Ethanol metabolism

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Metabolism of Ethanol

Introduction

The Catabolic Pathway

Human Metabolic Physiology

Introduction to Metabolism

In many ways, alcohol metabolism is intimately linked with the carbohydrate / glucose pathway. Alcohol enters into the normal glycolytic pathway toward the end of the normal glycolysis pathway as Acetyl-CoA. This is not atypical in metabolic systems - if glycolysis could only process glucose, the organism would have inadequate amount of energy. Therefore, the body has developed many important other pathways to convert certain chemicals into energy (ATP), such as ethanol.

Ethanol and Evolution

The catabolic degradation of Ethanol is essential to life, not only of humans, but of almost all living organisms. In fact, certain amino acid sequences in the enzymes used to oxidize ethanol are conserved all the way back to single cell bacteria^[1]. Such a functionality is needed because all organisms actually produce alcohol in small amounts by several pathways, primary amongst them Fatty Acid Synthesis^[2], Glycerolipid Metabolism^[3], and Bile Acid Biosynthesis.^[4] If the body had no mechanism for catabolizing the alcohols, they would build up in the body and become toxic. This could be an evolutionary rationale for alcohol catabolism.

Physiologic Structures

As is a basic organizing theme in biological systems, greater complexity of a body system, such as tissues and organs allows for greater specificity of function. This occurs for the processing of ethanol in the human body. We find that all the enzymes needed to accomplish the oxidation reactions are confined to certain tissues. In particular, we find much higher concentration of such enzymes in the kidneys and in the liver^[5], making such organs the primary site for alcohol catabolism. Because these reactions occur in the liver and kidney, they are much more likely to be damaged by super-reactive free radical reaction intermediaries such as acetaldehyde.

Thermodynamic Considerations

Common Misconceptions

There is a common misconception that drinking alcohol leads to weight gain. This has never been proven in the literature and is the subject of ongoing debate among experts.^[6] There are many complex theories in the literature which hope to explain why drinking alcohol (i.e. high concentration alcohol such as vodka, as opposed to an alcoholic beverage like beer) may not lead to weight gain, but none of the answers are conclusive. It is known that some or even most of the alcohol that is ingested is not catabolized entirely to H₂O and CO₂. Instead, much of the alcohol that is processed by the body ends up as acetic acid in the urine.

Free Energy Thermodynamics

Free Energy Calculations

The reaction from ethanol to Carbon Dioxide and Water is a complex one that proceeds in three steps. Below, the Gibbs Free Energy of Formation for each step is shown with ΔG_f values given in the CRC.^[7].

Complete Reaction: C₂H₆O(Ethanol)→C₂H₄O(Acetaldehyde)→C₂H₄O₂(Acetic Acid) →Acetyl-CoA→3H₂O+2CO₂.

ΔG_f = Σ ΔG_fp - ΔG_fo

Step One: Ethanol: -174.8 kJ/mol
Acetaldehyde: -127.6 kJ/mol
ΔG_f1 = -127.6 + 174.8 = 47.2 kJ/mol(Endergonic)
ΣΔG_f = 47.2 kJ/mol (Endergonic)

Step Two: Acetaldehyde: -127.6 kJ/mol
Acetic Acid: -389.9 kJ/mol
ΔG_f2 = -389.9 + 127.6 = -262.3 kJ/mol (Exergonic)
ΣΔG_f = -215.1 kJ/mol (Exergonic)

Step Four: (Because The Gibbs Free Energy is a State Function, we thus skip the Acetyl-CoA (step 3), for which themodynamic values do not exist).
Acetic Acid: -389.9 kJ/mol
3H₂O+2CO₂: -1500.1 kJ/mol
ΔG_f4 = -1500 + 389.6 = -1110.5 kJ/mol (Exergonic)
ΣΔG_f = -1325.3 kJ/mol (Exergonic)

Discussion of Calculations

If the catabolysis of alcohol goes all the way to completion, then, we have a very exothermic event yielding some 1325 kJ/mol of energy. If the reaction stops partway through the metabolic pathways, which happens because acetic acid is excreted in the urine after drinking, then not nearly as much energy can be derived from alcohol, indeed, only 215.1 kJ/mol. At the very least, the theoretical limits on Energy yield are determined to be 215.1 kJ/mol to 1325.3 kJ/mol. It is also important to note that step 1 on this reaction is endothermic, requiring 47.2 kJ/mol of Alcohol, or about 3 ATP / ethanol.

Organic Reaction Schema

Steps of the Reaction

The First three steps of the reaction pathways which lead from ethanol to Acetaldehyde to Acetic Acid to Acetyl-CoA are likely to be novel mechanisms to most readers. However, once Acetyl-CoA is formed, it is free to enter directly into the Citric acid cycle.

