TAG Synthesis

Q1: What are the two pathways for glycerol-3-phosphate synthesis, and which tissues use each?

A1: Pathway A: Glycerol + ATP → G3P (glycerol kinase) – liver, kidney, intestine (NOT adipose). Pathway B: DHAP + NADH + H⁺ → G3P (G3PDH) – all tissues (including adipose).

Q2: Why can’t adipose tissue use free glycerol for TAG synthesis?

A2: Adipose tissue lacks glycerol kinase.

Q3: Write the reaction for glycerol-3-phosphate formation from glycerol, including the enzyme.

A3: Glycerol + ATP → Glycerol-3-phosphate + ADP. Enzyme: Glycerol kinase (GK).

Q4: Write the reaction for glycerol-3-phosphate formation from DHAP, including the enzyme and coenzyme.

A4: Dihydroxyacetone phosphate (DHAP) + NADH + H⁺ → Glycerol-3-phosphate + NAD⁺. Enzyme: Glycerol-3-phosphate dehydrogenase (G3PDH).

Q5: What is the subcellular localization of TAG synthesis enzymes?

A5: Cytosol and endoplasmic reticulum (ER) membrane (hepatocytes and adipocytes).

Q6: Write the first acylation reaction in TAG synthesis, including enzyme and product.

A6: Glycerol-3-phosphate + Fatty acyl-CoA → Lysophosphatidic acid (LPA) + CoA-SH. Enzyme: GPAT (glycerol-3-phosphate acyltransferase).

Q7: Write the second acylation reaction in TAG synthesis, including enzyme and product.

A7: Lysophosphatidic acid (LPA) + Fatty acyl-CoA → Phosphatidic acid (PA) + CoA-SH. Enzyme: LPAAT (lysophosphatidic acid acyltransferase).

Q8: What is the structure of phosphatidic acid (PA)?

A8: Glycerol-3-phosphate with fatty acyl chains at C-1 and C-2.

Q9: Write the reaction for conversion of phosphatidic acid to diacylglycerol, including enzyme.

A9: Phosphatidic acid (PA) + H₂O → Diacylglycerol (DAG) + Pi. Enzyme: Phosphatidic acid phosphatase (PAP, also called lipin).

Q10: Write the final step of TAG synthesis, including enzyme.

A10: Diacylglycerol (DAG) + Fatty acyl-CoA → Triacylglycerol (TAG) + CoA-SH. Enzyme: Acyl-CoA:diacylglycerol acyltransferase (DGAT).

Q11: What is the rate-limiting enzyme of TAG synthesis, and why?

A11: DGAT (acyl-CoA:diacylglycerol acyltransferase). It catalyzes the final committed step, committing DAG to TAG vs. phospholipid synthesis.

Q12: Name the two isoforms of DGAT and their tissue distribution.

A12: DGAT1 (widely distributed) and DGAT2 (predominant hepatic isoform).

Q13: How does insulin regulate DGAT activity?

A13: Insulin activates DGAT via dephosphorylation (protein phosphatase 1), increases DGAT gene expression, and provides substrates (glucose, fatty acids).

Q14: How does glucagon regulate DGAT activity?

A14: Glucagon inhibits DGAT via PKA-mediated phosphorylation, decreases gene expression, and reduces substrate supply (promotes lipolysis and fatty acid oxidation).

Q15: Name two allosteric activators and two inhibitors of DGAT.

A15: Activators: Insulin, glucose, fatty acids, phosphatidic acid. Inhibitors: Glucagon, cAMP, epinephrine, high concentrations of long-chain fatty acyl-CoA.

Q16: What is the primary source of glycerol-3-phosphate in adipose tissue, and why is insulin required?

A16: Glucose via glycolysis. Insulin is required for GLUT4 translocation and glucose uptake.

Q17: What happens to glycerol released during lipolysis in adipose tissue?

A17: Glycerol is exported to the blood and transported to the liver for gluconeogenesis (cannot be re-esterified in adipose tissue).

Q18: What are the two sources of glycerol-3-phosphate in the liver?

A18: 1) Glucose via glycolysis (DHAP → G3P). 2) Free glycerol via glycerol kinase (glycerol + ATP → G3P).

Q19: Why is the liver able to re-esterify glycerol from lipolysis, but adipose tissue cannot?

A19: Liver has glycerol kinase; adipose tissue does not.

Q20: What is the energy density of TAG compared to carbohydrates and proteins?

A20: TAG = 9 kcal/g; Carbohydrates = 4 kcal/g; Proteins = 4 kcal/g.

Q21: List three advantages of TAG as an energy storage molecule.

A21: 1) Highly reduced (more C-H bonds = more energy). 2) Stored anhydrously (compact, no water). 3) Mobilizable via hormone-sensitive lipase.

Q22: How does malonyl-CoA regulate the balance between fatty acid storage and oxidation?

A22: High malonyl-CoA (fed state) inhibits CPT-I → fatty acids stay in cytosol → esterified to TAG. Low malonyl-CoA (fasted) activates CPT-I → fatty acids enter mitochondria → β-oxidation.

Q23: What is the role of insulin in promoting TAG storage in adipose tissue? (List 5 effects)

A23: 1) ↑ LPL activity (fatty acid uptake). 2) ↑ GLUT4 (glucose uptake → G3P). 3) ↑ fatty acid transporters. 4) ↓ lipolysis (inhibits HSL). 5) ↑ TAG synthesis enzymes (ACC, GPAT, DGAT).

Q24: What is the primary role of TAG synthesis in the liver versus adipose tissue?

A24: Liver – packaging TAG into VLDL for transport. Adipose tissue – long-term energy storage.

Q25: Compare glycerol-3-phosphate sources in liver vs. adipose tissue.

A25: Liver: glucose + glycerol (glycerol kinase present). Adipose: glucose only (no glycerol kinase).

Q26: What is VLDL, and how does it relate to TAG synthesis?

A26: VLDL (Very Low Density Lipoprotein) is assembled in the liver from TAG, apolipoprotein B-100, phospholipids, and cholesterol. It transports TAG to peripheral tissues.

Q27: How does fasting (high glucagon) affect TAG synthesis?

A27: Glucagon inhibits ACC (↓ malonyl-CoA → ↑ CPT-I → fatty acids oxidized), phosphorylates GPAT/DGAT (↓ activity), and reduces G3P supply (↓ glucose uptake). Net: ↓ TAG synthesis, ↑ lipolysis.

Q28: What is the role of insulin in VLDL synthesis?

A28: Insulin increases substrate availability (glucose, fatty acids) for TAG synthesis, thereby increasing VLDL production.

Q29: What is hepatic steatosis, and how does it develop?

A29: Fatty liver (TAG accumulation in hepatocytes). Causes: excessive fatty acid supply, increased de novo lipogenesis, and impaired VLDL secretion. Associated with insulin resistance.

Q30: Which enzymes in TAG synthesis are potential drug targets for obesity?

A30: DGAT inhibitors (reduce TAG synthesis), GPAT inhibitors (reduce hepatic TAG), and LPL modulators (control fatty acid uptake).