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Lipids Fatty, oily, or waxy organic compound Mostly made up of hydrocarbons hence they show very little tendency to dissolve in water and are “hydrophobic”. Cells use different kinds as their: 1) Main energy reservoirs. Fatty acids Fat : Triglyceride 2) Structural materials e.g. in the cell membrane and surface coatings. 3) Signaling molecules. Phospholipids Sterols Waxes Fatty acids Organic compound that consists of a chain of carbon atoms with an acidic carboxyl group at one end . Carbon chain of saturated types has single bonds only; that of unsaturated types has one or more double bonds Stearic acid : Saturated fatty acid Oleic acid : Monounsaturated fatty acid Linoleic acid : Polyunsaturated fatty acid Fats: Lipids that have one, two or three fatty acids attached to glycerol Animal fats : butter, lard etc. Plant fats : groundnut oil, sunflower oil, coconut oil etc. Triglycerides Most natural fats from animal and plant sources are made up of triglycerides: fats having three fatty acid tails attached to glycerol. http://www.familyhealthavenue.com/2009/ 04/know-something-about-triglycerides/ Phospholipids Lipid with a highly polar phosphate group in its hydrophilic head, and two nonpolar, hydrophobic fatty-acid tails. Main constituent of eukaryotic cell membranes which is made up of a bilayer of phospholipids. The heads of one layer are in the interior of the cell which is a watery environment and the heads of the other layer are dissolved in the fluid exterior of the cells. http://academic.brooklyn.cuny.edu/biology/bio4fv/page/phosphb.htm http://alevelnotes.com/Biological-Membranes/128 Understand the difference between fatty acids and fat Fatty acids are stored in adipose tissue as triacylglycerols (TAG) in which fatty acids are linked to glycerol with ester linkages. Fatty acids have four major functions: 1. Fatty acids are fuel molecules stored as triacylglycerols. 2. Fatty acids are components of phospholipids and glycolipids. 3. Fatty acids are attached to proteins to localize the proteins to membranes. 4. Fatty acids function as hormones and intracellular messengers. Fatty acid degradation and synthesis consist of four steps that are the reverse of each other with regard to their chemistry. Fatty acid degradation is an oxidative process that yields acetyl CoA. Fatty acid synthesis is a reductive process that begins with acetyl CoA. Triacylglycerols (TAG) are energy rich. Because they are hydrophobic and reduced, a gram of anhydrous fat stores more than six times the energy of a gram of hydrated glycogen. TAG are stored in large droplets in the cytoplasm of adipocytes. Adipose tissue is located throughout the body, with subcutaneous (below the skin) and visceral (around the internal organs) deposits being most prominent. Consider a typical 70-kg man, who has fuel reserves of 420,000 kJ (100,000 kcal) in triacylglycerols, 100,000 kJ (24,000 kcal) in protein (mostly in muscle), 2500 kJ (600 kcal) in glycogen, and 170 kJ (40 kcal) in glucose. Triacylglycerols constitute about 11 kg of his total body weight. If this amount of energy were stored in glycogen, his total body weight would be 64 kg greater. Migratory birds use fat stores to power long flights without the opportunity to feed. Triacylglycerols from the diet form lipid droplets in the stomach. Bile acids, secreted as bile salts by the gall bladder, insert into the lipid droplets, rendering them more accessible to digestion by lipases. Lipases, secreted by the pancreas, convert the triacylglycerols into two fatty acids and monoacylglycerol. The digestion products are carried as micelles to the intestinal epithelium cells for absorption. In the intestine, triacylglycerols are reformed from free fatty acids and monoacylglycerol and packaged into lipoprotein particles called chylomicrons. The chylomicrons eventually enter the blood so that the triacylglycerols can be absorbed by tissues. The fatty acids incorporated into triacylglycerols in adipose tissue are made accessible in three stages. 1. Degradation of TAG to release fatty acids and glycerol into the blood for transport to energyrequiring tissues. 2. Activation of the fatty acids and transport into the mitochondria for oxidation. 