Fatty Acid Catabolism

Fatty Acid Catabolism

• Hummingbirds store and use fatty acids to enable long migratory journeys.
• Focus on how fatty acids are catabolized and how organisms capture the inherent energy.

Energy Storage

• Fatty acids are used for energy storage for two reasons:
  • Carbon in fatty acids is reduced, thus its oxidation yields maximal energy.
  • Fatty acids are not hydrated, so denser packing is possible in storage tissues.

Dietary Fat Mobilization

• Dietary and adipose fats are generally triglycerides.
• Triglycerides are a major energy source in the modern American diet.
• Triglycerides are a main form of stored energy.
• Hormones like glucagon, epinephrine, and ACTH release fatty acids from adipose tissue.

Fat Digestion

• Pancreatic lipases and PLA2 break down triacylglycerols (TAGs).
• After absorption, TAGs are reconstituted.

Fatty Acid Breakdown

• Knoop determined fatty acids are degraded via 2-carbon unit removal.
• Albert Lehninger showed this process occurs in the mitochondria.
• F. Lynen and E. Reichart identified that the 2-carbon unit released is acetyl-CoA.
• Beta oxidation starts with the oxidation of the beta-carbon relative to the carboxyl carbon.

CoA Activation

• Acyl-CoA synthetase condenses fatty acids with CoA, hydrolyzing ATP into AMP and PPi.
• CoA ester formation is energetically expensive.
• Free energy change barely breaks even with ATP hydrolysis.
• Subsequent PPi hydrolysis strongly drives the reaction forward.
• The mechanism involves an acyl-adenylate intermediate.

Carnitine's Role

• Carnitine transports fatty acyl groups across mitochondrial membranes.
• Short-chain fatty acids enter the mitochondrial matrix directly.
• Long-chain fatty acids are converted into acyl-carnitines for transport into the mitochondria.
• Acyl-CoA esters form inside the inner mitochondrial membrane from transported acylcarnitines.

Beta Oxidation

• Strategy: Beta-carbon creates a carbonyl group.
• The first three reactions create a carbonyl group for the fourth to cleave the "beta-keto ester" in a reverse Claisen condensation.
• Products: Acetyl-CoA and a fatty acid shortened by two carbons are produced.
• Initial three reactions form the basis of other pathways.

Acyl-CoA Dehydrogenases

• Acyl-CoA dehydrogenases are a family of membrane-bound and soluble matrix enzymes.
• As a fatty acyl chain is shortened via beta-oxidation, it shifts from membrane-bound to soluble matrix enzymes.
• Acyl-CoA dehydrogenase mechanism: proton abstraction, double-bond formation, and hydride removal by FAD.
• Electrons transfer to electron transfer flavoprotein (ETF), then to the electron transport chain.

Enoyl-CoA Hydratase

• Enoyl-CoA hydratase adds water across the double bond, yielding L-hydroxyacyl-CoA.
• Enoyl-CoA hydratase converts trans-enoyl-CoA derivatives to L-beta-hydroxyacyl-CoA.

Hydroxyacyl-CoA Dehydrogenase

• L-Hydroxyacyl-CoA dehydrogenase oxidizes the beta-hydroxyl group.
• The third reaction is oxidation of the hydroxyl group at the beta-position, yielding a beta-ketoacyl-CoA derivative.
• L-hydroxyacyl-CoA dehydrogenase requires NAD+ as a coenzyme.
• Each NADH created drives the synthesis of 2.7 ATP in mammalian mitochondria.

Thiolase Reaction

• Cysteine thiolate on enzyme attacks the beta-carbonyl group, releasing acetyl-CoA.
• A new CoA thiol group attacks the shortened chain, forming a new acyl-CoA.
• Functions as the reverse of a Claisen condensation that attacks the enolate of acetyl-CoA to a thioester.
• Reaction favors the formation of a new thioester driving the other three.

Trifunctional Enzyme Complex

• Oligomeric associations of enzymes enhance metabolic pathway efficiency.
• A trifunctional enzyme (TFE) has a critical role in the beta-oxidation pathway.
• Bacterial and human complexes feature similar structures with two subunit types.
• Alpha-subunit: enoyl-CoA hydratase (ECH) and hydroxyacyl-CoA dehydratase (HCAD) activities.
• Beta-subunit: ketoacyl thiolase (KACT) activity.
• A substrate channel facilitates substrate channeling.

