By the end of this activity, students will be able to:

• Identify the main compounds involved in ethanol metabolism

• Arrange the correct biochemical sequence of reactions

• Identify the enzymes responsible for each step

• Explain the role of NAD⁺ and NADH in ethanol metabolism

• Describe the final products and energy outcome

KAMANZI Private

ISHIMWE Jean Luc

HITAYEZU Jean Pierre

NIYIGIRIMBABAZI Modestine

GIRAMATA ODILE (MBIOT 043/25)

NTAGARA YVONNE(MBIOT 047/25)

RAFIKI NORBERT (MBIOT 041/25)

HAKIZIMANA BAVUZE LEOPOLD (MBIOT 037/25)

1. Ethanol (C₂H₅OH) : the starting substrate

2. Acetaldehyde (CH₃CHO) : intermediate compound

3. Acetate (CH₃COO⁻) :oxidized product of acetaldehyde

4. Acetyl-CoA : activated form entering central metabolism

5. NAD⁺ / NADH : redox cofactors involved in electron transfer

These compounds link ethanol metabolism with major metabolic pathways such as the citric acid cycle (TCA cycle).

Step 1: Oxidation of Ethanol to Acetaldehyde

C2H5OH + NAD+ CH3CHO + NADH + H+

• Enzyme: Alcohol Dehydrogenase (ADH)

• Reaction type: Oxidation

• Location: Cytoplasm in many organisms

• Significance: Ethanol is converted to acetaldehyde while NAD⁺ is reduced to NADH.

Step 2: Oxidation of Acetaldehyde to Acetate

CH3CHO + NAD+ + H2O CH3COOH + NADH + H+

• Enzyme: Aldehyde Dehydrogenase (ALDH)

• Reaction type: Oxidation

• Significance: Toxic acetaldehyde is converted into acetate.

Step 3: Conversion of Acetate to Acetyl-CoA

CH3COO- + CoA + ATP Acetyl Coa + AMP + PPi

- AMP (Adenosine monophosphate)

- PPᵢ (Pyrophosphate)

• Enzyme: Acetyl-CoA Synthetase

• Reaction type: Activation reaction requiring ATP

• Significance: Acetyl-CoA enters the citric acid cycle for energy production or biosynthesis.

1 Ethanol → Acetaldehyde Alcohol Dehydrogenase (ADH)

2 Acetaldehyde → Acetate Aldehyde Dehydrogenase (ALDH)

3 Acetate → Acetyl-CoA Acetyl-CoA Synthetase

These enzymes are crucial in microbial fermentation systems and also in biotechnological ethanol metabolism studies.

Key roles:

1. Electron carrier : NAD⁺ accepts electrons during oxidation reactions.

2. Energy metabolism link : NADH carries electrons to the electron transport chain in aerobic conditions.

3. Redox balance : maintains cellular oxidation reduction balance.

During ethanol metabolism:

• NAD⁺ is reduced to NADH during oxidation of ethanol and acetaldehyde.

• NADH can later be oxidized back to NAD⁺, producing ATP through oxidative phosphorylation (in aerobic conditions).

Thus, NAD⁺/NADH cycling is essential for continuous metabolic flux.

Main products:

• Acetyl-CoA

• NADH

• CO₂ and ATP (if acetyl-CoA enters the TCA cycle)

Energy outcome:

1. Each oxidation step produces NADH.

2. NADH can generate approximately 2.5–3 ATP per molecule through oxidative phosphorylation.

3. Acetyl-CoA entering the TCA cycle produces additional:

o NADH

o FADH₂

o ATP (or GTP)

Therefore, ethanol metabolism can contribute significant cellular energy when oxygen is available.

The Glycolysis pathway consists of ten enzyme-catalyzed reactions divided into two phases: the energy investment phase and the energy payoff phase. During the energy investment phase, glucose is phosphorylated and converted into fructose-1,6-bisphosphate through the consumption of two ATP molecules. This modification prepares the six-carbon glucose molecule for cleavage into two three-carbon intermediates.

Fructose-1,6-bisphosphate is then split into glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). DHAP is rapidly converted into G3P, resulting in two molecules of G3P that proceed through the energy payoff phase. In this phase, oxidation and substrate-level phosphorylation reactions occur, generating ATP and reducing NAD⁺ to NADH.

