Fatty Acid Metabolism

Fatty Acid Metabolism Topics:

1. Fatty acids: High Energy Reserves 2. Introduction to Fatty Acid Metabolism 3. Fatty Acids as Fuels: Mechanism 4. Regulation and Control 5. Transport and Digestion

1. Fatty Acids: High Energy Reserves Fatty acids: Fatty acids form the building blocks of phospholipids and glycolipids. These are amphipathic molecules and this characteristic feature makes them as a significant section of the biological membranes. Proteins which need to serve the signaling purposes are modified by the fatty acids by the formation of covalent bonding between the two. This facilitates these proteins to reach their targets located on the membrane cells accurately. Fatty acids are the top fuel molecules inside the body which release enormous energy on burning. Neutral fats: When Fatty acids are not in readymade use, they are stored as triacylglycerols or triglycerides. Since they can be utilized for the future fuel requirements, they are termed as ‘neutral fats’. These are uncharged esters of fatty acids with glycerol. Triacylglycerols are highly concentrated with energy since they are anhydrous and in reduced condition. The non polar nature of the triacylglycerols helps them to be stored in an anhydrous form. On the other hand, carbohydrates and protein molecules are polar in nature and hence mostly present in hydrated conditions.

The glycogen store of an organism has the capacity to maintain the energy for biological activities of the cell at most for about 24 hours, but triacylglycerols stores supply energy nearly for a week. When the fatty acids assemble from triacylglycerols get oxidized, energy needed for the organism can be easily obtained. A gram of anhydrous fat can store nearly six times greater energy than a gram of anhydrous glycogen reserve. Moreover, fatty acid derivatives serve as hormones and intracellular messengers also.

2. Introduction to Fatty Acid Metabolism A fatty acid molecule consists of a long hydrocarbon chain and a terminal carboxylate group. ƒ ƒ

Fatty acid metabolism includes the fatty acid synthesis and degradation. Fatty acid degradation and synthesis are comparatively simple processes that are fundamentally the reverse of each other.

Fatty acid degradation: The process of degradation converts an aliphatic compound into a set of activated acetyl units (acetyl CoA) that can be processed by the citric acid cycle. ƒ Such an activated fatty acid undergoes oxidation and a double bond is introduced. ƒ This double bond is hydrated to bring in oxygen. ƒ The alcohol thus formed is oxidized to a ketone. ƒ Finally the four carbon fragment is cleaved by coenzyme A to yield acetyl CoA and a fatty acid chain which is two carbons shorter. Activated acyl group (polymer with additional two carbon atoms) ↓ Oxidation ↓ Hydration ↓ Oxidation ↓ Cleavage ↓ [Activated acyl group] + Activated acetyl group Condensed down (Monomer) To two carbon atoms) In case, if the fatty acid has an even number of carbon atoms and is saturated, the process will be repeated again until the fatty acid is completely converted into acetyl CoA units.

Fatty acid synthesis: Fatty acid synthesis is effectively the reverse of degradation process. The final synthesized product is a polymer, and hence obviously the process is initiated with the collecting of monomers together. Considering the case of an activated acyl group and malonyl units, the malonyl unit is condensed with the acetyl unit and results in the formation of a four-carbon fragment. The carbonyl is reduced in order to produce the required hydrocarbon chain. The process will proceed exactly opposite to that of degradation process. [Activated acyl group (monomer) + Activated malonyl group (monomer) ] ↓ Condensation ↓ Reduction ↓ Dehydration ↓ Reduction ↓ Activated acyl group (polymer with additional two carbon atoms) The resulted four-carbon fragment will be reduced, dehydrated, and reduced again for carrying the carbonyl group to the level of a methylene group along with butyryl CoA formation. Also, another activated malonyl group condenses with the butyryl unit and the process will continue until a C16 fatty acid is synthesized.

3. Fatty acids as fuels: Mechanism Hibernating animals are best examples for utilizing fat reserve as fuel. Bears go on hibernation for about 7 months. In this entire period, the energy is derived from degradation of fat stores in its body. Migratory birds also illustrate the utilization of triacylglycerols when they fly great distances without eating. Ruby-throated Hummingbirds fly non-stop between New England and West Indies (approximately 2400 km) across the Gulf of Mexico. The golden plover flies from Alaska to the Southern tip of South America without any feed over the ocean. This can be achieved only because of the stored fat in their bodies. Fatty acid degradation: This is the process in which fatty acids are broken down into their metabolites thereby releasing energy to the target cells. Three major steps are involved in this. (i) Lipolysis of and release from adipose tissue,

(ii) (iii)

Activation and transport into mitochondria, β-oxidation

Triacylglycerides are the foremost energy reserves in the body. On hydrolysis, in presence of lipases, it can be split into glycerol and fatty acids.