3. Degradation of the fatty acids to acetyl CoA for processing by the citric acid cycle. Triacylglycerols are stored in adipocytes in a lipid droplet. Epinephrine and glucagon, acting through 7TM receptors, stimulate lipid breakdown or lipolysis. Protein kinase A phosphorylates perilipin, which is associated with the lipid droplet, and hormone-sensitive lipase. Phosphorylation of perilipin results in the activation of adipocyte triacylglyceride lipase (ATGL). ATGL initiates the breakdown of lipids. The glycerol released during lipolysis is absorbed by the liver for use in glycolysis or gluconeogenesis. Fatty acids are transported in the blood bound to albumin. Glycerol is absorbed by the liver, phosphorylated and converted into dihydroxyacetone phosphate or glyceraldehyde 3-phosphate. Upon entering the cell cytoplasm, fatty acids are activated by attachment to coenzyme A, in a reaction catalyzed by acyl CoA synthetase (fatty acid thiokinase). The reaction proceeds through an acyl adenylate intermediate. The reaction occurs in two steps: 1. The formation of the acyl adenylate. 2. The reaction of acyl adenylate with CoA to form acyl CoA. The reaction is rendered irreversible by the action of pyrophosphatase. After being activated by linkage to CoA, the fatty acid is transferred to carnitine, a reaction catalyzed by carnitine acyltransferase I, for transport into the mitochondria. A translocase transports the acyl carnitine into the mitochondria. In the mitochondria, carnitine acyltransferase II transfers the fatty acid to CoA . The fatty acyl CoA is now ready to be degraded. A number of diseases have been traced to a deficiency of carnitine, the transferase, or the translocase. Inability to synthesize carnitine may be a contributing factor to the development of autism in males. The symptoms of carnitine deficiency range from mild muscle cramping to severe weakness and even death. In general, muscle, kidney, and heart are the tissues primarily impaired. Muscle weakness during prolonged exercise is a symptom of a deficiency of carnitine acyltransferases because muscle relies on fatty acids as a long-term source of energy. Medium-chain (C8–C10) fatty acids are oxidized normally in these patients because these fatty acids can enter the mitochondria, to some degree, in the absence of carnitine. These diseases illustrate that the impaired flow of a metabolite from one compartment of a cell to another can lead to a pathological condition. Fatty acid degradation consists of four steps that are repeated: an oxidation, a hydration, another oxidation, followed by thiolysis. Fatty acid degradation is also called β-oxidation because oxidation occurs at the βcarbon atom. 1. Oxidation of the β carbon, catalyzed by acyl CoA dehydrogenase, generates trans-Δ2-enoyl CoA and FADH2. The electrons travel from FADH2 to electron-transferring flavoprotein (ETF). ETF-ubiquinone reductase transfers electrons from ETF to ubiquinone. 2. Hydration of trans-Δ2-enoyl CoA by enoyl CoA hydratase yields L-3hydroxyacyl CoA. 3. Oxidation of L-3-hydroxyacyl CoA by L-3-hydroxyacyl CoA dehydrogenase generates 3-ketoacyl CoA and NADH. 4. Cleavage of the 3-ketoacyl CoA by thiolase forms acetyl CoA and a fatty acid chain two carbons shorter. The reaction for one round of β-oxidation is The complete reaction for C16 palmitoyl CoA is Processing of the products of the complete reaction by cellular respiration would generate 106 molecules of ATP. β-oxidation alone cannot degrade unsaturated fatty acids. When monounsaturated fatty acids are degraded by β-oxidation, cis-Δ3-enoyl CoA is formed, which cannot be processed by acyl CoA dehydrogenase. Cis-Δ3-enoyl CoA isomerase converts the double bond into trans-Δ2 enoyl CoA, a normal substrate for β-oxidation. When polyunsaturated fatty acids are degraded by β-oxidation, cis-Δ3 enoyl CoA isomerase is also required. 2,4-Dienoyl CoA is also generated, but cannot be processed by the normal enzymes. 