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Home , Fatty acid degradation, Fatty acid synthesis, Lipid metabolism, Lipolysis

Section 12: and Chapter 27: Chapter 28:

By the end of this section, you should be able to: Ø Identify the repeated steps of fatty acid degradation. Ø Describe bodies and their role in metabolism. Ø Explain how fatty acids are synthesized. Ø Explain how is regulated. Lecture: Ch27-28-29

CHAPTER 27 Fatty Acid Synthesis

CHAPTER 28 Fatty Acid Degradation Chapter 27 Outline Fatty acid degradation is a key source for mammals during hibernation.

• Fatty acids are stored in as triacylglycerols (TAG) in which fatty acids are linked to with linkages.

• Adipose tissue is located throughout the body, with subcutaneous (below the skin) and visceral (around the internal organs) deposits being most prominent. Lipid degradation

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 for transport to energy-requiring 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 cycle. • Triacylglycerols are stored in as a . • and , acting through 7TM receptors, stimulate lipid breakdown or . • kinase A phosphorylates perilipin, which is associated with the lipid droplet, and -sensitive . • of perilipin results in the activation of triacylglyceride lipase (ATGL). • ATGL initiates the breakdown of . Chanarin-Dorfmam syndrome, characterized by dry skin, enlarged and muscle, and mild cognitive disability, results if ATGL activity is compromised. • The glycerol released during lipolysis is absorbed by the liver for use in or . Triacylglycerols in adipose tissue are converted into free fatty acids in response to hormonal signals

The phosphorylation of perilipin restructures the lipid droplet and releases the coactivator of ATGL. The activation of ATGL by binding with its coactivator initiates the mobilization. Hormone-sensitive lipase releases a fatty acid from diacylglycerol. completes the mobilization process.

Abbreviations: 7TM, seven transmembrane; ATGL, adipose lipase; CA, coactivator; HS lipase, hormone-sensitive lipase; MAG lipase, monoacylglycerol lipase; DAG, diacylglycerol; TAG, triacylglycerol. Lipolysis generates fatty acids and glycerol

Acyl CoA is an activated form of fatty acid Acyl

After being activated by linkage to CoA, the fatty acid is transferred to carnitine, a reaction catalyzed by carnitine 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.

Clinical Insights: Muscle, , and use fatty acids as a fuel. Pathological conditions results if the acyltransferase or the translocase are deficient.

Carnitine deficiencies can be treated by carnitine supplementation. Fatty acid degradation consists of four steps that are repeated. 1. Oxidation of the β carbon, catalyzed by acyl CoA dehydrogenase, generates trans-Δ2-enoyl CoA and FADH2.

2. Hydration of trans-Δ2-enoyl CoA by enoyl CoA hydratase yields L-3-hydroxyacyl CoA.

3. Oxidation of L-3-hydroxyacyl CoA by L-3- hydroxyacyl dehydrogenase generates 2- ketoacyl CoA and NADH.

4. Cleavage of the 3-ketoacyl CoA by forms acetyl CoA and a fatty acid chain two carbons shorter.

Fatty acid degradation is also called β-oxidation. The reaction sequence for the degradation of fatty acids

Fatty acids are degraded by the repetition of a four-reaction sequence consisting of oxidation, hydration, oxidation, and thiolysis.

Two carbon units are sequentially removed from the carboxyl end of the fatty acid

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 would generate 106 molecules of ATP. Answer: The steps are (1) oxidation by FAD; (2) hydration; (3) oxidation by NAD+; (4) thiolysis to yield acetyl CoA. In symbolic notation, the β-carbon atom is oxidized. • 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 .

• 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. Even number 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 carboxylase, a , adds a carbon to propionyl CoA to form methylmalonyl CoA

Succinyl CoA, a component, is subsequently formed from methylmalonyl CoA by methylmalonyl CoA mutase, a B12 requiring enzyme. • —acetoacetate, 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.

• 3-Hydroxybutyrate is formed upon the reduction of acetoacetate. Acetone is generated by the spontaneous of acetoacetate.

