Fermentation pathways

Fermentation is a metabolic process occurring in organisms under anaerobic conditions, where glucose is partially broken down to generate energy and regenerate electron carriers. The primary purpose is to allow glycolysis to continue by recycling NADH back to NAD⁺, in the absence of oxygen, enabling ATP production even without aerobic respiration.

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Fermentation in Organisms: Stepwise Process

Step 1: Glycolysis

• Glucose (C₆H₁₂O₆) is transported into the cytoplasm.
• It undergoes a series of enzyme-catalyzed reactions, splitting into two molecules of pyruvate (C₃H₄O₃).
• During this, 2 ATP molecules are used in the preparatory phase, and 4 ATP molecules are generated in the payoff phase, yielding a net gain of 2 ATP.
• Additionally, 2 molecules of NAD⁺ are reduced to NADH (electron carriers).
• This step does not require oxygen and occurs in almost all living cells.

Step 2: Pyruvate Conversion (Fermentation Pathways)

In the absence of oxygen, pyruvate is metabolized anaerobically to regenerate NAD⁺ from NADH.

The conversion depends on the organism and the type of fermentation:

A. Alcoholic Fermentation (in yeast and some bacteria)

• Decarboxylation of Pyruvate to Acetaldehyde

Each pyruvate molecule produced in glycolysis is transported to the cytosol where it undergoes enzymatic decarboxylation.

The enzyme pyruvate decarboxylase catalyzes removal of one carbon dioxide molecule (CO₂) from pyruvate, producing a 2-carbon compound called acetaldehyde.

The release of CO₂ is responsible for the bubbles seen in alcoholic beverages and rising dough.

• Reduction of Acetaldehyde to Ethanol

Acetaldehyde is then reduced to ethanol by the enzyme alcohol dehydrogenase.

This reduction requires electrons and protons, which are provided by NADH molecules generated in glycolysis.

During this reaction, NADH is oxidized back to NAD⁺.

Regeneration of NAD⁺ is critical because it replenishes the supply of NAD⁺ needed to keep glycolysis running, thus allowing continuous ATP production under anaerobic conditions.

• Excretion of Ethanol and CO₂

The ethanol produced accumulates in the cytoplasm and is eventually excreted into the surrounding environment.

CO₂ released during pyruvate decarboxylation diffuses out, causing effervescence in fermented beverages or leavening in dough.

B. Lactic Acid Fermentation (in muscle cells and some bacteria)

• Pyruvate Reduction to Lactate

In the absence of oxygen, aerobic respiration cannot proceed, so cells divert pyruvate into fermentation.

Pyruvate serves as an electron acceptor.

The enzyme lactate dehydrogenase (LDH) catalyzes:

• The transfer of electrons (hydrogen atoms) from NADH to pyruvate.
• Pyruvate is reduced to lactate (lactic acid).

This regenerates NAD⁺ by oxidizing NADH back to NAD⁺, which is crucial to sustain glycolysis.

• NAD⁺ Recycling

NAD⁺ is essential as an electron acceptor for glycolysis.

Without fermentation regenerating NAD+, NAD+ would be depleted, halting glycolysis and ATP production.

The recycling of NAD⁺ through lactate formation ensures continuous ATP supply under anaerobic conditions.

• Lactate Fate

Lactate accumulates in muscle cells during intense exercise when oxygen delivery is limited.

It diffuses into the bloodstream and is transported to the liver.

In the liver, during recovery phases, lactate is converted back to pyruvate and glucose in the Cori cycle (gluconeogenesis).

The process does not produce CO₂; it is solely a reduction of pyruvate to lactate.

C. Other fermentation types (e.g., mixed acid, propionic acid fermentation) exist in specific microbes producing various organic acids, alcohols, and gases.

Mixed acid fermentation is a complex anaerobic process carried out by certain bacteria, notably Escherichia coli and Clostridium species, which produce a diverse array of organic acids, gases, and alcohols from sugars such as glucose.

• Pyruvate Conversion to Diverse End Products

Pyruvate undergoes various transformations catalyzed by different enzymes:

• Pyruvate formate-lyase (PFL): Converts pyruvate into formate and acetyl-CoA.
• Formate hydrogen lyase (FHL): Converts formate into hydrogen gas and carbon dioxide.
• Fumarate reductase: Reduces fumarate to succinate using electrons from NADH or reduced ferredoxin.​

• Formation of Organic Acids and Gases

These enzymes facilitate the formation of:

• Acetic acid (via acetyl-CoA conversion).
• Succinate (by fumarate reduction).
• Lactic acid or ethanol under specific conditions.
• Gases like hydrogen (H₂) and carbon dioxide (CO₂), contributing to flatus and gas production.​

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In sum, fermentation enables cells to produce ATP anaerobically by glycolysis coupled with NAD⁺ regeneration via diverse pathways relative to organism type. While less efficient than aerobic respiration, it allows survival and energy extraction under oxygen-limited conditions.