2,4-Dienoyl CoA is converted into trans-Δ3-enoyl CoA by 2,4-dienoyl CoA reductase, and the isomerase converts this product to trans-Δ2-enoyl CoA, a normal substrate. Unsaturated fatty acids with odd numbers of double bonds require only the isomerase. Those with even numbers of double bonds require both the isomerase and reductase. β-Oxidation of fatty acids with odd numbers of carbons generates propionyl CoA in the last thiolysis reaction. Propionyl CoA carboxylase, a biotin enzyme, adds a carbon to propionyl CoA to form methylmalonyl CoA. Succinyl CoA, a citric acid cycle component, is subsequently formed from methylmalonyl CoA by methylmalonyl CoA mutase, a vitamin B12-requiring enzyme. Cobalamin (vitamin B12)-requiring enzymes catalyze three types of reactions: 1. Intramolecular rearrangements 2. Methylations 3. Reduction of ribonucleotides to deoxyribonucleotides In mammals, vitamin B12 is required only for the conversion of L-methylmalonyl CoA into succinyl CoA and for the synthesis of methionine. The core of cobalamin is a corrin ring with a cobalt atom. Coenzyme B12 catalyzes exchanges of two groups bonded to adjacent carbon atoms. The mutase reaction begins with the generation of a 5-deoxyadenosyl radical and the Co2+ form of the coenzyme. The radical removes a hydrogen atom from the substrate, generating a substrate radical, which spontaneously rearranges: the carbonyl CoA migrates to the adjacent carbon atom that relinquished the hydrogen atom. This product radical removes a proton from 5-deoxyadenosine, regenerating the deoxyadenosyl radical and forming succinyl CoA. Oxidation of long-chain fatty acids can occur in peroxisomes. Oxidation halts with the formation of octanoyl CoA. The first dehydration in peroxisomal fatty acid degradation requires a flavoprotein dehydrogenase that generates H2O2, which is converted into water and oxygen by catalase. Subsequent steps are identical to β-oxidation. Peroxisomes do not function in patients with Zellweger syndrome. Liver, kidney, and muscle abnormalities usually lead to death by age 6. The syndrome is caused by a defect in the import of enzymes into the peroxisomes. Here we see a pathological condition resulting from an inappropriate cellular distribution of enzymes. What happens if Acetyl CoA generated during beta-oxidation cannot enter TCA cycle? Glycolysis, gluconeogenesis and fatty acid degradation Ketone bodies The acetyl CoA formed in fatty acid oxidation enters the citric acid cycle only if fat and carbohydrate degradation are appropriately balanced. Acetyl CoA must combine with oxaloacetate to gain entry to the citric acid cycle. The availability of oxaloacetate, however, depends on an adequate supply of carbohydrate. Oxaloacetate is normally formed from pyruvate, the product of glucose degradation in glycolysis, by pyruvate carboxylase. If carbohydrate is unavailable or improperly utilized, the concentration of oxaloacetate is lowered and acetyl CoA cannot enter the citric acid cycle. This dependency is the molecular basis of the adage that fats burn in the flame of carbohydrates. Steps of Gluconeogenesis Ketone bodies—acetoacetate, D-3-hydroxybutyrate and acetone—are synthesized from acetyl CoA in liver mitochondria and secreted into the blood for use as a fuel by some tissues such as heart muscle. D-3-Hydroxybutyrate is formed upon the reduction of acetoacetate. Acetone is generated by the spontaneous decarboxylation of acetoacetate. Heart muscle, Kidney (renal cortex), brain – when desperate In tissues using ketone bodies, 3-hydroxybutyrate is oxidized to acetoacetate, which is ultimately metabolized to two molecules of acetyl CoA. Ketone bodies are moderately strong acids, and excess production can lead to acidosis. An overproduction of ketone bodies can occur when diabetes, a condition resulting from a lack of insulin function, is untreated. The resulting acidosis is called diabetic ketosis. If insulin is absent or not functioning, glucose cannot enter cells. All energy must be derived from fats, leading to the production of acetyl CoA. Acetyl CoA builds up because oxaloacetate, which can be generated from glucose, is not available to replenish the citric acid cycle. Moreover, fatty acid release from adipose tissue is enhanced in the absence of insulin function. 