Lipid Anabolism and Fatty Acid Biosynthesis

Lipid Anabolism

Importance of Carbohydrates

• Carbohydrates constitute a large proportion of food intake for humans.

• Humans can only store a limited amount of energy as glycogen: up to 190 grams.

• Overeating Scenario: After acing a 160 exam, if an individual consumes:

  • 2 slices of pizza, roughly 35 g of carbs per slice: 70 g total

  • 3 cups of rice, each cup containing approximately 45 g of carbs: 135 g total

  • 3 glasses of iced tea or a similar beverage, with each glass providing about 25 g of carbs: 75 g total

Metabolism of Excess Carbohydrates

• Excess carbohydrates are catabolized via glycolysis and the Pyruvate Dehydrogenase (PDH) reaction.

• This metabolic pathway results in an increased production of acetyl CoA which then leads to:

  • Synthesis of fatty acids.

  • Formation of triglycerides (TAGs) in adipose tissue:

  • Fatty acids (FAs) cannot remain in the bloodstream long term.

Fatty Acid Biosynthesis

• Occurs in the cytosol of the cell and requires three key substrates:

  • Acetyl CoA

  • Malonyl CoA

  • NADPH

Fatty Acid Synthase Complex Components

1. Acetyl CoA-ACP transacetylase

2. Malonyl CoA-ACP transferase

3. β-Ketoacyl-ACP synthase

4. β-Ketoacyl-ACP reductase

5. β-Hydroxyacyl-ACP dehydratase

6. Enoyl-ACP reductase

Step 1A: Production of Cytosolic Acetyl CoA

• Derived from the excess glucose via the glycolysis pathway:

  1. Glycolysis results in Pyruvate.

  2. Pyruvate is converted to Acetyl CoA by the PDH complex.

  3. Acetyl CoA is produced in the mitochondria, then transported to the cytosol as citrate.

Step 1B1: Carboxylation of First Acetyl CoA

• The process involves biotin-dependent conversion:

  • Acetyl CoA is carboxylated into Malonyl CoA using the enzyme Acetyl CoA carboxylase.

  • Requires ATP, during which it transforms:

Acetyl ext{ } CoA + CO_2 + ATP
ightarrow ext{Malonyl } CoA + ADP + Pi

Step 1B2: Attachment of Malonyl CoA to ACP

• Malonyl CoA is transferred to the Acyl Carrier Protein (ACP) through the action of malonyl transacylase.

Step 1C: Attachment of Second Acetyl CoA to ACP

• The Acetyl CoA is attached to ACP via the enzyme Acetyl CoA-ACP transacetylase.

Step 1D: Condensation Reaction

• The Malonyl ACP combines with Acetyl ACP:

  • This process is catalyzed by β-Ketoacyl-ACP synthase, and results in:
    
    Malonyl-ACP + Acetyl-ACP
    ightarrow ext{β-Ketoacyl-ACP} + CO_2
    

Step 2: Reduction of β-Ketoacyl-ACP

• Catalyzed by β-Ketoacyl-ACP reductase, using NADPH:
  
  eta-Ketoacyl-ACP + NADPH
  ightarrow ext{D-3-hydroxybutyryl-ACP} + NADP^+
  

Step 3: Dehydration Reaction

• The 3-hydroxyacyl-ACP undergoes dehydration to form Crotonyl ACP:
  
  ext{D-3-hydroxybutyryl-ACP}
  ightarrow ext{Crotonyl-ACP} + H_2O
  

Step 4: Final Reduction Step

• Catalyzed by Enoyl-ACP reductase:
  
  ext{Crotonyl-ACP} + NADPH
  ightarrow ext{Butyryl-ACP} + NADP^+
  

Iterative Cycle of Fatty Acid Lengthening

• Butyryl ACP will undergo additional rounds of steps 1 - 4.

  • Each cycle adds 2 carbon atoms to the fatty acyl chain.

  • Examples of gradual elongation include:

  • C4 fatty acyl-ACP

  • C6 fatty acyl-ACP

  • C10 fatty acyl-ACP

  • C12 fatty acyl-ACP

  • C14 fatty acyl-ACP

  • Resulting in C16 fatty acyl-ACP

Cleavage of Fatty Acyl-ACP

• Fatty acyl-ACP is cleaved by Thioesterase to yield palmitate (C16:0).

