Chapter 4-7. Lipid Synthesis
Recommended Post: 【Biology】 Chapter 4. Cells and Energy Metabolism
2. Anabolism
1 . Acyl-Carnitine Cycle
⑴ 1st. Formation of Acetyl-CoA from pyruvate in the mitochondrial matrix.
① Saturated fatty acids
○ Acyl coA synthetase on the outer mitochondrial membrane forms acyl-coA by attaching coA to fatty acids after removing ppi from ATP.
○ 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 in the inner membrane.
○ Acyl carnitine is converted back to acyl coA by carnitine acyltransferase 2 in 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 + Oxaloacetate → Citrate
② 2nd - 2nd. Citrate can directly pass through the mitochondrial inner membrane.
③ 2nd - 3rd. Decomposes into oxaloacetate and acetyl-CoA in the cytoplasm.
2. Anabolism
⑴ Anabolism 1. Fatty acid synthesis
① Animal cells: In the cytoplasm, acetyl-CoA utilizes ATP and NADPH for fatty acid synthesis.
○ In animal cells, NADPH is synthesized via the pentose phosphate pathway.
○ Acetyl coA + ATP + HCO3- → malonyl coA + ADP + Pi
○ Enzyme: Acetyl coA carboxylase
○ Cofactor: Biotin
② Plant Cells: Fatty acid synthesis occurs in the chloroplasts, plastids.
⑵ Anabolism 2. Saturated fatty acid synthesis
① Animal Cells
○ In the cytoplasm, fatty acids can be synthesized up to C-16, which is palmitic acid, in terms of carbon number.
○ The elongation of palmitic acid into longer fatty acids occurs in the smooth endoplasmic reticulum and, to some extent, in mitochondria.
② Type 1. Lauric acid: (12:0)
○ 12 carbons, 0 double bonds
③ Type 2. Myristic acid: (14:0)
○ 14 carbons, 0 double bonds
④ Type 3. Palmitic acid: (16:0)
○ 16 carbons, 0 double bonds
⑤ Type 4. Stearic acid: (18:0)
○ 18 carbons, 0 double bonds
⑶ Anabolism 3. Unsaturated fatty acid synthesis
① Animal cells: Desaturase enzymes that form double bonds are located in the smooth endoplasmic reticulum.
② Plant cells: Desaturase enzymes that form double bonds are located in the smooth endoplasmic reticulum and chloroplasts.
③ Type 1. Palmitoleic acid: (16:1)
○ 16 carbons, 1 double bond
○ ω-7: First alkene formation at the 7th carbon from the methyl end.
④ Type 2. Oleic acid: (18:1)
○ 18 carbons, 1 double bond
○ ω-9: First alkene formation at the 9th carbon from the methyl end.
⑤ Type 3. Linoleic acid: (18:2)
○ 18 carbons, 2 double bonds
○ ω-6: First alkene formation at the 6th carbon from the methyl end.
⑥ Type 4. α-Linolenic acid: (18:3)
○ 18 carbons, 3 double bonds
○ ω-3: First alkene formation at the 3rd carbon from the methyl end.
○ Found in vegetables and vegetable oils (corn oil).
○ Prevents skin diseases and has growth factors.
⑦ Type 5. Arachidonic acid: (20:4)
○ 20 carbons, 4 double bonds
○ ω-6: First alkene formation at the 6th carbon from the methyl end.
○ Found in animal fats and prevents skin diseases.
⑧ Omega 3 fatty acids
○ Unsaturated fatty acids forming the first alkene at the 3rd carbon from the methyl end.
○ Promote red blood cell aggregation and prostaglandin production, preventing heart disease: Also thought to prevent cancer.
○ EPA, DHA: Omega 3 fatty acids abundant in fish.
○ DHA: Good for brain development in young children.
⑨ Linoleic acid → Arachidonic acid
⑩ Arachidonic acid → PG, Thromboxane
○ Mediating enzyme: C.O.X
○ PG: Increases blood clotting, headaches, uterine contractions, set point, and gastric mucosa formation.
○ Thromboxane: Forms platelets.
○ Arachidonic acid → Leukotriene
○ PG, Thromboxane, Leukotriene: Self-degrade.
⑷ Anabolism 4. Cholesterol Synthesis
Figure 1. Cholesterol Synthesis
① Cholesterol synthesis reaction: Occurs in the liver.
○ Step 1. 2 × acetyl-CoA → acetoacetyl CoA
○ Step 2. acetyl-CoA + acetoacetyl CoA + H2O + 2NADPH → HMG-coA (cytoplasm)
○ Step 3. HMG-coA → mevalonate: Rate-determining step
○ Step 4. Formation of isoprene (C5): Mevalonate → isopentenyl pyrophosphate (PPP) by decarboxylation
○ Step 5. Isopentyl pyrophosphate (C6) → squalene (C30)
○ Step 6. Cyclic formation of squalene → cholesterol
② Cholesterol acts as a precursor for various substances.
○ Bile salts
○ Steroid synthesis
○ Cholesterol → Androstenedione → (Aromatization) → Estrone
③ Mitochondria: Mevalonate → HMG-coA → acetoacetyl coA + acetyl coA (decomposition reaction)
④ Bacteria lack the ability to synthesize cholesterol.
○ Instead, they produce hopanoids to regulate membrane fluidity.
○ However, Mycoplasma obtains cholesterol from the environment and uses it to regulate membrane fluidity.
⑸ Anabolism 5. Ketone body synthesis
① Ketone bodies form when acetyl-CoA accumulates.
② Ketone bodies are converted back to acetyl-CoA in the brain, heart, kidneys, etc., and used to generate ATP.
⑹ Anabolism 6. Phospholipid synthesis
① 1st. Fatty acids move to the smooth endoplasmic reticulum membrane.
② 2nd. Converted to fatty acid-acyl-CoA.
③ 3rd. G3P synthesized in the cytoplasm moves to the smooth endoplasmic reticulum membrane.
④ 4th. A substituent combines with two fatty-acid-acyl-CoAs in the smooth endoplasmic reticulum membrane to form phosphatidic acid.
⑤ 5th. Phosphatidic acid is used for the synthesis of triacylglycerol or phospholipids.
Entry: 2019.03.28 13:38