Question

Identify \(\mathrm{A}\) and \(\mathrm{B}\) in the given reaction. Pyruvic acid \(+\mathrm{COA}+\mathrm{NAD}^{+} \frac{\mathrm{Mg}^{2+}}{\text { Pyruvate deydrogenase }}\) \(\mathrm{A}+\mathrm{B}+\mathrm{NADH}+\mathrm{H}^{+}\) A B (a) PEP \(\mathrm{CO}_{2}\) (b) Acetyl CoA \(\mathrm{CO}_{2}\) (c) \(\mathrm{CO}_{2}\). \(\mathrm{H}_{2} \mathrm{O}\) (d) Acetyl CoA \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

The correct option is (b) Acetyl CoA and CO2.

Step-by-step solution

Understanding the Reaction

Recognize that the given reaction describes the conversion of pyruvic acid to another molecule with the aid of coenzyme A (CoA), NAD+, and the enzyme pyruvate dehydrogenase in presence of Mg2+ ion. Acetyl CoA is formed, which will then enter the citric acid cycle, and NAD+ is reduced to NADH.

Identify the Products

Based on knowledge of cellular respiration and metabolic pathways, know that pyruvic acid is converted to Acetyl CoA and CO2 is released in the process. Also, NAD+ is reduced to NADH + H+.

Match the Products to the Answer Choices

Looking at the choices given, Acetyl CoA is a product which is mentioned in options (b) and (d). However, since CO2 is also released during this reaction, the correct set of products is Acetyl CoA and CO2, which is option (b).

Key concepts

Acetyl CoA Formation

When we delve into the metabolic journey of sugar breakdown, a pivotal player emerges: acetyl Coenzyme A, or Acetyl CoA. This molecule is the linchpin between glycolysis, which breaks down glucose to pyruvic acid, and the citric acid cycle, where the real energy harvest begins.

Pyruvic acid, having been produced in glycolysis, enters the mitochondrion where it undergoes a transformational process. This process is facilitated by a complex enzyme called pyruvate dehydrogenase, which requires the presence of cofactors such as NAD+ and the coenzyme A (CoA). In this enzymatic concert, pyruvic acid donates a carbon dioxide molecule (decarboxylation) and attaches to CoA to form Acetyl CoA—consider it a molecular makeover of sorts. The crucial Mg2+ ions act as cofactors, ensuring the reaction proceeds smoothly.

This conversion is not just a simple change; it's akin to a relay race handoff where pyruvic acid passes the baton of potential energy to Acetyl CoA. This energetic molecule is now primed to enter the citric acid cycle, marking a significant milestone in cellular respiration.

Citric Acid Cycle

Also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, the citric acid cycle is a series of chemical reactions used by all aerobic organisms to generate energy. It's an eight-step odyssey that takes place within the mitochondria and represents the central hub in the wheel of metabolism.

Each spin of the cycle begins with Acetyl CoA donating its acetyl group to oxaloacetate, producing citrate. From there, the cycle churns through a sequence of enzyme-mediated transformations. These steps whittle away carbon atoms, in the form of carbon dioxide, and capture high-energy electrons in the form of NADH and FADH2. ATP or GTP (depending on the cell type) is also synthesized, providing immediate energy to the cell.

Amidst these developments, water is consumed and proton (H+) gradients are established—fundamental for ATP production later on in oxidative phosphorylation. It's essential to grasp that the citric acid cycle is not just about energy production; it's also a wellspring of biosynthetic precursors for amino acids, nucleotides, and other vital compounds.

NADH Production

NADH production stands out as one of the marquee events in cellular respiration. NAD+, a coenzyme or 'helper' molecule, operates much like a shuttle service for electrons during metabolic reactions. In the process described in our exercise, NAD+ gets its passenger, in the form of high-energy electrons, from pyruvic acid when it is converted into Acetyl CoA.

The transfer of electrons from pyruvic acid to NAD+ reduces it to NADH. This is not merely a change in acronym letters; NADH is now a carrier of potential energy, ready to be cashed in during the electron transport chain. It's a currency of energy that'll eventually contribute to synthesizing ATP, which is the energy coin of the realm in the cell.

In the grand tapestry of metabolism, each NADH molecule is akin to a drop of high-octane fuel. During oxidative phosphorylation, these drops will fall on the turbine of ATP synthase, leading to the production of ATP. The guide in this metabolic journey—NAD+—ensures our cellular engines keep running efficiently by continually cycling between NAD+ and NADH, fulfilling a pivotal role in our existence.