Question

Which of the following processes has the following net reaction? $$\begin{array}{c} 2 \text { acety } \mid-C_{O A}+6 N A D^{+}+2 F A D+2 G D P+2 P_{i}+6 H_{2} O \rightarrow 4 C O_{2}+6 \\ N A D H+2 F A D H_{2}+2 G T P+6 H^{+}+2 C O A-S H \end{array}$$ (A) Pyruvate decarboxylation (B) Fermentation (C) Tricarboxylic acid cycle (D) Electron transport chain

Short answer

Tricarboxylic acid cycle

Step-by-step solution

Identify the given net reaction

The given net reaction involves acetic acid in combination with various reactants like NAD+, FAD, GDP, and more to produce products like CO2, NADH, FADH2, and GTP. The reaction suggests a complex biochemical transformation.

Understand the components of the reaction

The presence of NAD+, FAD, and production of NADH, FADH2, and GTP are significant hints. Analyze what each component generally does in metabolic processes. NAD+ and FAD are common electron carriers that are involved in oxidation-reduction reactions.

Match the reaction with known metabolic processes

Compare the reaction with known biochemical pathways: Pyruvate decarboxylation (a), fermentation (b), the tricarboxylic acid cycle (c), and the electron transport chain (d). Note that TCA cycle (c) involves the oxidation of acetyl-CoA to CO2 and the reduction of NAD+ to NADH and FAD to FADH2, which matches our given reaction.

Finalize the identification

Given the complex nature of the reaction, producing CO2, NADH, FADH2, and GTP, the Tricarboxylic acid cycle (TCA cycle or Krebs cycle) is the most fitting. It efficiently describes all the reactants and products in the net reaction provided.

Key concepts

TCA Cycle

The Tricarboxylic Acid Cycle (TCA cycle), also known as the Krebs cycle or citric acid cycle, is a fundamental metabolic pathway that harnesses biochemical energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The TCA cycle occurs in the mitochondrial matrix of eukaryotic cells. The primary function of the TCA cycle is to generate high-energy electron carriers, NADH and FADH2, and to create GTP, which can be converted to ATP, the cell’s energy currency.

The cycle begins when acetyl-CoA combines with oxaloacetate to form citrate. Citrate then undergoes multiple transformations, reducing NAD+ to NADH and FAD to FADH2 while releasing CO2 as a byproduct. By the end of the cycle, the initial acetyl-CoA has been fully oxidized.

The TCA cycle is crucial because it links to many other metabolic pathways, ensuring a continuous supply of energy for cellular functions. Any disruption in the TCA cycle can significantly impact cellular respiration and energy levels.

Biochemical Pathways

Biochemical pathways are series of connected chemical reactions within a cell. Each pathway consists of multiple steps, each catalyzed by a specific enzyme, leading to the transformation of molecules within the organism. The TCA cycle is an integral part of these pathways, interlinking with glycolysis and the electron transport chain.

For instance, glycolysis breaks down glucose into pyruvate, which is then converted to acetyl-CoA before entering the TCA cycle. The electron carriers produced in the TCA cycle (NADH and FADH2) proceed to the electron transport chain, resulting in ATP production.

Thus, the TCA cycle is part of a larger network that maintains cellular energy homeostasis. It demonstrates how different biochemical pathways are interconnected to sustain life processes.

NADH and FADH2 Production

In the TCA cycle, NADH and FADH2 are essential molecules produced during the oxidation of acetyl-CoA. These molecules are crucial in cellular respiration because they act as electron carriers.

At specific steps in the TCA cycle, electrons are transferred to NAD+ to form NADH and to FAD to form FADH2. Each NADH molecule typically results from the extraction of two hydrogen atoms, while each FADH2 molecule captures electrons and protons.

NADH and FADH2 then donate these electrons to the electron transport chain, located in the inner mitochondrial membrane. This process creates a proton gradient that drives ATP synthesis, making NADH and FADH2 key components in the production of cellular energy.

Acetyl-CoA Oxidation

Acetyl-CoA oxidation is the starting point of the TCA cycle. Acetyl-CoA, formed from carbohydrates, fats, and proteins, enters the cycle by combining with oxaloacetate to form citrate.

This combination marks the initiation of the complete oxidation of acetyl-CoA, where its carbon atoms are gradually released as carbon dioxide. The energy from this oxidation is then extracted and stored in the form of NADH, FADH2, and GTP.

Oxidation of acetyl-CoA is vital for meeting the energy requirements of the cell, as it integrates the breakdown products of various nutrients into one central metabolic pathway. This process not only provides high-energy molecules but also supplies intermediates for other metabolic pathways, highlighting its multifaceted role in cellular metabolism.