• Overall reaction:
  
  8 ext{ Acetyl CoA} + 7 ext{ ATP} + 14 ext{ NADPH} + 14 H^+
  ightarrow ext{Palmitate} + 8 ext{ CoA} + 7 ext{ ADP} + 7 ext{ Pi} + 14 ext{ NADP}^+ + 6 ext{ H}_2 ext{O}
  

Further Processing of Palmitate

• Palmitate can undergo further enzymatic processes:

  • Desaturation:

  • Example products include:

    • Palmitoleate (16:1 Δ9)

    • Stearate (18:0)

  • Elongation:

  • Produces longer saturated fatty acids like:

    • Oleate (18:1 Δ9)

    • Additional unsaturated fatty acids (Longer PUFAs)

    • Example: Linoleate (18:2 Δ9,12)

    • a-Linolenic acid (18:3 Δ6,9,12)

    • Arachidonic acid (20:4 Δ5,8,11,14)

Triglyceride Biosynthesis

• Fatty acids are stored in adipose tissues as TAGs (Triglycerides):
  ext{DAG} + 2 ext{ FA} + ext{H}_2 ext{O}
  ightarrow ext{TAG} + ext{Pi}

• Involves enzymatic actions of:

  • Glycerol 3-P-acyl transferase

  • Phosphatase

  • Diglyceride acyl transferase

Cholesterol Biosynthesis Overview

• Cholesterol is synthesized in the cytosol:

  1. Initial stage involves the formation of Mevalonate from Acetyl CoA:

  2. Key enzymes include HMG-CoA synthase and HMG-CoA reductase.

Cholesterol Synthesis Pathway

• From Acetyl CoA to HMG-CoA to Mevalonate:

  • HMG-CoA reductase is the committed step in this pathway, also inhibited by cholesterol through feedback inhibition.

Three Stages of Cholesterol Biosynthesis

1. Formation of Mevalonate:

   • 6 Acetyl-CoA form 6 Mevalonate through a series of reactions.

2. Polymerization of Mevalonate:

   • Produces **Squalene.

3. Cyclization of Squalene:

   • Final product is Cholesterol.

Summary of Cholesterol Precursors

• Acetate, through several steps, is transformed into:

  • Mevalonate

  • Isoprene derivatives

  • Ultimately yields Cholesterol.

Regulation of Cholesterol Biosynthesis

• Involves multiple feedback mechanisms:

  • ACAT: Converts cholesterol into storage form (Cholesteryl esters).

  • Regulation primarily through HMG-CoA reductase, which is influenced by levels of insulin and glucagon.

  • LDL-cholesterol uptake via receptor-mediated endocytosis regulates cholesterol levels.

Conclusion

• Overview of lipid anabolism highlights how excess carbohydrates are converted to lipids, emphasizing the interconnectedness of metabolic pathways and regulatory mechanisms.

LIPID ANABOLISM

Comprehensive Reviewer

Fatty Acid Synthesis, TAG Formation & Cholesterol Biosynthesis

Chapter 1: Introduction to Lipid Anabolism

Overview

• Lipid anabolism is the synthesis of lipids from simpler precursors

• Essential for storing excess energy from carbohydrates and proteins

• Occurs when energy is abundant (fed state)

• Humans can only store ~190 g of glycogen; excess carbohydrates are converted to fat

Why Convert Carbohydrates to Lipids?

• Limited glycogen storage: Only 190 g total (liver + muscle)

• Energy density: Lipids provide 9 kcal/g vs. carbohydrates at 4 kcal/g

• Excess acetyl-CoA: Cannot be stored; is converted to fatty acids

• Blood glucose regulation: Prevents excessive glucose accumulation

Pathway Overview

Excess Glucose → Glycolysis → Pyruvate → Acetyl-CoA → Malonyl-CoA → Fatty Acids → TAGs

Chapter 2: Fatty Acid Biosynthesis

Location and Requirements

• Location: Cytosol

• Substrates: Acetyl-CoA, Malonyl-CoA, NADPH

• Cofactor: Acyl Carrier Protein (ACP)

• Complex: Fatty Acid Synthase (FAS) - multienzyme complex

Fatty Acid Synthase Complex

• Vertebrates: All 7 enzymatic activities on one large polypeptide

• Yeast: 7 activities on two separate polypeptides

• Bacteria/Plants: 7 separate polypeptides

Seven Enzymatic Activities in FAS Complex

Chapter 3: Detailed Fatty Acid Synthesis Reactions

Stage 1A: Citrate Transport & Acetyl-CoA Production

• Acetyl-CoA produced in mitochondria cannot cross inner membrane

• Solution: Citrate-malate shuttle transports acetyl units

Stage 1B: Formation of Malonyl-CoA

• Rate-limiting step of fatty acid synthesis

• Requires biotin cofactor

• Malonyl-CoA is a signaling molecule that inhibits fatty acid oxidation

Stage 1C: Primer Formation

Stage 1D: Condensation Reaction

• Releases CO₂ from malonyl group (provides driving force)

• Forms 2-carbon extension

Stage 2: Reduction of Ketone

Stage 3: Dehydration

Stage 4: Reduction of Double Bond

Cycle Repeats with Chain Elongation

• C₄ fatty acyl-ACP enters Stage 1D again

• Another malonyl-ACP attaches

• Process continues: C₆ → C₈ → C₁₀ → C₁₂ → C₁₄ → C₁₆ (palmitate)