• In tissues using ketone bodies, 3-hydroxybutyrate is oxidized to acetoacetate, which is ultimately metabolized to two molecules of acetyl CoA. The formation of ketone bodies The utilization of D-3- hydroxybutyrate and acetoacetate as a fuel

The ketone bodies—acetoacetate, D-3-hydroxybutyrate, and acetone—are formed from acetyl CoA primarily in the liver. Enzymes catalyzing these reactions are (1) 3-ketothiolase, (2) hydroxymethylglutaryl CoA synthase, (3) hydroxymethylglutaryl CoA cleavage enzyme, and (4) D-3-hydroxybutyrate dehydrogenase. Acetoacetate spontaneously decarboxylates to form acetone

Ketogenic diets, rich in and low in but with adequate , lead to formation of substantial amounts of ketone bodies.

Ketogenic diets may have therapeutic properties: For reasons not yet established, such diets reduce the seizures in children suffering from drug-resistant epilepsy. 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 .

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 regenerated.

High levels of acetoacetate in the blood signify an abundance of acetyl units and lead to a decrease in the rate of lipolysis in adipose tissue. Answer: D-3-Hydroxybutyrate is more energy rich because its oxidation potential is greater than that of acetoacetate. After having been absorbed by a , d-3-hydroxybutyrate is oxidized to acetoacetate, generating high- energy electrons in the form of NADH. The acetoacetate is then cleaved to yield to acetyl CoA. • Ketone bodies are moderately strong acids, and excess production can lead to .

• An overproduction of ketone bodies can occur when diabetes, a condition resulting from a lack of function, is untreated. The resulting acidosis is called diabetic .

• 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 released from adipose tissue is enhanced in the absence of insulin function. Diabetic ketosis results when insulin is absent

In the absence of insulin, fats are released from adipose tissue, and glucose cannot be absorbed by the liver or adipose tissue. The liver degrades the fatty acids by b-oxidation, but cannot process the acetyl CoA because of a lack of glucose-derived oxaloacetate (OAA). Excess ketone bodies are formed and released into the blood. Abbreviation: CAC, citric acid cycle. • Glucose is the predominant fuel for the brain.

• During , protein degradation is initially the source of carbons for gluconeogenesis in the liver. The glucose is then released into the blood for the brain to use.

• After several days of fasting, the brain begins to use ketone bodies as a fuel.

• Ketone body use curtails (reduces) protein degradation and thus prevents tissue failure. Moreover, ketone bodies are synthesized from fats, the largest energy store in the body. Fuel choice during starvation

Saturated and trans unsaturated fatty acids are synthesized commercially to enhance the shelf life and heat stability of fats for food preparation.

Studies suggest that excess consumption of these fats results in obesity, heart disease and type 2 diabetes. Lecture: Ch27&Ch28

CHAPTER 28 Fatty Acid Synthesis Outline 1. The first stage of fatty acid synthesis is transfer of acetyl CoA out of the mitochondria into the . Citrate is transported into the cytoplasm and cleaved into oxaloacetate and acetyl CoA.

2. The second state is the activation of acetyl CoA to form malonyl CoA.

3. The third stage is the repetitive addition and reduction of two carbon units to synthesize C16 fatty acid. Synthesis occurs on an acyl carrier protein, a molecular scaffold.

Citrate, synthesized in the mitochondria, is transported to the cytoplasm and cleaved by ATP-citrate lyase to generate acetyl CoA for fatty acid synthesis. The transfer of acetyl CoA to the cytoplasm

PFK

Acetyl CoA is transferred from mitochondria to the cytoplasm, and the reducing potential of NADH is concomitantly converted into that of NADPH by this series of reactions • Fatty acid synthesis requires reducing power in the form of NADPH.

• Some NADPH can be formed from the oxidation of oxaloacetate, generated by ATP-citrate lyase, by the combined action of cytoplasmic and malic enzyme.

• Pyruvate formed by malic enzyme enters the mitochondria where it is converted into oxaloacetate by .

• The sum of the reactions catalyzed by malate dehydrogenase, malic enzyme, and pyruvate carboxylase is:

• Additional NADPH is synthesized by the pentose phosphate pathway. PATHWAY INTEGRATION: Fatty acid synthesis

Fatty acid synthesis requires the cooperation of various metabolic pathways located in different cellular compartments. • Fatty acid synthesis starts with the carboxylation of acetyl CoA to malonyl CoA, the activated form of acetyl CoA

• Malonyl CoA is synthesized by acetyl CoA carboxylase (ACC), a biotin- requiring enzyme.

The formation of malonyl CoA occurs in two steps:

ACC

(the activated form of CO2) (the activated form of acetyl CoA) • , a complex of enzymes, catalyzes the formation of fatty acids from from acetyl CoA, malonyl CoA, and NADPH is called fatty acid synthase.

