Fatty Acid Metabolism & Beta Oxidation

• All reactions in fatty acid metabolism occur between the α and β carbons.
• Fatty acid oxidation is also known as Beta Oxidation.
• Enzymes are found in the mitochondria and peroxisomes.
• Fatty acid metabolism generates more ATP per carbon than sugars.
• The process proceeds two carbons at a time.

Preparation for Oxidation

• Fatty acids must be activated and transported to the mitochondria before oxidation.
• Activation begins in the cytoplasm.
• Long Chain Fatty Acyl-CoA Ligase facilitates the transformation of a Fatty Acid with ATP + CoASH into Fatty Acyl CoA using AMP + PPi + H2O

Carnitine Acyltransferase

• Carnitine Acyltransferase I and II are involved in the transport of Acyl-CoA across the inner mitochondrial membrane.
• This is facilitated by the Translocase Antiport.
• Acyl-CoA and CoA are converted to Acyl-Carnitine and Carnitine respectively, and passed through the inner mitochondrial membrane

Four Steps in Fatty Acid Oxidation

• Dehydrogenation (Oxidation): Acyl-CoA becomes trans-Δ²-Enoyl-CoA with the help of FAD becoming FADH₂
• Hydration: trans-Δ²-Enoyl-CoA becomes L-3-Hydroxyacyl-CoA using H₂0.
• Oxidation: L-3-Hydroxyacyl-CoA becomes 3-Ketoacyl-CoA in an NAD⁺ to NADH reaction
• Thiolytic Cleavage: 3-Ketoacyl-CoA converts to Acetyl-CoA and Acyl-CoA

Beta Oxidation - Reaction 1

• Oxidation creates a trans intermediate that is unrelated to trans fat.
• Three forms of enzymes are present for this reaction.
• A medium form problem can occur in SIDS (Sudden Infant Death Syndrome).
• This reaction is used to generate ATP in oxidative phosphorylation.
• Acyl-CoA will become trans-Δ²-Enoyl-CoA in the presence of Acyl-CoA Dehydrogenase and with FAD converting to FADH₂

Beta Oxidation Reaction 2

• Similar to the Fumarase Reaction of the Citric Acid Cycle.
• Prepares the molecule for the next oxidation step.
• trans-Δ²-Enoyl-CoA is converted to L-3-Hydroxyacyl-CoA using Enoyl-CoA Hydratase + H₂0.

Beta Oxidation Reaction 3

• Analogous to the Malate Dehydrogenase Reaction of the Citric Acid Cycle.
• Reaction used to generate ATP in Oxidative Phosphorylation
• L-3-Hydroxyacyl-CoA converts to 3-Ketoacyl-CoA using Hydroxyacyl-CoA Dehydrogenase, and NAD⁺ to NADH + H⁺

Beta Oxidation Reaction 4

• The Thiolase enzyme catalyzes the reverse reaction when the R-group is a hydrogen.
• This is important for ketone body synthesis.
• This reaction will cleave 3-Ketoacyl-CoA into Acyl-CoA and Acetyl-CoA in the presence of CoA-SH and Thiolase.

Fatty Acid Oxidation - Summary

• Each round of oxidation produces one FADH₂, one NADH, one Acetyl-CoA and shortens the fatty acid by two carbons.
• Each Acetyl-CoA released into the mitochondrial matrix is readily oxidized in the Citric Acid Cycle.

Similarity of Fatty Acid Oxidation and Citric Acid Cycle

• Both Fatty Acid Oxidation and the Citric Acid Cycle include Dehydrogenation (Oxidation), Hydration, and Oxidation steps
• As well as FAD to FADH2, Fumarate, Malate, and NAD+ to NADH

Unsaturated Fatty Acid Oxidation

• Molecules with double bonds at the 2,3 and 4,5 positions are removed in 3 rounds of Beta Oxidation to yield 2,3 Trans Bond and 3,4 Double Bond molecules
• The 2,3 Trans Bond molecules with 3,4 Double Bonds are converted with Enoyl-CoA Isomerase

Other Fatty Acid Oxidation

• Long Chain Fatty Acids (20-22 Carbons or Greater) are Oxidized in Beta Oxidation Reactions in Peroxisomes
• Propionyl-S-CoA becomes Methyl malonyl-S-CoA when carboxyl is added, and ATP is converted to ADP + P
• Some amino acids become Propionyl-S-CoA
• Valine becomes Methyl malonyl-S-CoA
• Methyl malonyl-S-CoA will then move carboxyl onto Succinyl-S-CoA to oxidize in the citric acid cycle

Regulation of Beta Oxidation

• Lipolysis and β-oxidation of fatty acids are closely regulated under hormonal influence.
• Insulin inhibits Lipolysis of Adipose Fat (TAG) and mobilization of Free Fatty acids
• Insulin decreases β-oxidation of fatty acids.
• When cellular or blood glucose levels lower, Glucagon and Epinephrine stimulate Lipolysis.
• Glucagon stimulates the enzyme hormone-sensitive Lipase and hydrolyzes depot Fat (TAG).
• Glucagon mobilizes free fatty acids into blood circulation
• Glucagon increases β-oxidation of fatty acids

Regulation of Beta Oxidation at Two Levels

• Carnitine Shuttle
• Beta Oxidation Proper
• Transport of Fatty Acyl CoA from the cytosol into the mitochondrial matrix via the carnitine shuttle is the rate-limiting step.
• Malonyl-CoA, an intermediate of lipogenesis, is an inhibitor of Carnitine Acyl Transferase I.
• Control of fatty acid oxidation is exerted mainly at the step of fatty acid entry into the mitochondria.
• Acyl-CoA Dehydrogenase is a key regulatory enzyme of Beta Oxidation of Fatty Acids

Significance of Beta Oxidation

• Beta oxidation cycles help in cleaving and shortening of a long-chain fatty acid.
• Oxidation of the beta carbon atom of a fatty acid transforms a stronger bond between the alpha and beta carbon atoms to a weaker bond.
• Transformation to a weaker bond helps in easy cleavage between the alpha and beta carbons.
• During β-oxidation, beta carbon atom undergoes dehydrogenation (CH₂ to C=O).
• Hydrogen atoms are removed during beta oxidation with oxidized coenzymes(FAD and NAD+).
• Oxidized coenzymes are converted to temporarily accepted by reduced coenzymes
• Reduced coenzymes enter the ETC (electron transport chain) and get reoxidized
• The byproduct of the ETC is ATP.
• Thus, Beta oxidation of fatty acids metabolizes a long chain fatty acid with liberation of ATP for cellular activities.
• Palmitic acid yields eight Acetyl-CoAs.
• Complete β-oxidation of one Palmitic acid yields 106 molecules of ATP.
• Large energy yield results from the highly reduced state of carbon in fatty acids.
• This makes fatty acid the fuel of choice for migratory birds and many other animals.