Mobilization of fats from adipose tissue
Triacylglycerols stored in adipocytes during the postabsorptive period are mobilized during fasting and intense exercise as an energy source. The signals for lipolysis are glucagon, adrenaline, and noradrenaline. These hormones bind to G protein-coupled receptors; the α-subunit dissociates and activates adenylyl cyclase, which catalyzes the conversion of ATP to cAMP. cAMP activates protein kinase A.
Protein kinase A carries out two regulatory events:
- phosphorylates hormone-sensitive lipase (TAG lipase), converting it into the active form;
- phosphorylates the perilipin protein coating the lipid droplet; phosphorylated perilipin leaves the droplet surface, exposing the substrate to lipases.
Stages of triacylglycerol lipolysis
TAG hydrolysis occurs in a stepwise manner:
- Hormone-sensitive lipase (TAG lipase) cleaves off the first fatty acid, forming diacylglycerol. Additionally, adiponutrin (ATGL) participates in the initiation of lipolysis.
- Diacylglycerol lipase cleaves off the second fatty acid to form monoacylglycerol.
- Monoacylglycerol lipase breaks down monoacylglycerol into glycerol and the third fatty acid.
Free fatty acids enter the blood, bind to albumin, and are transported to consumer tissues.
Fatty acid activation and the carnitine shuttle
In the cytosol of the target cell, the fatty acid is activated by the enzyme acyl-CoA synthetase:
R–COOH + ATP + HS-CoA → R–CO–S–CoA + AMP + H₄P₂O₇
Two high-energy bonds are consumed (ATP → AMP + PPi).
The inner mitochondrial membrane is impermeable to acyl-CoA, so the carnitine shuttle is used:
- Carnitine palmitoyltransferase I (CPT-I) on the outer membrane transfers the acyl group from CoA to carnitine, forming acylcarnitine; CoA is released.
- Carnitine-acylcarnitine translocase transports acylcarnitine into the matrix.
- Carnitine palmitoyltransferase II (CPT-II) returns the acyl group to matrix CoA; free carnitine is transported back.
CPT-I is the main regulatory enzyme of the pathway.
Beta-oxidation reactions
The process takes place in the mitochondrial matrix under aerobic conditions and is coupled with the respiratory chain. Each cycle shortens the fatty acid by 2 carbon atoms and yields one acetyl-CoA molecule. Oxidation occurs at the β-carbon atom, hence the name.
- Dehydrogenation: acyl-CoA dehydrogenase removes hydrogen between the α- and β-carbons, reducing FAD → FADH₂. Enoyl-CoA is formed.
- Hydration: enoyl-CoA hydratase adds water across the double bond, forming β-hydroxyacyl-CoA.
- Dehydrogenation: β-hydroxyacyl-CoA dehydrogenase oxidizes the OH group, reducing NAD⁺ → NADH. This yields β-ketoacyl-CoA.
- Thiolysis: β-ketoacyl thiolase cleaves the bond between the α- and β-carbons by adding a second CoA. Acetyl-CoA and an acyl-CoA shortened by 2 carbons are released, the latter entering a new cycle.
Energy balance of beta-oxidation
For a fatty acid with an even number of carbon atoms n:
- number of β-oxidation cycles = n/2 − 1;
- each cycle yields FADH₂ (2 ATP) and NADH (3 ATP) — a total of 5 ATP per cycle;
- n/2 molecules of acetyl-CoA are formed, each yielding 12 ATP in the TCA cycle;
- activation of the fatty acid consumes the equivalent of 2 ATP.
Final formula: ATP = (n/2) · 12 + (n/2 − 1) · 5 − 2. For palmitic acid (C₁₆): 8·12 + 7·5 − 2 = 129 ATP.
Regulation of beta-oxidation
The rate of fatty acid oxidation depends on several factors:
- Oxygen availability — the process is strictly aerobic; in working muscle, β-oxidation becomes a significant energy source only after 10–20 minutes when blood flow increases.
- Substrate availability — the concentration of fatty acids in the blood and in the mitochondrial matrix.
- Energy status of the cell — oxidation is activated when there is an ATP deficit.
- Inhibition of CPT-I by malonyl-CoA — the key mechanism of reciprocal exclusion between lipogenesis and lipolysis.
Malonyl-CoA — the product of the reaction catalyzed by acetyl-CoA carboxylase. In the liver and adipose tissue, insulin dephosphorylates acetyl-CoA carboxylase via a phosphatase, activating it; the accumulation of malonyl-CoA blocks CPT-I and the entry of acyls into the matrix. Thus, synthesis and oxidation of fatty acids do not occur simultaneously.
Regulation in muscle tissue
Muscles do not synthesize fatty acids, but they contain an acetyl-CoA carboxylase isozyme that produces malonyl-CoA to regulate CPT-I. This isozyme is inactivated by phosphorylation via two kinases:
- Protein kinase A, activated by adrenaline via cAMP;
- AMP-activated protein kinase (AMPK), activated during an increase in the AMP/ATP ratio, i.e., during an energy deficit.
Phosphorylated acetyl-CoA carboxylase is inactive, the concentration of malonyl-CoA drops, the inhibition of CPT-I is lifted, and acyl transport into the matrix and β-oxidation increase. Thus, adrenaline and a low energy status couple muscle work with active fatty acid burning.
Key points
- Lipolysis is triggered by glucagon, adrenaline, and noradrenaline via cAMP-dependent activation of protein kinase A.
- Hormone-sensitive lipase and perilipin phosphorylation ensure the stepwise hydrolysis of TAG to glycerol and fatty acids.
- Fatty acid activation by acyl-CoA synthetase costs two high-energy ATP bonds.
- The carnitine shuttle (CPT-I, translocase, CPT-II) transports acyls into the mitochondrial matrix.
- One cycle of β-oxidation = dehydrogenation, hydration, dehydrogenation, thiolysis; yields FADH₂, NADH, and acetyl-CoA.
- Complete oxidation of palmitate (C₁₆) yields 129 ATP molecules.
- The main regulator is the inhibition of CPT-I by malonyl-CoA; insulin activates acetyl-CoA carboxylase, while adrenaline and AMPK inactivate it.
- Fatty acid synthesis and β-oxidation are reciprocally regulated and do not occur simultaneously.