In citric acid cycle, substrate level phosphorylation takes place in which of the following step ?

Question

Accepted Answer

The correct option is 3: Succinyl CoA → Succinate

The conversion of Succinyl CoA to Succinate is the sole reaction in the Citric Acid Cycle (TCA Cycle) where energy is captured directly in the form of a nucleoside triphosphate, a process known as **Substrate-Level Phosphorylation (SLP)**. This step is catalyzed by the enzyme **Succinyl-CoA Synthetase** (also known as Succinate Thiokinase). The high-energy thioester bond linking CoA to succinate is cleaved, releasing a significant amount of free energy. This liberated energy is harnessed directly by the enzyme to drive the synthesis of Guanosine Triphosphate (GTP) from Guanosine Diphosphate (GDP) and inorganic phosphate ($\text{P}*{i}$): $\text{Succinyl-CoA} + \text{GDP} + \text{P}*{i} \rightarrow \text{Succinate} + \text{GTP} + \text{CoA-SH}$. In some organisms, ATP is produced directly instead of GTP. Since GTP is energetically equivalent to Adenosine Triphosphate (ATP) and can be readily converted to ATP by the enzyme Nucleoside Diphosphate Kinase, this step represents the single direct ATP yield from one turn of the TCA cycle. Therefore, it is the classic and only example of substrate-level phosphorylation within the cycle during aerobic respiration.

The incorrect option is 1: Succinate → Fumarate

This step is an oxidation reaction catalyzed by **Succinate Dehydrogenase**, which is unique because it is the only TCA enzyme embedded in the inner mitochondrial membrane and is simultaneously **Complex II** of the Electron Transport Chain. The reaction oxidizes Succinate to Fumarate by transferring electrons and protons to the coenzyme **FAD**, reducing it to $\text{FADH}*{2}$. The $\text{FADH}*{2}$ produced is a high-energy electron carrier, and its energy will be used later to produce ATP through the Electron Transport Chain and **Oxidative Phosphorylation**. Because the energy released is used to reduce a coenzyme for later use rather than directly generating a nucleoside triphosphate, it is not a substrate-level phosphorylation step.

The incorrect option is 2: Malate → Oxaloacetate

This is the final step of the Citric Acid Cycle, a critical oxidation reaction catalyzed by the enzyme **Malate Dehydrogenase**. In this reaction, Malate is oxidized to regenerate the starting molecule, Oxaloacetate, a process that simultaneously reduces the electron carrier $\text{NAD}^{+}$ to **NADH**. The $\text{NADH}$ is a major reducing equivalent that carries electrons to the Electron Transport Chain, where its energy will be used to generate a significant amount of ATP via **Oxidative Phosphorylation**. As with most other oxidative steps in the cycle, the energy is not captured by directly transferring a phosphate group from the substrate to ADP/GDP; hence, it does not qualify as substrate-level phosphorylation.

The incorrect option is 4: Isocitrate $ightarrow$ $\alpha$-Ketoglutarate

This reaction is the first of two oxidative decarboxylation steps in the TCA cycle, catalyzed by **Isocitrate Dehydrogenase**. The reaction results in the formation of $\alpha$-Ketoglutarate and the release of the first molecule of **$\text{CO}\_{2}$** from the original Acetyl-CoA. Crucially, this step is a major checkpoint for the cycle and a powerful source of reducing power, as the enzyme reduces $\text{NAD}^{+}$ to **NADH**. The energy from the Isocitrate is conserved in the form of $\text{NADH}$, making it a step for **indirect** ATP synthesis via the Electron Transport Chain, not for the direct synthesis of ATP/GTP via substrate-level phosphorylation.