The final product of glycolysis is Pyruvate, with a net yield of two ATP molecules, two NADH molecules, and two pyruvate molecules per glucose.

Under anaerobic conditions in yeast, pyruvate cannot enter the Tricarboxylic Acid Cycle and is instead metabolized through Alcoholic Fermentation.

In this process, pyruvate is first converted to acetaldehyde and carbon dioxide by Pyruvate Decarboxylase. Acetaldehyde is then reduced to ethanol by Alcohol Dehydrogenase, using NADH as a reducing agent. This reaction regenerates NAD⁺, which is essential for sustaining glycolysis and enabling continuous ATP production in anaerobic environments.

Ethanol metabolism is an essential biochemical process through which the body breaks down alcohol and converts it into compounds that can be further used for energy production or safely eliminated. This process mainly occurs in the liver and involves a sequence of enzyme-catalyzed reactions that transform ethanol into intermediate molecules such as acetaldehyde and acetate before eventually forming acetyl-CoA, which enters cellular energy pathways. Throughout this process, important coenzymes such as NAD⁺ and NADH participate in oxidation–reduction reactions that help transfer electrons and support cellular metabolism. Understanding ethanol metabolism is important for explaining how the body processes alcohol and how energy is produced from metabolic reactions.

Therefore, this activity will examine the key aspects of ethanol metabolism by identifying the main compounds involved, outlining the correct biochemical sequence of reactions, recognizing the enzymes responsible for each step, explaining the role of NAD⁺ and NADH in the process, and describing the final products and overall energy outcome of the pathway.

Several important biochemical compounds participate in ethanol metabolism. Each molecule plays a specific role in breaking down ethanol and converting it into forms that can be used in cellular energy production.

Ethanol (C₂H₅OH)
Ethanol is the alcohol molecule that enters the metabolic pathway after alcohol consumption. It is a small organic compound with the chemical formula C₂H₅OH. Ethanol can easily diffuse across cell membranes and is mainly metabolized in the liver. Although ethanol itself is relatively stable, it undergoes oxidation reactions in the body that convert it into other compounds involved in metabolism.

Acetaldehyde (CH₃CHO)
Acetaldehyde is the first intermediate produced during ethanol metabolism. It is formed when ethanol (C₂H₅OH) is oxidized by the enzyme alcohol dehydrogenase. The chemical formula of acetaldehyde is CH₃CHO. This compound is highly reactive and toxic, and it can damage proteins, DNA, and cell membranes if it accumulates. Because of its toxicity, the body rapidly converts acetaldehyde into a less harmful substance.

Acetate (CH₃COO⁻)
Acetate is produced when acetaldehyde (CH₃CHO) is further oxidized by the enzyme aldehyde dehydrogenase. Its chemical formula is CH₃COO⁻ (the acetate ion). Acetate is less toxic than acetaldehyde and can circulate in the bloodstream to other tissues where it can be used for energy metabolism. It serves as the precursor for the formation of acetyl-CoA.

Acetyl-CoA (C₂₃H₃₈N₇O₁₇P₃S)
Acetyl-CoA is a central molecule in cellular metabolism. It is formed when acetate combines with coenzyme A (CoA) through the action of the enzyme acetyl-CoA synthetase. Acetyl-CoA contains an acetyl group (CH₃CO–) attached to coenzyme A. This molecule enters the citric acid cycle (Krebs cycle) where it is oxidized to produce energy in the form of ATP.

NAD⁺ (Nicotinamide Adenine Dinucleotide)
NAD⁺ is an important coenzyme that functions as an electron carrier in many metabolic reactions. During ethanol metabolism, NAD⁺ accepts electrons and hydrogen atoms when ethanol and acetaldehyde are oxidized. This process reduces NAD⁺ to NADH. NAD⁺ is therefore essential for enabling the oxidation reactions that break down ethanol.

NADH
NADH is the reduced form of NAD⁺ and carries high-energy electrons generated during ethanol metabolism. These electrons are transported to the electron transport chain in the mitochondria, where they help produce ATP through oxidative phosphorylation. However, excessive production of NADH during heavy alcohol metabolism can disturb the normal metabolic balance of cells and may lead to metabolic effects such as fat accumulation in the liver.

