Problem 85 - Objective Biology for NEET

Question

The formation of acetyl coenzyme-A from pyruvic acid is the result of its (a) Reduction (b) Dehydration (c) Dephosphorylation (d) Oxidative decarboxylation

Step-by-Step Solution

Answer

The formation of acetyl coenzyme-A from pyruvic acid is the result of its oxidative decarboxylation.

Step 1: Understanding the terms

Before making a choice, it is necessary to understand what each term means. When a compound is reduced, it gains electrons. Dehydration is a type of condensation reaction in which water is eliminated from the reacting molecule. Dephosphorylation is the removal of a phosphate group from an organic compound by hydrolysis. Oxidative decarboxylation is the process where a molecule is decarboxylated (lost its carboxyl group) and oxidized at the same time.

Step 2: Identifying the process

The process of forming acetyl coenzyme-A from pyruvic acid involves the removal of a carboxyl group from pyruvic acid, which then bonds with Coenzyme A. Simultaneously, this process is also an oxidation process, as NAD+ (an oxidizing agent) is reduced to NADH.

Step 3: Making the decision

Comparing this process to the terms defined in the first step, it's clear that oxidative decarboxylation is the matching synthetic pathway.

Key Concepts

Acetyl Coenzyme-A

Acetyl Coenzyme-A, often abbreviated as Acetyl-CoA, plays a central role in cellular metabolism. It acts as a key intermediary in several biochemical pathways. Its primary function is to deliver the acetyl group to the citric acid cycle, or Krebs cycle, which is crucial for energy production in cells.
This molecule is formed when pyruvic acid undergoes oxidative decarboxylation, where a carboxyl group is removed and Coenzyme A is added. This reaction is catalyzed by the enzyme complex called pyruvate dehydrogenase. The high energy bonds within Acetyl-CoA are essential for driving further biochemical reactions.

• Acts as a metabolic hub, linking glycolysis and the citric acid cycle.
• Its production is a gateway for the start of fatty acid synthesis.
• Involved in the synthesis of several other molecules like cholesterol and acetylcholine.

Acetyl Coenzyme-A is critical not just for energy production but also for the synthesis and metabolism of various compounds.

Pyruvic Acid

Pyruvic acid, or pyruvate, is a pivotal organic compound in cellular respiration. It is a key product of glycolysis, which is the process where glucose is broken down in the cell's cytoplasm.
At the end of glycolysis, one molecule of glucose yields two molecules of pyruvic acid. Pyruvic acid is then transported into the mitochondria, where it enters different pathways depending on the cell's oxygen availability.

• In aerobic conditions, pyruvic acid undergoes oxidative decarboxylation to form acetyl coenzyme-A.
• In anaerobic conditions, it can be converted into lactic acid or ethanol in certain organisms.

Pyruvic acid is vital for the continuation of the energy extraction processes in cells, providing a link between external glucose molecules and the internal energy production systems.

Cellular Respiration

Cellular respiration is a series of metabolic processes that occur within a cell, with the primary goal of converting biochemical energy from nutrients into adenosine triphosphate (ATP), and then releasing waste products.
The whole process consists of several stages including glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation.

• Glycolysis occurs in the cytoplasm and generates pyruvic acid.
• The citric acid cycle takes place in the mitochondria where acetyl coenzyme-A is further oxidized.
• Oxidative phosphorylation involves the electron transport chain and the production of a significant amount of ATP.

These processes can be aerobic (requiring oxygen) or anaerobic (occurring without oxygen), but aerobic respiration produces a much higher yield of ATP, thereby being the more efficient energy pathway.

NAD+ Reduction

NAD+ reduction is a critical process within cellular respiration. NAD+ (Nicotinamide adenine dinucleotide) is a coenzyme found in all living cells that acts as an electron carrier.
During oxidative decarboxylation, NAD+ is reduced to form NADH. This involves the gaining of electrons by NAD+, which is why it is called "reduction". NADH then carries the electrons to the electron transport chain in the mitochondria.

• NAD+ is crucial for accepting electrons during glycolysis and the citric acid cycle.
• NADH is responsible for transporting electrons and a proton (H+), contributing to the proton gradient used for ATP synthesis.
• The conversion of NAD+ to NADH is fundamental for maintaining the cell's redox balance.

Ultimately, NADH plays a vital role in the oxidative phosphorylation stage, helping to generate the energy currency of cells, ATP.