Ethanol metabolism

Ethanol is metabolized through a very complex catabolic metabolic pathway.

Human metabolic physiology

Ethanol and evolution

The average human digestive system produces approximately 3g of ethanol per day merely through fermentation of its contents.^{[citation needed]} Catabolic degradation of ethanol is thus 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, primarily along the fatty acid synthesis,^[2] glycerolipid metabolism,^[3] and bile acid biosynthesis pathways.^[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 also by sulfotransferase.

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. All the enzymes needed to accomplish the oxidation reactions are confined to certain tissues. In particular, much higher concentration of such enzymes are found in the kidneys and in the liver,^[5] making such organs the primary site for alcohol catabolism.

Thermodynamic considerations

Energy Thermodynamics

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.^[6]

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
Ethanal(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: Ethanal: −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 energy is a state function, we can skip the Acetyl-CoA (step 3), for which themodynamic values are not known).
Acetic Acid: −389.9 kJ/mol
3H₂O+2CO₂: −1 500.1 kJ/mol
ΔG_f4 = −1 500 + 389.6 = −1 110.5 kJ/mol (Exergonic)
ΣΔG_f = −1 325.3 kJ/mol (Exergonic)

Discussion of calculations

If catabolism of alcohol goes all the way to completion, then, we have a very exothermic event yielding some 1 325 kJ/mol of energy. If the reaction stops part way 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 1 325.3 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 molecules of ATP (adenosine triphosphate) per molecule of ethanol.

Organic reaction schema

Steps of the reaction

The first three steps of the reaction pathways lead from ethanol to acetaldehyde to acetic acid to acetyl-CoA. 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.^[7] 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

Ethanol to acetaldehyde in human adults

In human adults, ethanol is oxidized to acetaldehyde mainly via the hepatic enzyme 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.".^[8]

Ethanol to acetaldehyde in human fetuses

In human embryos and fetuses, ethanol is not metabolized via this mechanism as ADH enzymes are not yet expressed to any significant quantity in human fetal liver (the induction ADH only starts after birth, and requires years to reach adult levels).^[9] Accordingly, the fetal liver cannot metabolize ethanol or other low molecular weight xenobiotiocs. In fetuses, ethanol is instead metabolized at much slower rates by different enzymes from the cytochrome P-450 superfamily (CYP), in particular by CYP2E1. The low fetal rate of ethanol clearance is responsible for the important observation that the fetal compartment retains high levels of ethanol long after ethanol has been cleared from the maternal circulation by the adult ADH activity in the maternal liver.^[10] CYP2E1 expression and activity have been detected in various human fetal tissues after the onset of organogenesis (ca 50 days of gestation).^[11] Exposure to ethanol is known to promote further induction of this enzyme in fetal and adult tissues. CYP2E1 is a major contributor to the so called Microsomal Ethanol Oxidation Pathway (MEOS)^[12] and its activity in fetal tissues is thought to contribute significantly to the toxicity of maternal ethanol consumption.^[13][14] In presence of ethanol and oxygen, CYP2E1 is known to release superoxide radicals and induce the oxidation of polyunsaturated fatty acids to toxic aldehyde products like 4-hydroxynonenal (HNE).

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 (Vitamin C) and Vitamin B₁ (thiamine). 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.^[15] 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.^[16]

"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 aldehyde dehydrogenase, 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 East Asians have the cytosolic isozyme but not the mitochondrial isozyme. A remarkably higher frequency of acute alcohol intoxication among East Asians than among Caucasians could be related to the absence of a catalytically active form of the mitochondrial isozyme. The increased exposure to acetaldehyde in individuals with the catalytically inactive form may also confer greater susceptibility to many types of cancer. This gene encodes a mitochondrial isoform, which has a low Km for acetaldehydes, and is localized in mitochondrial matrix. Alternative splicing results in multiple transcript variants encoding distinct isoforms." ^[17]

Acetic acid to acetyl-CoA

Two enzymes are associated with the conversion of acetic acid to acetyl-CoA the first is ACSS2 or acetyl CoA synthase-1; which is expressed by gene 6.2.1.1 located on chromosome 20 locus q11.22. "This gene encodes a nuclear-cytosolic enzyme that catalyzes the activation of acetate for use in lipid synthesis and protein acetylation reactions. The second enzyme is ACSS1 (acetyl CoA synthase-2) which is localized to mitochondria and is used for energy generation via the tricarboxylic acid cycle. The proteins act as monomers and produce acetyl-CoA from acetate in a reaction that requires ATP. Expression of ACSS2 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."^[18]

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.

References