Oxidation of Lipids III

Suaib Luqman

Objectives
- v To understand the functions of the enzymes of fatty acid oxidation

v How the energy is liberated in fatty acid

Concept Map

3. Description

Many organisms depend on oxidation of long-chain fatty acids for their energy requirements. Organs like mammalian heart and liver get 80% of their energy from the conversion of fatty acids into acetyl CoA. ATP is synthesized by the electrons obtained during fatty acid oxidation when they get ahead of the respiratory chain. In the TCA cycle, the Acetyl CoA formed from the fatty acids may be completely oxidized to carbon dioxide , further ensuing in the conservation of energy. Apart from this acetyl CoA may also be converted into ketone bodies in liver that can be transported to brain when glucose is not accessible. In higher plants, Acetyl CoA serves up principally as a biosynthetic predecessor, merely secondarily as a fuel. Essentially, the mechanism of the fatty acid oxidation remains the same, although the biological role differs from organism to organism. Triacylglycerols are neutral fats and thus are very suitable to be stored as fuels in cells. The hydrocarbons on complete oxidation are good energy moieties as compared to carbohydrates and proteins of the same weight. Moreover the “lipid droplets” in which these triacylglycerols aggragate themselves are higly inert chemical entities, insoluble in water and show reluctant behavior towards osmolarity of the cytosol. Thus these physical and chemical features of these TAGs make them the best kind of molecules to be stored as fuel in large quantities in cell. In the intestine, the ingested TAGs must be emulsified before being digested by the water soluble enzymes and TAGs mobilized from the storage tissues gets absorbed in the intestine and must be carried in the blood bound to the proteins that counteract their insolubility. The entire oxidation of fatty acids to carbon dioxide and water takes place in three stages:

1. β oxidation: The oxidation of long-chain fatty acids to 2C fragment Acetyl CoA

2. TCA Cycle: The oxidation of Acetyl CoA to carbon dioxide.

3. Mitochondrial Respiratory Chain: The transfer of electrons from reduced electron carriers.

Vertebrates, gets the fats from their diet, muster fats stored in adipocytes (specialized cells of adipose tissues), and, in the liver, convert excess dietary carbohydrates to fats for export to other tissues. In highly industrialized countries, 40% or more (on an average) of the daily energy prerequisite of the human being is supplied by dietary TAGs. TAGs afford more than half of the energy requirements of several organs, particularly the resting skeletal muscle, heart and liver. Stored TAGs are practically the solitary energy source in migrating birds and hibernating animals. Protists get hold of fats by devouring organisms inferior in the food chain, and a few also accumulate fats as cytosolic lipid droplets. During germination, vascular plants mobilize fats stored in seeds, but do not otherwise depend on fats for energy.

The Enzymes of Fatty Acid Oxidation

The fatty acids oxidation in Gram positive and Gram negative bacteria as well as in the mitochondrial system of eukaryotes crop up with the aid of four identical enzymes. Peroxisomes and glyoxysomes have two extra enzymes for this process. The intra-mitochondrial enzymes of the β-oxidation spiral are considered to be articulated in all the tissues and are not acknowledged to have any tissue-specific isoforms. However, there are two isoforms of CPTI that vary in their regulatory properties and in their expression levels among the tissues. lCPTI, the liver isoform of CPTI, is also expressed in the pancreatic islets, kidney, brain and intestine, whereas mCPTI, the muscle isoform of CPTI, is established in cardiac and skeletal muscle, in brown adipocytes and in the testis. The liver and muscle CPTI isoforms have noticeably dissimilar kinetic distinctiveness: lCPTI has a low Km for carnitine whereas mCPTI is exceedingly much more receptive to malonyl CoA than the lCPTI. Cardiac muscle articulates both m and l CPTI isoforms, but the comparative proportions of the isoforms transformed during development as a result of which the by and large sensitivity to malonyl CoA and affinity for carnitine alters.

