Question

The ol' two step plus one. The conversion of pyruvate into acetyl CoA consists of three steps. What are these steps?

Short answer

Decarboxylation, oxidation, and formation of acetyl CoA.

Step-by-step solution

Decarboxylation

The first step involves the removal of a carboxyl group from pyruvate as a molecule of carbon dioxide is released. This step is catalyzed by the enzyme pyruvate dehydrogenase. As pyruvate (a 3-carbon molecule) loses a carbon, it forms an acetyl group, which is a 2-carbon molecule.

Oxidation

In the second step, the remaining 2-carbon fragment is oxidized, transforming NAD extsuperscript{+} into NADH. This oxidation process helps in the removal of electrons which are captured by NAD extsuperscript{+}, forming NADH, a high-energy electron carrier used in other metabolic pathways.

Formation of Acetyl CoA

The final step involves the combination of the acetyl group with coenzyme A (CoA) to form acetyl CoA. This high-energy compound then enters the citric acid cycle (Krebs cycle) where it is further oxidized for energy production.

Key concepts

Decarboxylation

Decarboxylation is the first step in the conversion of pyruvate into acetyl CoA, and it's a crucial process in cellular respiration. During this step, pyruvate, which is a 3-carbon molecule, loses one of its carbon atoms in the form of carbon dioxide. This process reduces pyruvate to a 2-carbon compound called an acetyl group. The enzyme responsible for this transformation is pyruvate dehydrogenase, which catalyzes the reaction in the mitochondria. This removal of a carbon is essential because it sets up the acetyl group for further metabolic processes, ultimately playing a critical role in energy production.

Pyruvate dehydrogenase

Pyruvate dehydrogenase is an intricate enzyme complex that plays a vital role in cellular metabolism. It's responsible for catalyzing the decarboxylation of pyruvate to form an acetyl group. This complex consists of multiple enzymes and cofactors working together to efficiently carry out this multi-step reaction.

Pyruvate dehydrogenase works by binding pyruvate in its active site, facilitating the removal of the carboxyl group. This step is followed by the transfer of the remaining acetyl group to a molecule of coenzyme A. By integrating into the core processes of energy metabolism, pyruvate dehydrogenase ensures that cells are able to produce energy through subsequent metabolic cycles.

NAD+ to NADH conversion

The conversion of NAD extsuperscript{+} to NADH is a key event in the second step of acetyl CoA synthesis. During this step, the 2-carbon acetyl group undergoes oxidation. This means electrons are removed from the group, which are then accepted by NAD extsuperscript{+}.

This process transforms NAD extsuperscript{+} into NADH, a high-energy electron carrier. NADH holds onto the extra electrons until they are transferred to the electron transport chain, which will ultimately produce ATP, the energy currency of the cell.

• NADH is crucial in carrying electrons.
• It influences further energy production steps.
• This conversion links glycolysis and the citric acid cycle.

By turning NAD extsuperscript{+} into NADH, cells effectively store energy from food molecules, which can then be released and utilized as needed.

Citric acid cycle

Once acetyl CoA is formed, it enters the citric acid cycle, also known as the Krebs cycle. This cycle takes place in the mitochondria and is central to energy production in aerobic organisms.

The citric acid cycle involves a series of chemical reactions that further break down the acetyl group from acetyl CoA into carbon dioxide, while simultaneously capturing high-energy electrons in the form of NADH and FADH extsubscript{2}. This cycle not only contributes to ATP production but also plays a role in various biosynthetic pathways.

• It helps generate electron carriers like NADH and FADH extsubscript{2}.
• It releases carbon dioxide as a byproduct.
• It provides precursors for amino acid and nucleotide biosynthesis.

This cycle is a critical hub in cellular metabolism, regenerating oxaloacetate to allow continuous processing of new acetyl groups from acetyl CoA, maintaining the energy flow within the cell.