Together, these compounds form the main components of the ethanol metabolism pathway, ensuring that ethanol is converted into molecules that can safely participate in cellular energy production.

Step 1: Oxidation of Ethanol to Acetaldehyde

The first step in ethanol metabolism is the oxidation of ethanol (C₂H₅OH) to acetaldehyde (CH₃CHO). This reaction occurs mainly in the cytosol of liver cells and is catalyzed by the enzyme alcohol dehydrogenase (ADH).

Reaction:

C₂H₅OH + NAD⁺ → CH₃CHO + NADH + H⁺

During this reaction, NAD⁺ accepts electrons and hydrogen atoms from ethanol, becoming NADH. This is an oxidation-reduction (redox) reaction, where ethanol is oxidized and NAD⁺ is reduced. The product acetaldehyde is highly toxic and reactive, so it must be rapidly converted into a less harmful compound.

Step 2: Oxidation of Acetaldehyde to Acetate

In the second step, acetaldehyde (CH₃CHO) is further oxidized to form acetate (CH₃COO⁻). This reaction occurs mainly in the mitochondria and is catalyzed by the enzyme aldehyde dehydrogenase (ALDH).

Reaction:

CH₃CHO + NAD⁺ + H₂O → CH₃COO⁻ + NADH + 2H⁺

In this reaction, NAD⁺ again acts as an electron acceptor, forming NADH. This step is extremely important because acetaldehyde is toxic and can cause cellular damage if it accumulates. The rapid conversion of acetaldehyde to acetate helps protect the body from its harmful effects.

Step 3: Formation of Acetyl-CoA

In the final step, acetate (CH₃COO⁻) is activated and converted into acetyl-CoA by the enzyme acetyl-CoA synthetase. This reaction requires ATP and coenzyme A (CoA).

Reaction:

CH₃COO⁻ + CoA + ATP → Acetyl-CoA + AMP + PPi

Acetyl-CoA is a central metabolic intermediate that enters the citric acid cycle (Krebs cycle) in the mitochondria. Within this cycle, acetyl-CoA is further oxidized, producing NADH, FADH₂, and ATP, which are essential for cellular energy production through oxidative phosphorylation.

Summary of the Pathway

Overall, ethanol metabolism can be summarized as:

Ethanol (C₂H₅OH) → Acetaldehyde (CH₃CHO) → Acetate (CH₃COO⁻) → Acetyl-CoA

Through these steps, ethanol is converted into a molecule that can participate in the cell’s main energy-producing pathways, linking alcohol metabolism to normal cellular respiration.

3.1 Alcohol Dehydrogenase (ADH)

Reaction: Ethanol → Acetaldehyde

Alcohol dehydrogenase (ADH) is the enzyme responsible for the first step of ethanol metabolism. It is mainly found in the cytosol of liver cells, although smaller amounts are present in other tissues such as the stomach and kidneys. The main function of ADH is to oxidize ethanol (C₂H₅OH) into acetaldehyde (CH₃CHO).

During this reaction, ADH removes hydrogen atoms from ethanol and transfers them to the coenzyme NAD⁺, which becomes NADH. This process is an oxidation-reduction reaction where ethanol is oxidized and NAD⁺ is reduced.

Simplified reaction:

C₂H₅OH + NAD⁺ → CH₃CHO + NADH + H⁺

This step is very important because it begins the breakdown of ethanol, but it also produces acetaldehyde, a highly toxic compound. Therefore, the body must quickly convert acetaldehyde into a safer molecule in the next step.

3.2 Aldehyde Dehydrogenase (ALDH)

Reaction: Acetaldehyde → Acetate

Aldehyde dehydrogenase (ALDH) catalyzes the second step of ethanol metabolism, where the toxic compound acetaldehyde is converted into acetate (CH₃COO⁻). This reaction occurs mainly in the mitochondria of liver cells.

Acetaldehyde is extremely reactive and can damage proteins, DNA, and cell membranes if it accumulates in the body. ALDH helps protect the body by rapidly detoxifying acetaldehyde and converting it into the less harmful acetate.

During this reaction, NAD⁺ again acts as an electron acceptor and is reduced to NADH.

