The reaction that follows is catalyzed by isocitrate dehydrogenase and occurs in two steps, the first of which (step A) is formation of an unstable intermediates (shown in brackets).

b. In which step is CO₂ evolved and a hydrogen ion added?

Verified step by step guidance

1. Step 1: Understand the reaction context. The reaction involves isocitrate dehydrogenase, an enzyme that catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate in the citric acid cycle. This reaction occurs in two steps: (A) formation of an unstable intermediate and (B) subsequent decarboxylation.
2. Step 2: Analyze the reaction mechanism. In step A, isocitrate undergoes oxidation, facilitated by the enzyme, to form an unstable intermediate. This step does not involve the release of CO₂ or the addition of a hydrogen ion.
3. Step 3: Focus on step B. In this step, the unstable intermediate undergoes decarboxylation, which means a carbon dioxide (CO₂) molecule is released. Additionally, a hydrogen ion (H⁺) is added to the reaction system as part of the process.
4. Step 4: Identify the role of cofactors. During this reaction, NAD⁺ (or NADP⁺ in some cases) acts as an electron acceptor, becoming reduced to NADH (or NADPH) as it gains electrons and a proton (H⁺). This is a key part of the reaction mechanism.
5. Step 5: Conclude that CO₂ evolution and H⁺ addition occur in step B, which is the decarboxylation step. This step is critical for the progression of the citric acid cycle and energy production in cellular metabolism.

Key Concepts

Isocitrate Dehydrogenase

Isocitrate dehydrogenase is an enzyme that plays a crucial role in the citric acid cycle (Krebs cycle) by catalyzing the oxidative decarboxylation of isocitrate to alpha-ketoglutarate. This reaction is significant for energy production in cellular respiration, as it facilitates the conversion of isocitrate into a more energy-rich compound while releasing carbon dioxide (CO₂) as a byproduct.

Decarboxylation

Decarboxylation is a chemical reaction that involves the removal of a carboxyl group (-COOH) from a molecule, resulting in the release of carbon dioxide (CO₂). In the context of the citric acid cycle, this process is essential for transforming substrates into more reactive forms, contributing to the overall energy yield of cellular metabolism.

Hydrogen Ion Addition

The addition of a hydrogen ion (H⁺) during biochemical reactions often indicates a reduction process, where a substrate gains electrons. In the context of the reaction catalyzed by isocitrate dehydrogenase, the addition of H⁺ can be associated with the conversion of isocitrate to alpha-ketoglutarate, highlighting the importance of electron transfer in metabolic pathways.