The glycerol thus formed can be metabolized in the glycolytic pathway upon oxidation which occurs in the outer face of the inner mitochodrial membrane to form dihydroxyacetone phosphate. Both the electrons released during this oxidation are received by ubiquinone (Q), and are directly fed into the electron transport chain. Fatty acids undergo the different β-oxidation pathway which happens in the mitochondrion before which they needs to be activated. This activation reaction takes place in the cytoplasm. It involves the transformation of the fatty acid into its acyl-CoA derivative.

The thioester bonds are very energetic and hence an ATP gets hydrolyzed into AMP, which is equivalent to the hydrolysis of 2 ATP to 2 ADP in this process. Mithochondrial inner membrane is impermeable to acyl-CoA molecules. To overcome this difficulty and enter inside, these molecules will react with a "special" aminoacid, called carnitine, thereby releasing CoA. The sterified carnitine thus formed is carried into the mitochondial matrix with the help of a specific membrane-bound transport complex. Inside the mitochondrion, carnitine transfers the acyl group to another CoA molecule. The same transport complex helps the free carnitine in returning back to the cytoplasm also. In this process, there is no net CoA transport into the mitochondrion. Separate cytoplasmic and mitochondrial CoA pools are reserved. In simple terms, fatty acids β-oxidation is a cycle composed of three successive reactions, which are matching to the final part of the citric acid cycle: dehydrogenation, hydration of the newly formed C=C double bond, and oxidation of the alcohol to a ketone. From the product of these reactions, the enzyme thiolase releases acetyl-CoA and an acyl-CoA with two carbon atoms less than the original acyl-CoA. The consecutive rounds of the cycle eventually lead to the total degradation of evenchain fatty acids in acetyl-CoA, which can be later completely oxidized to CO2 through the citric acid cycle. In the last stage of β-oxidation, odd-chain fatty acids yield acetyl-CoA and propionylCoA. For propionyl-CoA to be used by the citric acid cycle it must gain an extra carbon atom, and this is accomplished by carboxilation.

Methylmalonyl-CoA formed in this reaction is then rearranged succinyl-CoA, in a cobalamine (a vitamin B12 derivative)-assisted reaction. Even-chain fatty acids cannot be used for net synthesis of oxaloacetate, and therefore are not a suitable substrate for gluconeogenesis. Succinyl-CoA is an intermediate in the citric acid cycle and also a precursor of heme biossynthesis. It can be oxidized by the citric acid cycle to malate. When malate diffuses into the cytoplasm, it can be utilized in gluconeogenesis. In the cytoplasm, malate can also be decarboxylated to pyruvate by the malic enzyme, with associated NADPH production. Pyruvate formed as a result of this reaction can enter the mitochondrion and be oxidized completely to CO2 by the citric acid cycle. Much of the acetyl-CoA produced by fatty acid β-oxidation in liver mitochodria is converted in acetoacetate and ketone bodies. These molecules can be used by the heart and skeletal muscle to produce energy. The sole energy source of brain is glucose during normal conditions. It can also use ketone bodies during a long fasting period like more then two or three days. Ketogenesis refers to the synthesis of ketone bodies. Ketogenesis starts with the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA.

Condensation of another acetyl-CoA molecule yields 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA). The basic mechanism of this reaction is identical to the condensation of oxaloacetate with acetyl-CoA to produce citrate which forms the first step in the citric acid cycle. HMG-CoA is afterwards cleaved in acetoacetate and acetyl-CoA. Acetoacetate is transported into the bloodstream where it is distributed to the tissues. If it is absorbed, it reacts with succinyl-CoA in mitochondria, to form succinate and acetoacetyl-CoA, which can be cleaved by thiolase into two molecules of acetyl-CoA.

Fatty acid synthesis: When acetyl-CoA is abundant, liver and adipose tissue starts synthesizing fatty acids. The synthesis pathway is quite similar to the reverse of β-oxidation, but consists of several significant differences. Fatty acid synthesis takes place in the cytoplasm, rather than in the mitochondrion. Here NADPH serves as the electron donor and the acyl carrier group is Acyl Carrier Protein (ACP) instead of coenzyme A. Fatty acids synthesis uses acetyl-CoA as main substrate. Still the process is quite endergonic and requires the activation of acetyl-CoA. This happens through carboxylation process. Like other carboxylases, carboxilase uses biotin as a prosthetic group.