Organic Reactions

The reactions that transform Ethanol into an Aldehyde and then into a carboxylic acid are examples of Oxidation reactions, which in organic chemistry, are typically characterized by the addition of oxygen onto a functional group.^[8] The third reaction, the enzyme mediated formation of Acetyl-CoA from Acetic Acid is an example of an enzymatic synthetase reaction where, through a complex intramolecular interaction a product molecule is formed from reactants.

Gene Expression and Ethanol Metabolism

Glycolysis Pathway

Ethanol to Acetaldehyde

Ethanol will not convert to Acetaldehyde under normal conditions because such a transition is kinetically unfavorable. Therefore, an enzyme is needed to transition ethanol into a high energy intermediary. This enzyme is alcohol dehydrogenase IB (class I), beta polypeptide (ADH1B). The gene coding for this enzyme is 1.1.1.1 on chromosome 4, locus 4q21-q23. The enzyme "encoded by this gene is a member of the alcohol dehydrogenase family. Members of this enzyme family metabolize a wide variety of substrates, including ethanol, retinol, other aliphatic alcohols, hydroxysteroids, and lipid peroxidation products. This encoded protein, consisting of several homo- and heterodimers of alpha, beta, and gamma subunits, exhibits high activity for ethanol oxidation and plays a major role in ethanol catabolism. Three genes encoding alpha, beta and gamma subunits are tandemly organized in a genomic segment as a gene cluster." ^[9]

Acetaldehyde to Acetic Acid

Acetaldehyde is a highly unstable compound and quickly forms free radical structures which are highly toxic if not quenched by antioxidants such as ascorbic acid and Vitamin B1. These free radicals can result in damage to embryonic Neural Crest cells and can lead to severe birth defects. Prolonged exposure of the kidney and liver to these compounds in chronic alcoholics can lead to severe damage. The literature also suggests that these toxins may have a hand in causing some of the ill effects associated with hang-overs.

The enzyme associated with the chemical transformation from Acetaldehyde to Acetic Acid is aldehyde dehydrogenase 2 family (ALDH2). The gene encoding for this enzyme is 1.2.1.3 and is found on chromosome 12, locus q24.2^[10].

"This enzyme is alcohol dehydrogenase 1A (class I), alpha polypeptide. This protein belongs to the aldehyde dehydrogenase family of proteins. Aldehyde dehydrogenase is the second enzyme of the major oxidative pathway of alcohol metabolism. Two major liver isoforms of this enzyme, cytosolic and mitochondrial, can be distinguished by their electrophoretic mobilities, kinetic properties, and subcellular localizations. Most Caucasians have two major isozymes, while approximately 50% of Asians have only the cytosolic isozyme, missing the mitochondrial isozyme. A remarkably higher frequency of acute alcohol intoxication among Asians than among Caucasians could be related to the absence of the mitochondrial isozyme. This gene encodes a mitochondrial isoform, which has a low km for acetaldehydes, and is localized in mitochondrial matrix." ^[11]

Acetic Acid to Acetyl-CoA

The enzyme associated with the conversion of acetic acid to Acetyl-CoA is ACSS2; it is expressed by gene 6.2.1.1 located on chromsome 20 locus q11.22. "This gene encodes a cytosolic enzyme that catalyzes the activation of acetate for use in lipid synthesis and energy generation. The protein acts as a monomer and produces acetyl-CoA from acetate in a reaction that requires ATP. Expression of this gene is regulated by sterol regulatory element-binding proteins, transcription factors that activate genes required for the synthesis of cholesterol and unsaturated fatty acids. Two transcript variants encoding different isoforms have been found for this gene."^[12] Gene 6.2.1.1 on Chromosome 20

Acetyl-CoA to Water and Carbon Dioxide

Once Acetyl-CoA is formed it enters the normal Citric acid cycle.

Citric Acid cycle

References

1. ^ Comparison of Nucleotide Residues Between Humans and Bacteria
2. ^ Fatty Acid Synthesis
3. ^ Glycerolipid Metabolism
4. ^ Bile Acid Biosynthesis
5. ^ Polymorphism of alcohol-metabolizing genes affects drinking behavior and alcoholic liver disease in Japanese men
6. ^ See Eric Jéquier, "Alcohol intake and body weight: a paradox," American Journal of Clinical Nutrition 69:2, 173-174 for a quick overview of the literature and many good secondary references
7. ^ CRC Handbook of Chemistry and Physics, 81st Edition, 2000
8. ^ J. McMurry, Organic Chemistry 6t ed. (United States: Thomson, 2004), 587-854.
9. ^ ADH1B at NIH
10. ^ NCBI sequence: locus NC_000012;43439 bp;DNA
11. ^ ALDH2 NIH
12. ^ ACSS2 NIH