1. Interestingly, diets that promote ketone-body formation, called ketogenic diets, are frequently used as a therapeutic option for children with drug-resistant epilepsy. Ketogenic diets are rich in fats and low in carbohydrates, with adequate amounts of protein. In essence, the body is forced into starvation mode, where fats and ketone bodies become the main fuel source. How such diets reduce the seizures suffered by the children is currently unknown. Research Atkins Diet. Fats are converted into acetyl CoA which is then processed by the citric acid cycle. Oxaloacetate, a citric acid cycle intermediate, is a precursor to glucose. However, acetyl CoA derived from fats cannot lead to the net synthesis of oxaloacetate or glucose because, although two carbons enter the cycle when acetyl CoA condenses with oxaloacetate, two carbons are lost as CO2 before oxaloacetate is generated. Saturated and trans unsaturated fatty acids (“trans fat”) are commercially synthesized from polyunsaturated fatty acids to increase stability for storage and cooking. Consumption of large amounts of saturated and trans fat has been linked to obesity, type 2 diabetes and atherosclerosis. 1. Fatty acid synthesis occurs in the cytoplasm, whereas degradation occurs in the mitochondrial matrix. 2. Intermediates in synthesis are linked to the sulfhydryl group of acyl carrier protein (ACP), whereas intermediates in degradation are linked to the sulfhydryl group of CoA. 3. Fatty acid synthase in higher organisms is a single polypeptide containing all of the required enzyme activities. The enzymes for degradation do not appear to be associated with one another. 4. The new fatty acid grows by the sequential addition of two carbon units from malonyl ACP, which is derived from acetyl CoA. The decarboxylation of malonyl CoA powers synthesis. 5. The reductant in fatty acid synthesis is NADPH, whereas the oxidants in degradation are NAD+ and FAD. 6. The isomeric form of the hydroxyacyl intermediate differs: the L form is found in degradation and the D form in synthesis. Malonyl CoA is synthesized by acetyl CoA carboxylase 1, a biotin-requiring enzyme. The formation of malonyl CoA occurs in two steps: Acyl carrier protein (ACP) is a 77 amino acid protein. Phosphopantetheine, a component of CoA also, is attached to a serine residue of the protein. Fatty acid synthesis occurs on the acyl carrier protein (ACP), a polypeptide linked to CoA. Intermediates are linked to the sulfhydryl group of the CoA attached to ACP. Acetyl transacylase and malonyl transacylase attach substrates to the ACP. β-Ketoacyl synthase catalyzes the condensation of acetyl ACP and malonyl ACP to form acetoacetyl ACP. The next three steps—a reduction, dehydration, and another reduction—convert the keto group at carbon 3 to a methylene group (-CH2-), forming butyryl ACP. The corresponding enzymes are β-ketoacyl reductase, 3-hydroxylacyl dehydratase, and enoyl reductase. NADPH is the source of reducing power. The second round of synthesis begins with the condensation of malonyl CoA with the newly synthesized butyryl ACP, forming C6-β-ketoacyl ACP. The reduction, dehydration, reduction sequence is repeated. Synthesis continues until C16-acyl ACP is formed; this is cleaved by thioesterase to yield palmitate. The elongation cycles continue until C16-acyl ACP is formed. This intermediate is a good substrate for a thioesterase that hydrolyzes C16-acyl ACP to yield palmitate and ACP. The thioesterase acts as a ruler to determine fatty acid chain length. The bacterial enzyme that catalyzes this step, enoyl reductase, can be inhibited by triclosan, a broad-spectrum antibacterial agent that is added to a variety of products such as toothpaste, soaps, and skin creams. The reactions of fatty acid synthesis are similar in E. coli and animals. In animals, all of the enzymes required for fatty acid synthesis are components of a single polypeptide chain. The functional enzyme is composed of two identical chains. The enzyme consists of two distinct compartments. The selecting and condensing compartment binds the acetyl and malonyl substrates and condenses them. The modification compartment carries out the reduction and dehydration activities required for elongation. A catalytic cycle of mammalian fatty acid synthase involves seven steps. 1. ACP delivers an acetyl unit to the synthase (KS) and accepts a malonyl unit from MAT. 2. ACP delivers the malonyl unit to KS, which forms the keto acyl product, still attached to ACP. 3. ACP visits the reductase (KR), which reduces the keto group to an alcohol. 