Fatty acid synthesis happens in the cytoplasm, but acetyl-CoA is produced in the mitochondrion. Therefore acetyl-CoA must cross the inner mitochondrial membrane so that it can be used in fatty acid synthesis. This can be done by the citrate shuttle. Citrate is formed in the mitochondrion by condensing acetyl-CoA with oxaloacetate and diffuses through the membrane into the cytoplasm, where it gets cleaved by citrate-lyase into acetyl-CoA and oxaloacetate. When oxaloacetate reduces with malate, return to the mitochondrial matrix. Malate can also be used to produce part of the NADPH needed for fatty acid synthesis, through the action of the malic enzyme. The remainder of the NADPH needed for fatty acid synthesis has to be produced by the pentose phosphate pathway.

4. Regulation and Control Hormone-sensitive lipase (HSL) is the enzyme that hydrolyses triacylglycerides to free fatty acids from fats, the process simply termed as lipolysis. Even though, the recent studies revealed the fact that at most HSL converts triacylglycerides to monoglycerides and free fatty acids. Monoglycerides are then hydrolyzed by monoglyceride lipase. Adipose triglyceride lipase play a special role in converting triacylglycerides to diacylglycerides, since diacylglycerides are the best substrate for HSL which is regulated by the hormones insulin, glucagon, norepinephrine, and epinephrine. Glucagon delas with low blood glucose, and epinephrine is associated with increased metabolic demands. Energy is needed in both the conditions, and the oxidation of fatty acids is increased to meet that demand. Glucagon, norepinephrine, and epinephrine bind to the G protein-coupled receptor, which activates adenylate cyclase to produce the second messenger cyclic AMP. cAMP consequently activates protein kinase A, which phosphorylates and activates the hormone-sensitive lipase. When blood glucose level is high, lipolysis is inhibited by insulin. Insulin activates protein phosphatase 2A, which dephosphorylates HSL, thus inhibiting its activity. Insulin also activates the enzyme phosphodiesterase, which breaks down cAMP and stop the rephosphorylation effects of protein kinase A.

5. Transport and Digestion Fatty acids are usually ingested as triglycerides. These cannot be easily absorbed by the intestine. They are broken down into free fatty acids and monoglycerides by pancreatic lipase. This enzyme forms a 1:1 complex with a protein called colipase which is necessary for its activity. The activated complex can only work at a water-fat interface: and it is therefore very essential that fatty acids (FA) to be emulsified by bile salts for optimal activity of these enzymes. People, who have had their gallbladder removed due to gall stones, will have great difficulty in digesting fats. Most are absorbed as free fatty acids and 2-monoglycerides, but a small fraction is absorbed as free glycerol and as diglycerides. Once across the intestinal barrier, they are reformed into triglycerides and packaged into chylomicrons or liposomes, which are released into the lymph system and then finally into the blood. Eventually, they bind to the membranes of hepatocytes, adipocytes or muscle fibers, where they are either stored or oxidized for energy. When blood sugar is low, glucagon provides signals the adipocytes to activate hormone sensitive lipase, and to convert triglycerides into free fatty acids. These have very low

solubility in the blood; still the most abundant protein in blood, serum albumin, binds free fatty acids, increasing their effective solubility. Thus, serum albumin is capable of transporting fatty acids to organs such as muscle and liver for oxidation when blood sugar is low.

Points to remember: •

A fatty acid molecule consists of a long hydrocarbon chain and a terminal carboxylate group.

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Triacylglycerols are uncharged esters of fatty acids with glycerol called as ‘neural fats’.

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The energy yield from the complete oxidation of fatty acid is very high about 38 kJ g-1 in comparison with the energy yield of about 17 kJ g-1 in case of proteins and carbohydrates.

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The high calorific value of fatty acids is a result of their highly reduced nature unlike the hydrated proteins and carbohydrates.

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Fatty acid degradation is carried out in three steps: Lipolysis, Activation and transport into mitochondria and β-oxidation.

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Lipolysis is carried out by lipases.

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Fatty acids freed from glycerol enter blood and muscle fiber by the process of diffusion.

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Beta oxidation splits long carbon chains of the fatty acid into Acetyl CoA, which can eventually enter Citric Acid cycle.

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Fatty acid synthesis is carried out by the repeated happening of the following reactions in sequence: Condensation, reduction, dehydration, and reduction.

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High blood glucose level is lowered with the action of insulin hormone

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Glucagon helps in maintaining the blood glucose at normal levels when the glucose level goes low.

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Pancreatic lipase helps in breaking down of triglycerides into free fatty acids and monoglycerides by forming a complex along with the protein along with colipase, thus keeps itself in an activated state.