Ethanol metabolism

Metabolism of Ethanol

Introduction

Depth of Coverage

Much has been written in the primary literature regarding the catabolic metabolism of ethanol in humans; reference will be given to such where appropriate. In general, the following level of this article is directed toward a reader with an understanding of high school chemistry and some basic organic concepts.

Breadth of Coverage

A complete understanding of the Catabolic Pathway described herein requires:

1) An working knowledge of human physiology and metabolism as it pertains specifically to alcohol catabolism;

2) A basic understanding of the thermodynamic relations between the catabolic components;

3) An understanding of Organic Oxidation Reactions required to transform Alcohol functional groups to Aldehydes and Carboxylic Acids;

4) Appreciation for the gene expression required to create the enzymes necessary for the pathway to take place.

Each of these topic will be taken up in it’s own subheading.

The Catabolic Pathway

Human Metabolic Physiology

Introduction to Metabolism

Basic human Metabolism is covered in any basic biology book. In many ways, alcohol metabolism is intimately linked with the carbohydrate / glucose pathway taught in introductory texts. Indeed, ‘alcohol’ enters into the normal glycoltyic pathway toward the end of the normal glycolysis pathway as Acetyl-CoA. Such a state of affairs is not atypical in metabolic systems. If Glycolysis could only process glucose, the body would have a very hard time deriving enough 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. Such a functionality is needed because all organisms actually produces alcohol in small amounts by several pathways, primary amongst them Fatty Acid Synthasis, Glycerolipid Metabolism, and Bile Acid Biosynthesis. If the body had no mechanism for catabolizing the alcohols, they would build up in the body and become toxic. Thus, we begin to see an evolutionary rational for alcohol catabolosis.

Physiologic Structures

As is a basic organizing theme in biological systems, greater complexity of a body system, such as tissue and organs allows for greater specificty of function. This occurs for the processesing of ethanol in the human body. We find that all the enzymes needed to accomplish the oxidation reactions are confined to certain tissues. In perticular, we find much higher concentration of such enzymes in the kidneys, and even more so, in the liver. Making such organs the primary site for alcohol catabolis. Because these reactions occur in the liver and kindey, they are much more likely to be damaged by super-reactive free radical reaction intermidiaries like 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. There are many complex theories in the literature which hope to explain why drinking alcohol (pure alcohol, like vodka, not 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 injested 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 proceedes in three steps. Below, the Gibbs Free Energy of Formation for each step is shown with ΔG_f values given in the CRC..

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(Endothermic) ΣΔG_f = 47.2 kJ/mol (Endothermic)

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

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

Discussion of Calculations

If the catabolsis 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 enrgy 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 carboxcylic acid are examples of Oxidation reactions, which in organic chemistry, are typically charecterized by the addation of oxygen onto a functional group. The third reaction, the enzyme mediated formation of Acetyl CoA from Acetic Acid is an example of an ezamatic synthetase reaction where, through a complex intramolecular interaction a product molecule is fromed from reactants.

Gene Expression and Ethanol Metabolism

File:Gycolpathway.gif

Ethanol to Acetaldehyde

Ethanol will not convert to Acetaldehyde under normal conditions because such a transition is thermodynamically unfavoriable. Therefore, an enzyme is needed to transition ethanol into a this high energy intermidiary. 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."

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 Nueral Crest cells and can lead to severe birth defects. Prolonged exposure of the kidney and liver to these compounds in chronic alchohics can lead to severe damage. The literature also suggests that these toxins may have a hand in causing some of the ill effects assocaited with hang-overs.

The enzyme assocaited 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.

"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 Orientals have only the cytosolic isozyme, missing the mitochondrial isozyme. A remarkably higher frequency of acute alcohol intoxication among Orientals 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."

Acetic Acid to Acetyl CoA

The enzyme assocaited with the conversion os 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."

Acetyl CoA to Water and Carbon Dioxide

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