Comprehensive Study Notes on Fatty Acid Oxidation

Diversity of Roles: Fats (lipids) play multiple roles in living organisms. While they are inherently hydrophobic, this property can be modified through the addition of different chemical groups.
Triacylglycerol (TAG): * Primary Storage: TAG is the most significant form of fat storage in mammals, accounting for approximately $85\%$ of stored energy. * Dietary Prevalence: It is the most common type of fat found in food sources. * Energy Density Comparison: * TAG yields approximately $38\,kJ/g$ . * Glycogen yields approximately $17\,kJ/g$ . * Storage Efficiency: TAG is more energy-dense because it is anhydrous; in contrast, glycogen binds significant amounts of water, increasing its mass relative to its energy content.

Main Storage Sites: * White Adipose Tissue (WAT): The primary site for long-term fat storage. * Intramuscular Triacylglycerol (IMTG): Fat droplets found within certain types of muscle fibers ( $0.5\,\mu m$ scale seen in microscopy).
Synthesis and Transport: * Liver: Can synthesize fat and release it into the bloodstream. * Plasma Transport: Fats circulate in the blood in two forms: * As free fatty acids (FFAs) bound to the protein albumin. * As Triacylglycerols (TAG) packaged within lipoproteins.

Intracellular Lipases: * Function: Act on TAG stored within intracellular lipid droplets. * Activation Signals: * Adrenaline: Triggered during exercise. * Noradrenaline: Triggered by cold exposure. * Low Insulin: Triggered during fasting states. * Tissue Application: * Adipose Tissue: Releases FFAs into the blood during fasting and exercise to provide fuel for other tissues. * Skeletal Muscle: Releases FFAs for the muscle fiber itself during exercise (activated by adrenaline).
Lipoprotein Lipase (LPL): * Function: Acts on TAG carried within circulating lipoproteins in the capillaries. * Regulation: * Adipose Tissue: Expression is increased by insulin during the late fed state to facilitate fat storage. * Skeletal Muscle: Has a low $K_m$ (high affinity) to capture fatty acids for use during fasting or for replenishment post-exercise.

Activation Step: Before oxidation, fatty acids must be activated in the cytosol by attachment to Coenzyme A (CoA).
Energetics of Activation: * The reaction utilizes ATP and converts it to AMP and inorganic pyrophosphate ( $PP_i$ ). * Irreversibility: The enzyme pyrophosphatase hydrolyzes $PP_i$ into two inorganic phosphates ( $P_i + P_i$ ). This is a common strategy to drive a reaction to completion. * Net ATP Cost: Converting $1 \times \text{ATP} \rightarrow \text{AMP}$ is energetically equivalent to the consumption of $2 \times \text{ATP} \rightarrow \text{ADP}$ .
Coenzyme A (CoA): * Acyl groups are covalently attached via a sulphydryl group (-SH), forming a high-energy thioester linkage. * Attaching the coenzyme prevents the fatty acyl group from diffusing out of the cell.

General Characteristics: Also known as Fatty Acid Oxidation. It occurs in the mitochondrial matrix. It produces no ATP directly but generates reduced coenzymes and Acetyl CoA.
The Four-Step Cycle: 1. Oxidation: Fatty acyl CoA is acted upon by Fatty acyl CoA dehydrogenase. This produces FADH2 (which feeds into the Electron Transport Chain) and Enoyl CoA. 2. Hydration: Enoyl CoA hydratase adds $H_2O$ across the double bond to produce Hydroxy-acyl CoA. 3. Oxidation: Hydroxy-acyl CoA dehydrogenase oxidizes the molecule to Keto-acyl CoA, producing NADH. 4. Cleavage (Thiolysis): The enzyme Thiolase uses a molecule of CoA to cleave the bond, releasing one Acetyl CoA ( $2C$ ) and a new Fatty acyl CoA that is two carbons shorter than the original.

ETC Contribution: NADH and flavoprotein dehydrogenases contribute electrons to the Electron Transport Chain (ETC), provided $O_2$ is available.
Electron Transfer Flavoprotein (ETF): Electrons from FAD in Fatty acyl CoA dehydrogenase travel through ETF and ETF-Q oxidoreductase to the ubiquinone pool.
P:O Ratio Calculation for Fatty Acyl CoA DH: * Based on an ATP synthase with $8$ c-subunits: $8\,H^+$ are required for $3\,\text{ATP}$ synthesis, plus $3\,H^+$ for transport (phosphate/ATP translocase). * Total requirement: $11\,H^+$ for $3\,\text{ATP}$ , resulting in approximately $3.67\,H^+ / 1\,\text{ATP}$ . * Oxidation of FADH2 from the dehydrogenase results in the pumping of $6\,H^+$ ( $4$ at Complex III and $2$ at Complex IV). * $P:O \text{ ratio} = \frac{6}{3.67} \approx 1.6$ .

Tissue Restrictions: * Fatty acid oxidation is limited if cells have few mitochondria or insufficient oxygen supply. * Neurons in the Brain: Do not oxidize fatty acids because they have very low levels of the enzyme thiolase.
Pathway Interplay: * Fatty acid oxidation and glycolysis typically do not occur at high rates simultaneously (except in exercising muscle). * Fatty acid oxidation and fatty acid synthesis are mutually inhibitory.
Ketone Bodies: * During fasting or prolonged exercise, excess Acetyl CoA from fat oxidation is converted into ketone bodies ( $4C$ compounds). * Ketone bodies are highly water-soluble and serve as an alternative fuel for the brain.

Fed State: * Fatty acid oxidation is inhibited due to high rates of glycolysis and fat synthesis. * The brain uses only glucose. Adipose tissue performs little to no fat oxidation.
Fasting State: * Adipose tissue releases FFAs into circulation. Glycolysis is inhibited in most tissues to spare glucose for the brain. * Most tissues increase fat oxidation.
Starvation State: * The liver oxidizes fatty acids to produce ketone bodies from Acetyl CoA. * The brain adapts to use both glucose and ketone bodies for fuel.
Exercise State: * Adipose tissue mobilizes FFAs. Fatty acid oxidation increases in muscle as the supply of fatty acids increases.