4. The alcohol product is delivered to the dehydratase (DH), which introduces a double bond with the release of water. 5. The enoyl product visits the reductase (ER), which reduces the double bond. 6. ACP hands off the reduced product to KS and receives another malonyl from MAT. 7. KS condenses the two molecules on ACP, which is ready for another reaction cycle. Function of Fatty Acid Synthase is thematically very similar to Pyruvate Dehydrogenase Complex The stoichiometry for the synthesis of palmitate is The synthesis of the required malonyl CoA is described by the following reaction Thus, the stoichiometry for the synthesis of palmitate from acetyl CoA is Citrate, synthesized in the mitochondria, is transported to the cytoplasm and cleaved by ATPcitrate lyase to generate acetyl CoA for fatty acid synthesis. Fatty acid synthesis requires reducing power in the form of NADPH. Some NADPH can be formed from the oxidation of oxaloacetate, generated by ATPcitrate lyase, by the combined action of cytoplasmic malate dehydrogenase and malic enzyme. Pyruvate formed by malic enzyme enters the mitochondria where it is converted into oxaloacetate by pyruvate carboxylase. The sum of the reactions catalyzed by malate dehydrogenase, malic enzyme, and pyruvate carboxylase is Additional NADPH is synthesized by the pentose phosphate pathway. Tumors require large amounts of fatty acid synthesis to produce precursors for membrane synthesis. β-Ketoacyl ACP synthase inhibitors retard tumor growth. Mice treated with synthase inhibitors also showed dramatic weight loss, suggesting that such drugs may be used to treat obesity. Acetyl CoA carboxylase may also be a target for inhibiting cancer cell growth. Fatty acid synthase cannot generate fatty acids longer than C16 palmitate. Longer fatty acids are synthesized by enzymes attached to the endoplasmic reticulum. These enzymes extend palmitate by adding two-carbon units, using malonyl CoA as a substrate. The introduction of double bonds is catalyzed by a complex of three membranebound proteins: NADH-cytochrome b5 reductase, cytochrome b5 and a desaturase. Mammals lack the enzymes that introduce double bonds beyond carbon 9. Thus, linoleate and linolenate are essential fatty acids that must be obtained in the diet. Arachidonate, a 20-carbon fatty acid with four double bonds, is derived from linoleate. Arachidonate is a precursor for a variety of signal molecules 20 carbons long, collectively called the eicosanoids. These signal molecules, which include prostaglandins, are local hormones because they are short-lived and only affect nearby cells. Aspirin blocks access to the active site of the enzyme that converts arachidonate into prostaglandin H2. Because arachidonate is the precursor of other prostaglandins, prostacyclin, and thromboxanes, blocking this step interferes with many signaling pathways. Aspirin’s ability to obstruct these pathways accounts for its wide-ranging effects on inflammation, fever, pain, and blood clotting. The mammalian fatty acid synthase is a member of a family of complex enzymes called megasynthases. Such enzymes synthesize polyketides and nonribosomal peptides, some of which are important antibiotics. Acetyl CoA carboxylase 1 is subject to regulation on several levels. Carboxylase 1 is inhibited when phosphorylated by AMP-dependent kinase (AMPK). Inhibition due to phosphorylation is reversed by protein phosphatase 2A. Citrate activates carboxylase, in conjunction with a protein MIG12, by facilitating the formation of active polymers of the carboxylase. Citrate mitigates inhibition due to phosphorylation. Palmitoyl CoA, the end product of fatty acid synthase, inhibits carboxylase by causing depolymerization of the enzyme. Acetyl CoA carboxylase 2, a mitochondrial enzyme, inhibits fatty acid degradation because its product, malonyl CoA, prevents the entry of fatty acyl CoA into the mitochondria by inhibiting carnitine acyltransferase 1. Glucagon and epinephrine inhibit carboxylase by enhancing AMPK activity. Insulin stimulates the dephosphorylation and activation of carboxylase. The enzymes of fatty acid synthesis are regulated by adaptive control. If adequate fats are not present in the diet, the synthesis of enzymes required for fatty acid synthesis is enhanced.