A system of nomenclature of the enzyme of fatty acid metabolism was proposed at the Second International Conference on Biochemical Problem of Lipids held at Ghent, Belgium in 1955. The enzymes active in Reaction 1 was termed and proposed as thiokinases. There may be individual thiokinases for fatty acids of separate chain lengths. Reaction 2 is catalyzed by an Acyl dehydrogenase, Reaction 3 by Enoyl hydrases also termed Crotonases, Reaction 4 by β-hydroxyl Acyl dehydrogenases and Reaction 5 by β-keto Acyl thiolases (thiolases). These enzymes were isolated and studied by various workers.

A Schematic Representation of Energy Yield from Oxidation of Palmitoyl CoA

The Energy Aspect of Fatty Acid Oxidation

The Acetyl CoA oxidation through the TCA cycle is believed to yield a total of 12 high-energy bonds. In the process of β oxidation specifically in reaction 2 and 4, flavoprotein and NAD are reduced. The transfer of electrons from these coenzymes over the transfer system to cytochrome may be expected to yield additional high energy bonds, three may be assumed for NAD system and two for the flavoprotein system. Taking an example of palmitic acid (C16) , a total of eight C2 units will be produced yielding 8 X 12 = 96 high-energy bonds upon complete oxidation in the citric acid cycle. In addition, seven of these units will contribute to the high energy pool, through the NAD and flavoprotein system, as they are formed in the course of β-oxidation, this will produce at least 7 X 5 = 35 more bonds. The total is 35 + 96 = 131 bonds/mol of the palmitic acid (C16) oxidized. From this two bonds must be substracted as it was used for the initial activation of the fatty acid (reaction 1) yielding a net gain of 129 bonds/mol or 129 X 7.6 = 980 kilocalories. The caloric value of palmitic acid is 2340 kilocalories/mol. Therefore, the coupling of phosphorylation to oxidation of the fatty acid in the cell may be expected to capture about 41% (980/2340 X 100) of the total available energy.

The β-oxidation described above is mainly for the degradation of the saturated fatty acid having an even number of carbon moeities. However, additional steps are required for unsaturated fatty acid degradation and odd numbered saturated fatty acid.

ODD Number Fatty Acid Oxidation

In plants, microorganisms, many herbivorous animals and in some of the marine organisms, fatty acids with odd numbers of carbons are found and reported. The β-oxidation of the fatty acid present in them is subjected in the identical way as the fatty acids with even numbers of carbon units. For odd numbered saturated fatty acid degradation (relatively less common), same set of reactions occur but the final result yields propionyl coenzyme A along with acetyl CoA instead of 2 moles of acetyl CoA in the final thiolysis step. Therefore, propionyl CoA is converted to usable form by conversion to succinyl CoA through rearrangement reaction involving vitamine B12 and thus it enters citric acid cycle. Propionyl-CoA is a substrate for the biotin-dependent enzyme Propionyl-CoA carboxylase that utilizes the energy in form of ATP to add a carbon, resulting in the formation of D-methylmalonyl-CoA (4C). In the next step of the reaction, the stereochemistry at the chiral carbon of the substrate gets reversed by the action of Methylmalonyl-CoA epimerase, resulting in the formation of L-methylmalonyl-CoA. The final step of the chronological reaction in the pathway is catalyzed by Methylmalonyl CoA mutase that form Succinyl CoA from L-methylmalonyl CoA (the branched chain compound), a TCA cycle intermediate. Unlike Acetyl CoA, Succinyl CoA can be used as a substrate for gluconeogenesis. The production of Succinyl CoA can also be used to increase TCA aptitude. The reactions involved in the conversion of Propionyl CoA to Succinyl CoA are valuable for more than simply implementing the odd chain fatty acid metabolism; some amino acids metabolism and of several other compounds also results in Propionyl CoA assembly.

The long chain fatty acids of odd chain are also oxidized by the pathway similar to that of the even number of carbon chain starting at the carboxyl end. Conversely, the substrate for the final pass through the β-oxidation cycle is a fatty Acyl CoA with 5C fatty acid. The final products are Propionyl CoA and Acetyl CoA instead of 2 moles of Acetyl CoA formed in the customary β-oxidation spiral reaction cycle. The Acetyl CoA is oxidized through the TCA cycle but the Propionyl CoA oxidation masquerade an exhilarating problem and at first peek it emerges to be a substrate incongruous for β-oxidation.

