l08 Metabolism in Liver 2 - Fatty Acid Metabolism (Mod)

Topic 8 : Metab. Liver 2 1

Topic 8 : Metabolism in Liver 2• Objectives

Synthesis and Storage of fatty acids Comparison of synthesis and breakdown

of fatty acids Synthesis of triacylglycerol Ketone body formation Utilisation of ketone bodies

Topic 8 : Metab. Liver 2 2

Fatty Acid Synthesis Early studies showed that synthesis of fatty acids not a reversal of degradation, although pathways very similar.

The two processes occur in different compartments of cells.

Requirement for bicarbonate in synthesis but not incorporated into product.

• Fatty acid synthesis from acetyl-CoA and malonyl-CoA occurs by a series of reactions, catalyzed in bacteria by seven different enzymes. • In eukaryotes, synthesis catalysed by Fatty Acid Synthase System (FAS).• NADPH serves as electron donor in two reactions involving substrate reduction. The NADPH is produced mainly by the Pentose Phosphate Pathway.

Topic 8 : Metab. Liver 2 3

Conversion of Carbohydrate to Fat

Diet of humans mainly carbohydrate. Storage of carbohydrate limited because more

“bulky”. Instead stored as fat; weight for weight fats

yield more energy than carbohydrates. Excess carbohydrate converted to fats; hence

requirement for conversion of carbohydrate to fats.

Central metabolite is acetyl CoA, formed from oxidative decarboxylation of pyruvate and β-oxidation of fatty acids.

Topic 8 : Metab. Liver 2 4

Acetyl Co A as key Intermediate between fat and carbohydrate metabolism

Carbohydrates can be converted to fats but not vice-versa except propionyl CoA and glycerol.

PDH virtually irreversible → pyruvate cannot be formed from acetyl CoA

Transfer of Acetyl units and Reducing Equivalents for Fatty Acid Synthesis

Acetyl CoA is produced in the mitochondria whereas fatty acid synthesis occurs in the cytosol Acetyl CoA like all CoASH derivatives cannot penetrate the mitochondrial membrane. Shuttle mechanism exists which not only transports the acetyl units but also provides mechanism to supply some of the NADPH

1 = Citrate synthase2 = Citrate lyase3 = Malate dehydrogenase4 = Malic enzyme5 = Pyruvate carboxylase

1 2

3 3

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In fatty acid oxidation, all the intermediates are linked to a carrier molecule, coenzyme A (CoASH) Fatty acid synthesis involves a carrier molecule known as acyl carrier protein (ACP)

Acyl Carrier Protein

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Formation of Acetyl- and Malonyl–acyl carrier protein (ACP)

• First committed step in fatty acid synthesis is carboxylation of acetyl CoA to form malonyl CoA

• Catalysed by acetyl CoA carboxylase, an enzyme which is allosterically stimulated by citrate, contains biotin as prosthetic group.

• Elongation steps in FA synthesis involve all intermediates linked to terminal sulphydryl group of phosphopantetheine of ACP.

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Reactions of FA Synthesis• Cycle of reactions involve

Condensation of acetyl–ACP and malonyl–ACP with release of CO2.

Reduction with NADPH.Dehydration to yield a trans double

bond.Reduction with NADPH.

• First cycle yields butyryl–ACP (C4).• Cycle repeats with malonyl–ACP adding

two-carbon units.• Chain elongation until palmitoyl-ACP

(C16).• Molecule not accepted by acyl–malonyl–

ACP condensing enzyme.• Hydrolysed by thioesterase to give

palmitate and ACP.

Topic 8 : Metab. Liver 2 12

Enzymes in Fatty Acid Synthesis

• Overall reaction8Acetyl CoA + 7ATP + 14NADPH + 6H+ → Palmitate + 14

NADP+ + 8CoASH + 6H20 + 7ADP + 7 Pi

• In prokaryotes, reactions of FA synthesis carried out by separate enzymes.

