Chapter 4-6. Lipid Degradation
Recommended Reading: 【Biology】 Chapter 4. Cells and Energy Metabolism
1. Overview
3. Animal Cells: Terminal Oxidation and Subterminal Oxidation
4. Animal Cells: Beta Oxidation
5. Animal Cells: Acyl-Carnitine Cycle
1. Overview
⑴ Lipids are used as energy when carbohydrates are not available.
2. In Animal Cells
⑴ 1st. Adipocytes
① 1st - 1st. Hormones bind to receptors, generating cAMP.
② 1st - 2nd. cAMP activates PKA, which activates perilipin A and lipase.
③ 1st - 3rd. Perilipin A is phosphorylated and changes to a structure that makes fat easier to break down.
④ 1st - 4th. Various lipases break down triacylglycerol into glycerol and fatty acids.
⑤ 1st - 5th. Fatty acids are released from cells and travel via serum albumin.
⑵ 2nd. Plasma: Glycerol and fatty acids travel through the plasma.
⑶ 3rd. Skeletal muscle cells
① Glycerol: Rarely used in glycolysis in skeletal muscle cells.
② Fatty Acids
○ 3rd - 1st. As a result of beta oxidation, fatty acid is converted to Acetyl-CoA.
○ 3rd - 2nd. After the Acyl-Carnitine cycle, Acetyl-CoA passes directly through the mitochondrial inner membrane.
○ 3rd - 3rd. Acetyl-CoA is broken down into CO2 and H2O through the TCA cycle.
⑷ 4th. Liver cells
① Glycerol: Converted to G3P and immediately participates in glycolysis.
② Fatty Acids
○ Situation: Significant fasting state is implied for fatty acids to be broken down. Oxaloacetic acid in the body is used for gluconeogenesis in the brain.
○ There is a lot of Acetyl-CoA, which is not used in the TCA cycle or gluconeogenesis, so keton body formation reaction occurs.
○ Ketone body: Acetoacetate, D-β-Hydroxybutyrate, Acetone
○ Ketone bodies are used as an energy source by the heart, kidneys, muscles, and brain.
Figure 1. Ketone body formation process
3. Animal Cells: Terminal Oxidation and Subterminal Oxidation
⑴ Terminal oxidation and subterminal oxidation occur when alkanes are broken down.
Figure 2. Terminal Oxidation or Subterminal Oxidation
4. Animal Cells: Beta Oxidation
⑴ Location
① Animal Cells: 1/3 in peroxisomes, 2/3 in the mitochondrial intermembrane space.
② Plant Cells: 100% in peroxisomes.
③ Note that fatty acid tail synthesis occurs in the cytoplasm and lipid synthesis in the smooth endoplasmic reticulum.
⑵ Reaction
Figure 3. Beta Oxidation Reaction Cycle
① Only saturated fatty acids undergo beta oxidation.
② During the reaction, 1 FADH2 is first produced (1st oxidation), followed by 1 NADH (2nd oxidation).
③ As a result of one beta oxidation cycle, one Acetyl-CoA (2C) is produced with ② from the fatty acid.
④ Fatty acids with 2n carbons undergo n-1 beta oxidation cycles, producing n-1 Acetyl-CoA and 1 glycerol.
⑶ Application: After palmitic acid, a C16 saturated fatty acid, is converted to palmitoyl-CoA, 8 molecules of acetyl-CoA are generated through β oxidation.
Figure 4. Oxidation process of palmitic acid in animal muscle sells
5. Animal Cells: Acyl-Carnitine Cycle
⑴ 1st. Acetyl-CoA is produced from pyruvate in the mitochondrial matrix.
① Saturated Fatty Acids
○ The acyl coA synthetase on the mitochondrial outer membrane removes ppi from ATP and attaches coA to the fatty acid to form acyl-coA.
○ ppi is broken down into 2pi by pyrophosphatase.
○ Carnitine acyltransferase 1 on the outer membrane replaces coA with carnitine.
○ Acyl carnitine is transported by translocase on the inner membrane.
○ Acyl carnitine is converted to acyl coA by carnitine acyltransferase 2 on the inner membrane.
○ Acyl coA → trans-Δ2 enoyl coA → L-3-hydroxyacyl coA (Enzyme: Acyl coA dehydrogenase)
○ L-3-hydroxyacyl coA → 3-ketoacyl coA (Enzyme: L-3-hydroxyacyl coA dehydrogenase)
○ 3-ketoacyl coA → acyl coA (n-2) + acetyl coA (Enzyme: β-ketothiolase)
② Unsaturated fatty acids
○ Isomerase and reductase are additionally involved, producing propionyl coA and succinyl coA.
○ Propionyl coA (3C) + HCO3- → methylmalonyl coA (4C) (Enzyme: Propionyl coA carboxylase)
○ Methylmalonyl coA (4C) → Succinyl coA (Enzyme: mutase)
⑵ 2nd. Acyl-Carnitine Cycle: Acetyl-CoA cannot pass through the mitochondrial inner membrane.
① 2nd - 1st. Acetyl-CoA + oxaloacetic acid → citric acid
② 2nd - 2nd. Citric acid can directly pass through the mitochondrial inner membrane.
③ 2nd - 3rd. Citric acid that has passed through the mitochondrial inner membrane is broken down into oxaloacetic acid and acetyl-CoA.
6. In Plant Cells
⑴ Glyoxysome: Exists only in plants.
① Glyoxysome does not contain succinate dehydrogenase.
⑵ Glyoxylate Cycle
① Overview
○ Converts fatty acids into glucose.
○ Occurs across lipid bodies, glyoxysomes, and mitochondria.
② Reaction 1. Acetyl coA is produced by beta oxidation of fatty acids.
○ Since beta oxidation produces H2O2, glyoxysomes also contain catalase.
③ Reaction 2. Acetyl coA + OAA → Citrate
④ Reaction 3. Citrate → Succinate + Glyoxylate
○ Succinate moves to mitochondria and reacts to malate in the TCA cycle.
⑤ Reaction 4. Glyoxylate + Acetyl coA → Malate
⑥ Reaction 5. Malate → OAA
⑦ Reaction 6. OAA is converted into glucose through gluconeogenesis in the cytoplasm.
⑶ Lipids stored in seeds are an energy source until the young plant can photosynthesize.
① During early stages, photosynthesis is not performed, so glyoxysomes convert fatty acids into sugar.
⑷ Animal cells cannot perform gluconeogenesis from fatty acids.
Input: 2019.01.16 17:29
Modified: 2022.09.17 21:10