Palmitate Release

Overall Equation for Palmitate Synthesis

8 Acetyl-CoA + 7 ATP + 14 NADPH + 14 H⁺ → Palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP⁺ + 6 H₂O

Chapter 4: Fatty Acid Modification

Elongation and Desaturation

• Palmitate (C16:0) is end product of FAS

• Can be elongated to stearate (C18:0) and longer fatty acids

• Can be desaturated to introduce double bonds

• Desaturation requires O₂ and cytochrome P450

Common Fatty Acids Produced

Chapter 5: Triacylglycerol (TAG) Synthesis

Overview

• Storage form of excess fatty acids

• Synthesized in liver and adipose tissue

• Primary storage molecule for lipids

Step-by-Step TAG Synthesis

Overall TAG Synthesis

Glycerol-3-P + 3 Fatty Acyl-CoA → TAG + 3 CoA + Pi + H₂O

Chapter 6: Cholesterol Biosynthesis

Overview

• All carbon atoms from acetyl-CoA

• Primarily synthesized in liver (~70%), also in adrenal cortex and intestine

• Occurs in cytosol and ER

• ~900 mg synthesized daily; ~300-400 mg from diet

Three Stages of Cholesterol Synthesis

Stage 1: Formation of Mevalonate

Stage 2: Polymerization of Mevalonate

• 6 Mevalonate molecules → Squalene

• Forms 30-carbon isoprene unit (farnesyl-PPi)

• Two C₁₅ units condense to form C₃₀ squalene

Stage 3: Cyclization to Cholesterol

• Squalene → Lanosterol (first steroid intermediate)

• Multiple oxidation, reduction, and isomerization steps

• Forms the characteristic 4-ring steroid structure

Chapter 7: Regulation of Lipid Anabolism

Regulation of Fatty Acid Synthesis

Acetyl-CoA Carboxylase (Key Control Point)

• Allosteric activation: Citrate (fed state signal)

• Allosteric inhibition: Fatty acyl-CoA (product inhibition)

• Covalent modification: Phosphorylation (by AMPK) inactivates; dephosphorylation activates

• Hormonal control: Insulin activates (fed); glucagon/epinephrine inhibit (fasted)

Regulation of Cholesterol Synthesis

HMG-CoA Reductase (Rate-Limiting Enzyme)

• Feedback inhibition: Cholesterol inhibits its own synthesis

• Covalent modification: Phosphorylation (by AMPK) inactivates

• Transcriptional regulation: Cholesterol decreases enzyme expression

• Degradation: High cholesterol increases proteolytic degradation

Hormonal Regulation

• Insulin (fed state): Activates FAS and cholesterol synthesis

• Glucagon (fasted state): Inhibits FAS through phosphorylation of acetyl-CoA carboxylase

• Epinephrine: Inhibits synthesis; promotes lipid catabolism

Key Definitions

Acetyl-CoA Carboxylase

• Catalyzes the first committed step of fatty acid synthesis: conversion of acetyl-CoA to malonyl-CoA

Acyl Carrier Protein (ACP)

• Small protein that holds growing fatty acid chain during synthesis; contains phosphopantetheine prosthetic group

Fatty Acid Synthase (FAS)

• Large multienzyme complex that catalyzes synthesis of palmitate from acetyl-CoA and malonyl-CoA

HMG-CoA Reductase

• Rate-limiting enzyme of cholesterol synthesis; converts HMG-CoA to mevalonate using NADPH

Mevalonate

• 6-carbon intermediate formed from HMG-CoA; first committed precursor to cholesterol

Malonyl-CoA

• Activated form of acetyl-CoA; provides 2-carbon units for fatty acid chain extension

Palmitate

• 16-carbon saturated fatty acid (C16:0); direct end product of mammalian fatty acid synthase

Review Questions

1. Why are excess carbohydrates converted to fatty acids rather than stored as additional glycogen?

2. Where does fatty acid synthesis occur, and what are the three main substrates required?

3. Describe the role of citrate in transporting acetyl-CoA from the mitochondria to the cytosol.

4. Write the complete equation for the formation of malonyl-CoA from acetyl-CoA.

5. What are the seven enzymatic activities of the fatty acid synthase complex?

6. In the condensation reaction, why is CO₂ released from the malonyl group important?

7. How many NADPH molecules are required to synthesize one molecule of palmitate, and why?

8. How are palmitate and other saturated fatty acids further modified after synthesis?

9. Write the complete equation for the synthesis of palmitate from acetyl-CoA, ATP, and NADPH.

10. What are the three steps involved in TAG synthesis, and which enzymes catalyze each step?

11. Explain the three stages of cholesterol synthesis.

12. What is the rate-limiting enzyme of cholesterol synthesis, and how is it regulated?

13. How does insulin regulate fatty acid and cholesterol synthesis?

14. Why is acetyl-CoA carboxylase considered a critical control point in lipid anabolism?

15. How is the synthesis of fatty acids different in bacteria/plants versus vertebrates?

Quick Reference: Complete Enzyme Summary