Fatty Acid Catabolism

Energy Yield from fatty acid vs carbohydrate oxidation

• Complete oxidation of fatty acids yields 9kcal/g

• Carbohydrates and proteins yield 4kcal/g

What causes the difference

• Fatty acids are highly reduced, carbon atoms in FA are primary bonded to hydrogen

• Carbohydrates are partially oxidized through bonds to oxygen

• Fatty acid oxidation transfers more electrons to the ETC

• Reduction of more oxygen and the generation of more ATP

Fuel Reserves in a 70kg Individual

• Triacylglycerols = 100,000kcal

• Protein = 25,000kcal

• Glycogen = 600kcal

• Blood glucose: 40kcal

 

Structure and properties of fatty acid

• Linear hydrocarbon chain with a carboxylate at one end

  • Saturated has no double bonds

  • Unsaturated has at least 1 double bond

• Almost all naturally occurring FA's have an even number of carbon atoms (with carboxy carbon)

Location and Overall Reaction strategy of beta-oxidation

• Occurs in the mitochondrial matrix

• Fas tagged as fatty acyl-CoA (thioester)

• The acetyl will be eliminated as an enolate eventually

Cyclic process and self propagating

• Each round shortens chain the chain by 2C at carboxy end, release acetyl-CoA

• The shortened acyl-CoA re-enters cycle until fully converted

• Acyl-CoA is prepared for shrotening by beta oxidation where 2 hydrides are extracted from the beta carbon CH2 to make beta-keto acyl-CoA

• Acetyl-CoA enolate leaves, new CoA captures the beta-keto carbon and rest of FA chain

Example: Palmitoyl-CoA(C16) > 7 cycles > 8 acetyl-CoA

Role of CoA

• Activates fatty acids

• Prevent beta-keto acid decarboxylation

• Channels acetyl-CoA to TCA

In long chain FAs (more than 12 carbons)

• Last 3 steps is from trifunctional enzyme complex

Step 1: Dehydrogenated

Enzyme: Acyl-CoA dehydrogenase

Reaction: Palmitoyl-CoA (Any FA) + FAD >  trans-delta^2-Enyoyl-COA + FADH2

• Acyl-CoA dehydrogenase initiates Beta-oxidation by removing 2 hydrogen atoms between alpha and beta carbon of the fatty acyl-CoA

• Free energy is -10kJ/mol

Purpose

• Creates a conjugated doble bond to prime the molecule for hydration

Cofactor

• Enzyme has tightly bound FAD to accept electrons, reducing to FADH2

Connections to ETC

• Electrons within FADH2 is transferred to the electron transfer flavoprotein, an inner-membrane -associated carrier

• Will eventually deliver the reducing equivalents to ubiquinone to form ubiquinol

• Integrates fatty acid oxidation directly to ETC

 

Hydration

Enzyme: Enoyl-CoA hydratase

Reaction: trans-delta^2-enoyl-CoA > L-Beta-hydroxy-acyl-CoA (OH on Beta, H on alpha, syn addition)

• Enoyl-CoA hydratase catalyzes stereospecific addition of water across the trans doble bond of the enoy-lCo-A

• Converts alpha/beta doble bond carbs in a beta-hydroxyl group

• Free energy = 0

Purpose

• Converts nonpolar alkene into a hydroxylated intermediate that undergo further oxidation

Mechanism

• 2 active site glutamate act as a proton donor/acceptor pair

• Facilitates the position of the trans doble bond and activates water molecule for syn addition

• Facilitates the addition of the hydroxyl group to the beta-carbon and proton to the alpha carbon

• L-beta hydroxy is the only from recognized by beta-hydroxacyl-CoA dehydrogenase

 

Dehydrogenase

Enzyme: beta-hydroxacyl-CoA dehydrogenase

Reaction: L-Beta-hydroxy-acyl-CoA + NAD+> NADH + H+ + beta-Ketoacyl-Coa

• Enzyme is an NAD+ dependent oxidoreductase in the mitochondrial matrix

• Free energy is -10kJ/mol

Purpose

• Drives beta-oxidation

• Generates NADH and funny energy into ATP production by ETC

• Transforms reduced hydrocarbon into a reactive carbonyl, ready for bond cleaves

Mechanism

• Removes 2 hydrogens from the beta-hydroxyl group and one as a hydride

• Basic amino acid in the enzyme accepts proton from hydroxyl, NAD+ accepts hydride from beta-carbon

• Beta-hydroxyacyl-CoA is very reactive to drive beta-oxidation

Thiolysis

Enzyme: Thiolase

Reaction: beta-ketoacyl-COA + CoA-SH > Acyl-CoA + Acetyl-CoA

• Thiolase catalyze the cleavage of the bond between alpha and beta-carbon of the beta-ketoacyl-CoA intermediates

