Question

Write the net reaction for the citrate cycle. Briefly describe the eight chemical reactions in the citrate cycle and include the names of the relevant enzymes.

Short answer

Question: Describe the net reaction for the citrate cycle and briefly mention the eight chemical reactions involved. Answer: The net reaction for the citrate cycle is: Acetyl-CoA + 3 NAD+ + Q + GDP + Pi + 2 H2O → CoA-SH + 3 NADH + 3 H+ + QH2 + GTP + 2 CO2 The eight chemical reactions that constitute a complete cycle of the citrate cycle are: 1. Formation of citrate (catalyzed by Citrate synthase) 2. Isomerization of citrate to isocitrate (catalyzed by Aconitase) 3. Oxidation of isocitrate to α-ketoglutarate (catalyzed by Isocitrate dehydrogenase) 4. Oxidation of α-ketoglutarate to succinyl-CoA (catalyzed by α-Ketoglutarate dehydrogenase) 5. Conversion of succinyl-CoA to succinate (catalyzed by Succinyl-CoA synthetase) 6. Oxidation of succinate to fumarate (catalyzed by Succinate dehydrogenase) 7. Hydration of fumarate to malate (catalyzed by Fumarase) 8. Oxidation of malate to oxaloacetate (catalyzed by Malate dehydrogenase)

Step-by-step solution

Write the net reaction for the citrate cycle

The net reaction for the citrate cycle is: Acetyl-CoA + 3 NAD^+ + Q + GDP + P_i + 2 H_2O → CoA-SH + 3 NADH + 3 H^+ + QH_2 + GTP + 2 CO_2 Here, Acetyl-CoA, NAD^+, Q, GDP (Guanosine diphosphate), P_i (inorganic phosphate), and H_2O are reactants, and CoA-SH, NADH, H^+, QH_2, GTP (Guanosine triphosphate), and CO_2 are the products.

List and briefly describe the eight chemical reactions in the citrate cycle

1. Formation of citrate: Enzyme: Citrate synthase Reaction: Acetyl-CoA + Oxaloacetate → Citryl-CoA → Citrate + CoA-SH Citrate synthase catalyzes the condensation of Acetyl-CoA with Oxaloacetate to form Citrate. 2. Isomerization of citrate to isocitrate: Enzyme: Aconitase Reaction: Citrate → cis-Aconitate → Isocitrate Aconitase catalyzes the reversible conversion of Citrate into Isocitrate. 3. Oxidation of isocitrate to α-ketoglutarate: Enzyme: Isocitrate dehydrogenase Reaction: Isocitrate + NAD^+ → α-Ketoglutarate + NADH + H^+ + CO_2 Isocitrate dehydrogenase oxidizes Isocitrate to α-Ketoglutarate, producing NADH and releasing CO_2. 4. Oxidation of α-ketoglutarate to succinyl-CoA: Enzyme: α-Ketoglutarate dehydrogenase Reaction: α-Ketoglutarate + NAD^+ + CoA-SH → Succinyl-CoA + NADH + H^+ + CO_2 α-Ketoglutarate dehydrogenase converts α-Ketoglutarate to Succinyl-CoA, generating NADH and releasing CO_2. 5. Conversion of succinyl-CoA to succinate: Enzyme: Succinyl-CoA synthetase Reaction: Succinyl-CoA + GDP + P_i → Succinate + GTP + CoA-SH Succinyl-CoA synthetase catalyzes the conversion of Succinyl-CoA into Succinate, generating GTP in the process. 6. Oxidation of succinate to fumarate: Enzyme: Succinate dehydrogenase Reaction: Succinate + Q → Fumarate + QH_2 Succinate dehydrogenase catalyzes the oxidation of Succinate to Fumarate, reducing Q to QH_2. 7. Hydration of fumarate to malate: Enzyme: Fumarase Reaction: Fumarate + H_2O → Malate Fumarase catalyzes the reversible hydration of Fumarate to Malate. 8. Oxidation of malate to oxaloacetate: Enzyme: Malate dehydrogenase Reaction: Malate + NAD^+ → Oxaloacetate + NADH + H^+ Malate dehydrogenase oxidizes Malate to regenerate Oxaloacetate, producing NADH. These eight reactions constitute a complete cycle of the citrate cycle. Each step is catalyzed by a specific enzyme, producing high-energy molecules (NADH, GTP, and QH_2) and releasing CO_2.

Key concepts

Metabolic Pathways

In the bustling cellular city, metabolic pathways are like intricate road systems designed for biochemical traffic. They constitute sequences of chemical reactions, each step finely tuned and interconnected, facilitating the transformation of substances within a cell. The citrate cycle, also known as the Krebs or tricarboxylic acid (TCA) cycle, is a primary metabolic highway where crucial cellular fuel, in the form of Acetyl-CoA, is oxidized to release energy.

Think of the citrate cycle as a circular road where Acetyl-CoA enters as a traveler, embarking on a journey through several 'stations' or enzymatic reactions. As it travels, it generates energy-rich compounds like NADH and GTP, along with carbon dioxide as an exhaust. The cycle is a pivotal hub that not only generates energy but also provides building blocks for other important metabolic pathways, such as amino acid and lipid synthesis. This makes it a cornerstone in the metabolic network within cells.

Enzyme Catalysis

Enzymes are the diligent workers of the cellular world, acting as catalysts that speed up biochemical reactions without being consumed in the process. Like a series of specialized tools, each enzyme in the citrate cycle is designed to facilitate a specific reaction, ensuring that the cycle runs smoothly and efficiently.

For instance, citrate synthase expertly fuses Acetyl-CoA and Oxaloacetate into citrate, while isocitrate dehydrogenase transforms isocitrate into α-ketoglutarate, shedding some 'weight' in the form of CO₂. Each enzyme acts on its substrate with precision, reducing the activation energy required and propelling the cycle forward. Understanding enzyme catalysis is key to grasping how the citrate cycle maintains its speed and efficiency to meet the energy demands of the cell.

Cellular Respiration

Cellular respiration is the process by which cells harvest energy from the food we consume. It's a multi-lane expressway that includes glycolysis, the citrate cycle, and the electron transport chain. The citrate cycle is a central junction in this process, where the breakdown products of carbohydrates, fats, and proteins converge.

During this stage, Acetyl-CoA, which contains energy extracted from food, enters the cycle and undergoes a series of reactions, ultimately leading to the production of CO₂, NADH, and GTP. This is akin to combusting fuel in an engine to power a vehicle. The NADH and QH₂ created here are then channeled to the electron transport chain, where they are further processed to drive the synthesis of a substantial amount of ATP, the energy currency of the cell, making the citrate cycle a vital link in cellular energy production.

Biochemical Reactions

Biochemical reactions are the individual responses that cells use to convert substances into different forms, crucial for maintaining life. Each of the eight steps in the citrate cycle represents a unique biochemical reaction, complete with its own enzyme to catalyze the transformation. These reactions range from simple hydration, where water adds to fumarate to form malate, to more complex redox reactions, where molecules like isocitrate are oxidized, transferring electrons to NAD⁺ to form NADH.

The elegance of the citrate cycle lies in its orchestration of these biochemical reactions to not only produce energy carriers like NADH and GTP but also to regenerate its starting molecule, oxaloacetate, ensuring the cycle can continue without interruption. This self-sustaining feature exemplified by the citrate cycle is what makes it a marvel of biochemical engineering.