesis

ATP synthesis is the creation of adenosine triphosphate (ATP), an essential energy carrier in cells. During cellular respiration, various methods contribute to ATP production.

• Substrate-Level Phosphorylation: Occurs directly in the reactions of glycolysis and the citric acid cycle.
• Oxidative Phosphorylation: Takes place in the mitochondria during the electron transport chain.

In substrate-level phosphorylation, a phosphate group is directly transferred from a substrate molecule to ADP, forming ATP.
Oxidative phosphorylation involves ATP synthase, which uses the energy from a proton gradient created by the electron transport chain.
This two-step mechanism ensures cells have a stable supply of energy needed for maintaining cellular functions and supporting biological activities. The reaction involving succinyl-CoA in the citric acid cycle is a perfect example of substrate-level phosphorylation, highlighting direct ATP formation without an electron transport chain.