Simplified reaction:

CH₃CHO + NAD⁺ + H₂O → CH₃COO⁻ + NADH + 2H⁺

This enzyme is critical for preventing the buildup of acetaldehyde. In some individuals, genetic variations in ALDH reduce its activity, causing acetaldehyde accumulation and symptoms such as facial flushing, nausea, and rapid heartbeat after alcohol consumption.

3.3 Acetyl-CoA Synthetase

Reaction: Acetate → Acetyl-CoA

The final step of ethanol metabolism involves the enzyme acetyl-CoA synthetase, which converts acetate (CH₃COO⁻) into acetyl-CoA. This reaction takes place in the mitochondria and requires ATP and coenzyme A (CoA).

Simplified reaction:

CH₃COO⁻ + CoA + ATP → Acetyl-CoA + AMP + PPi

Acetyl-CoA is a central metabolic molecule that enters the citric acid cycle (Krebs cycle). In this cycle, it is further oxidized to produce energy in the form of ATP, NADH, and FADH₂, which are essential for cellular respiration.

Summary

These three enzymes work together in a coordinated metabolic pathway:

1. Alcohol Dehydrogenase (ADH) begins ethanol oxidation.

2. Aldehyde Dehydrogenase (ALDH) detoxifies the harmful intermediate acetaldehyde.

3. Acetyl-CoA Synthetase converts acetate into acetyl-CoA for energy production.

Through the action of these enzymes, ethanol is efficiently converted into a form that can participate in the body’s normal energy-producing metabolic pathways.

During the metabolism of ethanol, NAD⁺ acts as an electron acceptor in the first two major reactions. In the first step, when ethanol is oxidized to acetaldehyde by the enzyme alcohol dehydrogenase, NAD⁺ accepts electrons and hydrogen atoms released from ethanol. As a result, NAD⁺ is reduced to NADH. A similar process occurs in the second step when acetaldehyde is oxidized to acetate by the enzyme aldehyde dehydrogenase. Again, NAD⁺ accepts electrons and is converted into NADH.

The production of NADH is important because NADH carries high-energy electrons to the electron transport chain in the mitochondria. In this system, the electrons are passed through a series of proteins that ultimately help produce ATP (adenosine triphosphate) through the process of oxidative phosphorylation. ATP is the main energy currency of the cell, so the NADH generated during ethanol metabolism contributes to cellular energy production.

However, excessive alcohol consumption leads to large amounts of NADH being produced in the liver. When NADH accumulates, it disrupts the normal balance between NAD⁺ and NADH inside cells. This imbalance affects several metabolic pathways, including carbohydrate and fat metabolism. High levels of NADH can inhibit normal oxidation reactions and promote the conversion of fatty acids into triglycerides, which may lead to fat accumulation in the liver (fatty liver disease). Therefore, maintaining a proper NAD⁺/NADH balance is essential for healthy cellular metabolism.

Acetyl-CoA is formed after acetate is converted by the enzyme acetyl-CoA synthetase. This molecule is a central intermediate in metabolism and plays a key role in the citric acid cycle (Krebs cycle). In this cycle, acetyl-CoA is further oxidized, producing additional reduced coenzymes such as NADH and FADH₂, which store energy in the form of high-energy electrons.

The NADH molecules generated during ethanol metabolism are also important for energy production. NADH transports high-energy electrons to the electron transport chain located in the inner mitochondrial membrane. As electrons move through this chain, energy is released and used to generate ATP through oxidative phosphorylation. This process allows the cell to convert the energy stored in electrons into a usable form.

Although ethanol metabolism can contribute to energy production through the formation of acetyl-CoA and NADH, excessive alcohol metabolism can disturb normal metabolic processes. The large amount of NADH produced may alter metabolic balance, inhibit normal glucose production, and increase the formation of fatty acids and triglycerides in the liver. Over time, this metabolic imbalance can contribute to liver disorders such as fatty liver, alcoholic hepatitis, and liver cirrhosis.

In summary, ethanol metabolism not only converts alcohol into less harmful compounds but also links alcohol breakdown to the cell’s major energy-producing pathways, demonstrating the important relationship between metabolism, energy production, and physiological health.