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Chapter 4-6. Lipid Catabolism

Recommended Reading : 【Biology】 Chapter 4. Cells and Energy Metabolism

1. Overview

2. In Animal Cells

3. Animal Cells : Terminal Oxidation and Subterminal Oxidation

4. Animal Cells : Beta Oxidation

5. Animal Cells : Acyl-Carnitine Cycle

6. In Plant Cells

a. Lipid Synthesis

1. Overview

⑴ Lipids are used as energy when carbohydrates are not available

2. In Animal Cells

⑴ 1^st. Adipocytes

① 1^st - 1^st. Hormones bind to receptors, generating cAMP

② 1^st - 2^nd. cAMP activates PKA, which activates perilipin A and lipase

③ 1^st - 3^rd. Perilipin A is phosphorylated and changes to a structure that makes fat easier to break down

④ 1^st - 4^th. Various lipases break down triacylglycerol into glycerol and fatty acids

⑤ 1^st - 5^th. Fatty acids are released from cells and travel via serum albumin

⑵ 2^nd. Plasma : Glycerol and fatty acids travel through the plasma

⑶ 3^rd. Skeletal Muscle Cells

① Glycerol : Rarely used in glycolysis in skeletal muscle cells

② Fatty Acids

○ 3^rd - 1^st. As a result of Beta Oxidation, fatty acid is converted to Acetyl-CoA

○ 3^rd - 2^nd. After the Acyl-Carnitine Cycle, Acetyl-CoA passes directly through the mitochondrial inner membrane

○ 3^rd - 3^rd. Acetyl-CoA is broken down into CO₂ and H₂O through the TCA cycle

⑷ 4^th. 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 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 (β 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 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 : Palmitic acid, a C16 saturated fatty acid, is converted to palmitoyl-CoA and generates 8 molecules of Acetyl-CoA through β oxidation

Figure 4. Oxidation Process of Palmitic Acid in Animal Muscle Cells

5. Animal Cells: Acyl-Carnitine Cycle

⑴ 1^st. 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-hydroxylacl coA dehydrogenase)

○ 3-ketoacyl coA → acyl coA (n-2) + acetyl coA (enzyme : β-ketothiolase)

② Unsaturated Fatty Acids

○ Additional involvement of isomerase and reductase, 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 2^nd. Acyl-Carnitine Cycle : Acetyl-CoA cannot pass through the mitochondrial inner membrane

① 2^nd - 1^st. Acetyl-CoA + oxaloacetic acid → citric acid

② 2^nd - 2^nd. Citric acid can directly pass through the mitochondrial inner membrane

③ 2^nd - 3^rd. 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