• Fatty acid synthesis occurs on the acyl carrier protein (ACP), a polypeptide linked to CoA. Intermediates are linked to the sulfhydryl group of the pantothenate 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.

• 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, which is cleaved by thioesterase to yield palmitate. 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: • 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.

Ø The selecting and condensing compartment, which binds the acetyl and malonyl substrates and condenses them.

Ø The modification compartment, which carries out the reduction and dehydration activities required for elongation. Animal fatty acid synthase domain structure

Binds acetyl and malonyl substrates

Answer: Acetyl CoA is the basic substrate for fatty acid synthesis. It is transported out of mitochondria in the form of citrate. After the formation of acetyl CoA, the resulting pyruvate is transported back into the mitochondria with a concomitant formation of NADPH, the reducing power for fatty acid synthesis. Additional NADPH can be generated by the pentose phosphate pathway. Malonyl CoA, the ultimate substrate for fatty acid synthesis is formed by the carboxylation of acetyl CoA. • Tumors require large amounts of fatty acid synthesis to produce precursors for membrane synthesis.

• β-Ketoacyl synthase inhibitors retard tumor growth.

• Mice treated with β-Ketoacyl synthase inhibitors also showed dramatic weight loss, suggesting that such drugs may be used to treat obesity.

• Acetyl CoA carboxylase inhibitors may also be potential chemotherapy agents. • β-Hydroxybutyric acid, when attached to ACP or CoA, is a substrate in fatty acid synthesis and degradation, and is a ketone body as well.

• An isomer of this key biochemical, γ-hydroxybutyric acid is a potent, illegal drug. • Fatty acid synthase cannot generate fatty acids longer than C16 palmitate.

• Longer fatty acids are synthesized by enzymes attached to the .

• These enzymes extend palmitate by adding two carbon units, using malonyl CoA as a substrate. • Enzymes bound to the endoplasmic reticulum introduce double bonds into saturated fatty acids.

For instance:

• Mammals lack the enzymes that introduce double bonds beyond carbon 9.

• 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 .

• These signal molecules, which include , are local because they are short lived and only affect nearby cells. Arachidonate is the major precursor of hormones Structures of several eicosanoids • Aspirin prevents the use of arachidonate as a substrate for the enzyme that generates H2 .

• Blocking this step effects many signaling pathways, accounting for the wide range effects of aspirin. Acetyl CoA carboxylase 1 and 2 are subject to regulation on several levels.

Carboxylase 1, a cytoplasmic enzyme, is inhibited when phosphorylated by AMP- activated kinase (AMPK). Inhibition due to phosphorylation is reversed by protein phosphatase 2A.

Citrate actives carboxylase 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.

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 acyl transferase 1. The control of acetyl CoA carboxylase Dependence of the catalytic activity of acetyl CoA carboxylase on the concentration of citrate. • Glucagon and epinephrine inhibit carboxylase by enhancing AMPK activity, by which they prevent fatty acid synthesis .

• Insulin stimulates the dephosphorylation and activation of carboxylase, by which it stimulates fatty acid synthesis .

• The enzymes of fatty acid synthesis are regulated by adapative control. If adequate fats are not present in the diet, the synthesis of enzymes required for fatty acid synthesis is enhanced.

Answer Malonyl CoA, the substrate for fatty acid synthesis, inhibits carnitine acyl transferase I, thus preventing the transport of fatty acids into mitochondria for degradation. Palmitoyl CoA inhibits acetyl CoA carboxylase, the transport of citrate into the cytoplasm, and glucose 6- phosphate dehydrogenase, the controlling enzyme of the pentose phosphate pathway. • One pathway for ethanol processing consists of two steps and leads to excess production of NADH:

• Excess NADH inhibits gluconeogenesis and enhances lactate production, which may result in lactic acidosis.

• Excess NADH inhibits fatty acid degradation and stimulates fatty acid synthesis, leading to the accumulation of fats in the liver. • Liver can convert some of the acetate generated by aldehyde dehydrogenase into acetyl CoA, but the acetyl CoA cannot be processed by the citric acid cycle because of the paucity or lack of NAD+.

• The build-up of acetyl CoA can lead to ketone body secretion by the liver, which exacerbates the acidosis caused by lactate accumulation.

• If acetate cannot be processed, acetaldehyde accumulates. Acetaldehyde is very reactive and modifies reactive groups of proteins, causing a loss of protein function.

• As protein damage accumulates, liver function can fail.