• Produce acetyl-CoA and fatty acyl-CoA shortened by 2 carbons

• Free energy about 3kJ/mol

Purpose

• Acetyl-CoA feeds into TCA cycle for ATP generation

• Acyl-CoA sustains continuous fatty acid catabolism

• Thiolase ensures efficient chain shortening and energy extraction

Mechanism

• Uses a nucleophilic attack on the beta-keto carbon by the thiol group of free coenzyme-A

• Release 2 carbon acetyl-COA unit as an enolate and is quickly protonated

• Remaining fatty acyl-CoA is shortened by 2 carbon and is regenerated and primed to enter another cycle of beta-oxidation

 

Comparison to TCA cycle Recycling phase

• Overall reversible and has 2 key hydride extractions at the beta-carbon

  • Converts the substrate into trans-delta2-enoyl-CoA and generate FADH2

  • Form beta-ketoacyl-CoA and generate NADH

  • FADH2 and NADH will enter ETC at complex 2 and complex 1 respectively to produce ubiquinol

• TCA vs beta-oxidation equivalents

  • Acyl-CoA and succinate

  • Trans-delta2-enoyl-CoA and fumarate

  • L-beta-hydroxyacyl-CoA and malate

  • Beta-ketoacyl-CoA

 

Roles of CoA tag in fatty acid metabolism

• Tethering

  • The last 3 steps of beta oxidation is catalyzed by a trifunctional enzyme complex

  • Fatty acyl group is shuttled between active sites by CoA tether

• Tagging/activation

  • FA + CoA > thioester, a high energy bond

• Prevents decarboxylation

  • Beta acids spontaneously decarboxylate because the beta keto  stabilizes the carbanion formed during CO2 loss

  • Beta-keto thioester on FA ends presents it, ensures correct alpha-beta bond cleavage

 

Energy Yield and ATP calculation

NADH creates 2.5 ATP, FADH2 creates 1.5 ATP

Enzyme catalyzing the oxidation steps in complete breakdown of palmitoyl-CoA

Beta oxidation

• Acyl-CoA dehydrogenase

  • 7 FADH2 = 10.5 ATP

• Beta-hydroxyacyl-CoA dehydrogenase

  • 7 NADH = 17.5 ATP

• Product of acetyl-CoA enters the TCA

TCA

• Isocitrate DH

  • 8 NADH = 20 ATP

• Alpha-KG DH

  • 8 NADH = 20 ATP

• Succinyl-CoA synthase = 8 ATP

• Succinate dehydrogenase

  • 8 FADH2 = 12 ATP

• Malate DH

  • 8 NADH = 20

Total of 108 ATP produce

ATP per carbon

• Complete oxidation of palmitoyl-CoA generates 108 ATP

• 6.75 ATP per carbon compared with glucose which produces 32 ATP from 6 carbon, 5.3 ATP per carbon

Reason for higher yield

• FA are more reduced than carbs, each carbon contributes more electrons to the ETC so it provides more energy

• Used as long-term energy sotres

Takeaway

• Fatty acid oxidation maximizes energy capture per carbon

• Central role in sustained energy metabolism and cellular energetics

Hibernation and oxidize

• Bears hibernate for 7 months, keeps their body temp around 32 degrees celsius and burns 6,000kcal/day

  • It powers body temp, protein synthesis and cellular process from stored fat

  • Produce metabolic water to stay hydrated

• Glycerol from fat fuels gluconeogenesis, urea from amino acid catabolism is recycled by kidneys to maintain protein balance

Beta-oxidation in medicine

• Proper beta-oxidation is essential for energy homeostasis

• Breakdown of FA to acetyl-CoA for energy is critical during fasting, prolonged exercise and in organs relying on FA (skeletal, heart, liver)

Disorders

• Medium-chain acyl-CoA DH deficiency MCAD

  • Impaired oxidation of medium-chain FA = hypoglycemia from impaired gluconeogenesis (acetyl-CoA regulated pyruvate carboxylase)

  • Causes lethargy and sudden infant death

• Long chain FA oxidation defects

  • Cause cardiomyopathy, hepatomegaly and muscle weakness

 

Therapeutic implications: Dietary management

• Avoid fasting

• Have medium-chain triglycerides to bypass defective steps

Metabolic monitoring

• Blood acylcarnitine profiles used in diagnosing beta-oxidation defects

• Acyl-carnitines are present at low levels in health individual, but accumulate and appear at elevated levels in blood when beta-oxidation is impaired

Pharmacology

Drugs modulate FA oxidation are explored in metabolic disorders and heart disease