Nevertheless, the substrate is apprehended by two strikingly dissimilar pathways:

1. Methylmalonate Pathway: The pathway is reported only in animals and operates in the mitochondria of skeletal muscles, cardiac, kidney, liver, and other tissues. Propionate (Propionyl CoA) is also produced by the oxidation of some amino acid including isoleucine, methionine, threonine and valine. Acetyl CoA synthetase catalyses propionate to produce Propionyl CoA which is then carboxylated to form the D-stereoisomer of Methylmalonyl CoA by Propionyl CoA carboxylase with the cofactor biotin. The carbon dioxide (or hydrated ion, HCO3–) in this reaction is activated by biotin alliance before it gets transfer to the propionate moiety. The carboxy-biotin intermediate formation requires energy exploited by the cleavage of ATP to AMP and PPi. The epimerization of D-methylmalonyl CoA, thus formed, is enzymatically done to form L-Methylmalonyl CoA by the catalytic accomplishment of Methylmalonyl CoA epimerase. An intra-molecular rearrangement of the L-methylmalonyl CoA occurs to form Succinyl CoA by Methylmalonyl CoA mutase with cofactor Deoxyadenosyl cobalamin or Coenzyme B12. At equilibrium condition, Succinyl CoA formation is favored by a ratio of 20:1 over Methylmalonyl CoA. The so formed Succinyl CoA can then be oxidized via TCA and Succinate cycle to carbon dioxide and water. Diagonostically, in patients with vitamin B12 deficiency, both the methylmalonate and propionate are excreted in the urine in peculiarly hefty amounts. Two inheritable types of methylmalonic acidemia (aciduria) are linked in young children with failure to grow and mental retardness. In first type, the mutase protein is missing or malfunctioning since addition of coenzyme B12 to liver extracts does not reinstate the mutase activity. In the second type, nourishing huge doses of vitamin B12 alleviates the acidemia and aciduria, and addition of coenzyme B12 to liver extracts refurbishes the mutase activity. Propionic acidemia (aciduria) is another inheritable anarchy of propionate metabolism crop up due to a defect in Propionyl CoA carboxylase. Individuals with Propionic acidemia as well as those with methylmalonic acidemia are competent of oxidizing a quantity of propionate to carbon dioxide even in the deficiency of Propionyl CoA carboxylase

Methylmalonate Pathway of Propionate Metabolism in Animals

2. β-hydroxypropionate Pathway: This pathway is ubiquitous ly present in plants and is a customized form of β-oxidation spiral reactions. It adequately resolves the predicament of how plants can cope with propionic acid by a scheme not involving vitamin B12 as cobamide coenzyme. Since plants have no B12 functional enzymes, the methylmalonate pathway does not maneuver in them. So, β- hydroxypropionate pathway, thus, evades the B12 barricade in an effectual way.

Summary

In this lecture we learnt about:

The Enzymes of the fatty acid oxidation
The Energy aspects of the lipid oxidation
Odd Number Fatty Acid Oxidation

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Weblinks

Books

Lehninger Principles of Biochemistry by David L. Nelson, Albert‎ L. Lehninger, Michael‎ M. Cox. 2008. https://books.google.co.in/books?isbn=071677108X
Fatty acid metabolism and its regulation – Page v. 1984. https://books.google.co.in/books?isbn=0080860656
Lipid Metabolism by Salih Wakil. 2012. Page 43 https://books.google.co.in/books?isbn=0323144276
Regulation of Fatty Acid and Glycerolipid Metabolism: FEBS by Raymond Dils, Jens‎ Knudsen. 2014. Page 44 https://books.google.co.in/books?isbn=1483148769
Metabolism of Odd-numbered Carbon Fatty Acids in …by Eugene Marion Sabo. 1966. https://books.google.co.in/books?id=ip-hNwAACAAJ