Summary of Beta-Oxidation

• Repetition of the cycle yields a succession of acetate units.
• Palmitic acid yields eight acetyl-CoAs.
• Beta-oxidation of palmitic acid yields 106 ATP molecules.
• High energy yield from the reduced state of carbon in fatty acids.
• Migratory animals utilize fatty acids as a fuel source due to their low density.

Energy Accounting

• 1 Acetyl-CoA = 10 ATP (in TCA, ETC)
• 1 NADH = 2.5 ATP
• 1 FADH2 = 1.5 ATP
• An even FA with n carbons, has n/2 –1 beta oxidations, n/2 – 1(NADH + FADH2) and n/2 Acetyl-CoA.
• C16 = 8 Acetyl-CoA, 7(NADH +FADH2) – 2ATP invested to make initial CoAester

Migratory Birds

• Fatty acids serve as a concentrated energy source for long migratory flights.
• American golden plovers fly 3300 km from Alaska to Hawaii in 35 hours which is more than 250,000 wing beats.
• Ruby-throated hummingbirds fly nonstop across the Gulf of Mexico.
• Birds store large amounts of triacylglycerols for these feats.
• Birds are often 70% fat by weight when migration begins.

Animals and Metabolic Water

• Beta-oxidation produces significant metabolic water.
• Desert animals, seawater-drinking killer whales, and camels oxidize stored fatty acids to yield dietary water.
• The needed water as well as metabolic energy.

Odd-Carbon Oxidation

• Beta-oxidation of odd-carbon fatty acids yields propionyl-CoA.
• Odd-carbon fatty acids metabolize normally until a 3-C fragment, propionyl-CoA, is reached.
• Reaction: propionyl-CoA yields succinyl-CoA via three reactions.
• Needs biotin and vitamin B12.
• Succinyl-CoA undergoes pathway oxidation.

Unsaturated

• Processes monounsaturated fatty acids like oleic and palmitoleic acid.
• The process undergoes normal beta-oxidation cycles.
• Cis-Δ3 acyl-CoA is not utilized via acyl-CoA dehydrogenase.
• Enoyl-CoA isomeraseconverts this to trans-Δ2 acyl CoA
• Beta oxidation continues from this point.

Polyunsaturated

• Degradation of polyunsaturated fatty acids is more complex.
• The process mirrors oleic acid degradation through 3 cycles of beta-oxidation.
• Enoyl-CoA isomerase converts the cis-Δ3 double bond to a trans-Δ2 double bond.
• One beta-oxidation round is permitted.
• The resulting trans-Δ2, cis-Δ4 structure is a problem to overcome.
• Solved with 2,4-Dienoyl-CoA reductase.

Peroxisomal Beta-Oxidation

• Peroxisomes execute flavin-dependent oxidations regenerating oxidized flavins O2 produces H2O2
• Similar to mitochondrial beta-oxidation, acyl-CoA oxidase primarily responsible double bonds.
• Electrons transfer straight to O2
• Yields smaller ATPs

Branched-Chain Fatty Acids

• Branched chain FAs with branches at odd-number carbons are poor beta-oxidation substrates.
• Alpha-oxidation is an alternative.
• Phytanic acid alpha-oxidase oxidation decarboxylates at the alpha position.
• Beta-oxidation then happens past the branch

Key facts about Refsum's disease

• A rare inherited disorder, patients are absent of of a-oxidizing protein.
• Patients build of large quantities of pytanic acid.
• Leads severe symptoms, cerebellar ataxia, retinitis pigmentosa, nerve deafness and peripheral neuropathy.
• Can ameliorate symptoms with the strict restriction from dairy products and ruminant meat from the diet.

Omega-Oxidation

• Omega-oxidation yields small amounts of dicarboxylic acids of fatty acids.
• Dicarboxylic acid is performed with oxidation of omega carbon in a cytochrome P-450 reaction

Ketone Bodies

• Acetyl-CoA produces in fatty acid oxidation produces acetone, acetoacetate, and 𝛼-hydroxybutyrate.
• Ketone bodies act as fuel source for brain, heart, and muscle
• Energy of the brain as a major energy source of starvation.
• Known as transportable forms of fatty acids

Ketone Body Formation and Metabolism

• Synthesized primarily in liver mitochondria.
• the synthesis of ketone bodies only happens in the mitochondrial matrix