Lipid Synthesis Notes

Learning Goals

  • Detailed Mechanism of the seven enzymatic activities of fatty acid synthase (palmitate synthase)
  • Regulation of fatty acid synthesis
  • General Mechanism (and energy requirements) of fatty acid desaturation, diacylglycerol synthesis, and glycerophospholipid synthesis
  • Key Steps in cholesterol synthesis

Glycerophospholipid Synthesis Overview

  • Process Steps:
    • Produce 16-carbon fatty acid chains (palmitic acid product)
    • Lengthen acyl chain if required
    • Desaturate acyl chain if needed
    • Add glycerol backbone to form phosphatidic acid product
    • Incorporate a head group to complete glycerophospholipid

Fatty Acid Metabolism

  • Catabolism vs. Anabolism:
    • Catabolism
    • Produces acetyl-CoA (2C)
    • Generates reducing power (NADH, FADH2)
    • Occurs in the mitochondria
    • Anabolism
    • Requires acetyl-CoA (2C) and malonyl-CoA (3C)
    • Utilizes reducing power from 2 NADPH
    • Takes place in the cytosol in animals; in chloroplasts in plants

Synthesis of Malonyl-CoA

  • Reaction: Acetyl-CoA+Bicarbonate→Malonyl-CoA\text{Acetyl-CoA} + \text{Bicarbonate} \rightarrow \text{Malonyl-CoA}
    • Catalyzed by Acetyl-CoA Carboxylase (has three subunits)
    • Uses Biotin as a prosthetic group, enabling CO2 transfer
    • Reaction consumes ATP
  • Implications: Malonyl-CoA serves as a building block for fatty acid synthesis

Fatty Acid Synthase Complex

  • FAS I (in vertebrates):
    • Single polypeptide chain
    • Synthesizes palmitate (16:0) exclusively
  • FAS II (in plants and bacteria):
    • Made of separate, diffusible enzymes
    • Capable of producing various fatty acids (saturated, unsaturated, branched, various lengths)

Enzymatic Mechanism in Fatty Acid Synthesis

  • Active Sites of FAS I:
    1. KS (ketoacyl synthase)
    2. MAT (malonyl-acetyl-CoA transferase)
    3. KR (ketoacyl reductase)
    4. DH (dehydratase)
    5. ER (enoyl-ACP reductase)
    6. ACP (acyl carrier protein)
    7. TE (thioesterase) - releases final products

Mechanistic Steps in the Four-Step Cycle of Fatty Acid Synthesis

  1. Condensation: Acetyl-CoA and malonyl-CoA condense forming a β-keto thioester, releasing CO2.
  2. Reduction: The β-keto group is reduced to an alcohol using NADPH (catalyzed by KR).
  3. Dehydration: H2O is eliminated, forming a double bond (catalyzed by DH).
  4. Reduction: The double bond is reduced to yield fully saturated fatty acid (catalyzed by ER).

Synthesis of Palmitate

  • Steps:
    • Starts with an acetyl group and undergoes seven rounds of condensation with malonyl-CoA (resulting in palmitate, 16C)
    • Releases palmitate via thioester hydrolysis by TE
    • Consumes 2 NADPH per round of synthesis

Stoichiometry of Palmitate Synthesis

  1. Malonyl-CoA Formation:
    7Acetyl-CoA+7CO2+7ATP→7Malonyl-CoA+7ADP+7Pi7 \text{Acetyl-CoA} + 7 \text{CO}_2 + 7 \text{ATP} \rightarrow 7 \text{Malonyl-CoA} + 7 \text{ADP} + 7 \text{Pi}
  2. Synthesis Reaction:
    Acetyl-CoA+7Malonyl-CoA+14NADPH+14H+→Palmitate+7CO<em>2+8CoA+14NADP++6H</em>2O\text{Acetyl-CoA} + 7 \text{Malonyl-CoA} + 14 \text{NADPH} + 14 \text{H}^+ \rightarrow \text{Palmitate} + 7 \text{CO}<em>2 + 8 \text{CoA} + 14 \text{NADP}^+ + 6 \text{H}</em>2\text{O}

Regulation of Fatty Acid Synthesis

  • Allosteric and Hormonal Control:
    • Inhibitors: Palmitoyl-CoA inhibits malonyl-CoA formation.
    • Activators: Citrate promotes fatty acid synthesis.
    • Hormonal influence: Hormones like glucagon and epinephrine affect the activity via phosphorylation of Acetyl-CoA Carboxylase.

Fatty Acid Desaturation

  • Desaturation Mechanism: Involves mixed-function oxidase, requiring NADPH and cytochrome b5.
  • Types of Fatty Acids:
    • Mono-unsaturated (palmitoleate, oleate) and poly-unsaturated fatty acids (linoleate, linolenate) which are essential for mammals, sourced from plants.

Glycerophospholipid Synthesis

  • Precursor Molecules: Fatty acyl-CoA and glycerol 3-phosphate (resulting from glycolysis).
  • Diacylglycerol Formation: Involves acylation leading to phosphatidic acid, the key precursor in the synthesis.
  • Head Group Attachment: Various strategies for addition via CDP activation or head-group recycling.

Cholesterol Synthesis Overview

  • Key Steps:
    1. Synthesis of Mevalonate (6C) from acetyl-CoA.
    2. Conversion to activated isoprenes (5C).
    3. Condensation into squalene (30C).
    4. Conversion of squalene to cholesterol (final product, critical for membrane structure).
  • Regulation: Mevalonate synthesis is the key regulatory step in cholesterol biosynthesis.