• In eukaryotes, enzymes present in a single polypeptide chain, multifunctional enzyme complex called fatty acid synthase (FAS).

• Swinging arm of phosphopantetheine (PPT) brings acyl groups into contact with active sites of FAS

• Fatty acid synthase complex exits as a dimer and arranged in such a way as to increase the efficiency of FA synthesis.

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Fatty Acid ModificationSynthesis of odd-chain fatty acids• Propionyl –ACP (3-C) used as primer – occurs particularly in ruminantsChain Elongation In eukaryotes, elongation of fatty acids (beyond C16) occur both in the

mitochondria as well as endoplasmic reticulum, especially in the latter known as the microsomal system

Reactions similar to FAS except involve CoASH derivatives e.g. condensation of palmitoyl CoA (hence palmitate → palmitoyl CoA) and malonyl CoA

Unsaturation of Fatty Acids Involves microsomal system called fatty acyl-CoA desaturase Palmitoyl CoA + NADH + H+ + O2 → Palmitoeyl CoA + NAD+ + 2H2O Mammals cannot introduce double bonds beyond ∆9 in fatty acids Unsaturated fatty acids like linoleic acid (ω6, 18:2, ∆9,12) and linolenic acid

(ω3, 18:3, ∆9,12,15), are the only two known essential fatty acids in many animals, including humans; provided in the diet.

Topic 8 : Metab. Liver 2 14

Comparison of Fatty Acid Synthesis and Degradation

Dehydrogenation (FAD)

Reduction(NADPH)

Hydration Dehydration

Dehydrogenation (NAD+)

Reduction(NADPH)

Thiolytic cleavage

Condensa- tion

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Comparison of Fatty Acid Synthesis and Degradation

Dehydrogenation (FAD)

Reduction(NADPH)

Hydration Dehydration

Dehydrogenation (NAD+)

Reduction(NADPH)

Thiolytic cleavage

Condensa- tion

L-form D-form

Oxidative degradation Synthesis

Synthesis of Triacylglycerols

Fatty acids are converted to triacylglycerols (TAG) for storage especially in adipose tissue and, lesser extent, liver

Stored TAG in dynamic state – continuously broken down and synthesized

Glucose → → Dihydroxyacetone phosphate (Liver and adipose tissue)

Glycerol (liver)

Phosphatidic acid is key intermediate not only for TAG synthesis but also other lipids e.g. phospholipids

(1)

(2)

(1) = Glycerol 3-phosphate dehydrogenase(2) = Glycerol kinase (present in liver)

Ketogenesis – Formation of Ketone Bodies

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β-Ketothiolase

HMG-CoA synthase

HMG-CoA lyase

Β-Hydroxybutyrate dehydrogenase

Acetyl CoATCA Cycle Fatty acid

Ketogenesis Occurs when acetyl CoA accumulates beyond cell’s capacity to oxidise it or to synthesize fatty acids Occurs in the mitochondria, primarily in the liver Extremely important during starvation during which the brain adapts to using ketone bodies for fuel

(Minor)

CH3CO~SCoA

CoASH

-OOC-CH2

Ketogenesis – Formation of Ketone Bodies

Topic 8 : Metab. Liver 2 20

Utilisation of Ketone Bodies Ketone bodies transported from liver to other tissues and used for energy production.

Liver cannot convert acetoacetate to acetoacetyl CoA and hence cannot use former for fuel

Extrahepatic tissues including brain but not RBC (no mitochondria) efficiently oxidise acetoacetate and hydroxybutyrate for energy production

During starvation, glucagon secreted Adipose tissue, mobilisation of TAG → fatty acids

Liver, increase breakdown of fatty acids → acetyl CoA Gluconeogenesis stimulated (Pyruvate → OAA →→ Glucose)

Increase in acetyl CoA but low OAA, acetyl CoA accumulates → ketogenesis

Topic 8 : Metab. Liver 2 21

Metabolism of Ketone Bodies