Lipid Metabolism

Overview of Fatty Acid Breakdown

  • **Key Components: **
    • CoA-SH
    • Palmitoyl-CoA
    • Acetyl-CoA
  • **Process: **
    • Successive removal of acetyl-CoA (C₂) units from fatty acids.
  • Source: Murray RK et al., Harper's Illustrated Biochemistry, 29th Edition.

Oxidation of Fatty Acids

  • Fatty acids exist as:
    • Saturated (no double bonds)
    • Unsaturated (one or more double bonds)
  • Presence in Biological Molecules:
    • Majorly found in triacylglycerols and phospholipids.
    • In plants and animals, unsaturated fatty acids are predominant.
  • Pathway Variations:
    • Saturated fatty acids undergo standard β-oxidation.
    • Unsaturated fatty acids have pathway modifications.

Transport of Fatty Acids for β-Oxidation

  • Key Components:
    • FFA (Free Fatty Acids)
    • Acyl-CoA
    • Carnitine
  • Transport Mechanism:
    • Acyl-CoA Synthetase: Converts FFA to Acyl-CoA using ATP.
    • Carnitine Acyltransferase I: Facilitates Acyl-CoA transfer to carnitine across the outer mitochondrial membrane.
    • Carnitine Acylcarnitine Translocase: Transports acylcarnitine into the mitochondrial matrix.

β-Oxidation of Saturated Fatty Acids

  1. First step:

    • Enzyme: Acyl-CoA dehydrogenase (FAD-dependent)
    • Reaction: Dehydrogenation forms trans A²-enoyl-CoA and reduces FAD to FADH₂.
    • Isoenzymes: Specific for carbon chain lengths (short, intermediate, long).

  2. Second step:

    • Enzyme: Enoyl-CoA hydratase
    • Reaction: Hydration at the double bond gives β-hydroxyacyl-CoA.

  3. Third step:

    • Enzyme: β-Hydroxyacyl-CoA dehydrogenase
    • Reaction: Dehydrogenation forms β-ketoacyl-CoA (reduces NAD+ to NADH + H+).

  4. Fourth step:

    • Enzyme: Acyl-CoA acetyltransferase (thioester cleavage)
    • Reaction: Produces Acetyl-CoA and a shortened fatty acyl-CoA (n-2 carbons).
  • The process repeats, continuing until the entire fatty acid is converted to acetyl-CoA.
  • Acetyl-CoA enters the Krebs cycle.

β-Oxidation of Unsaturated Fatty Acids

  • Additional Enzymes Involved:
    • Isomerase and Reductase are essential for modifying trans double bonds.
  • Examples:
    • Oleic Acid: A monounsaturated fatty acid with 18 carbons and one cis double bond (C9-C10).
    • Linolenic Acid: A polyunsaturated fatty acid with 18 carbons containing two cis double bonds (C9-C10 and C12-C13).
  • Inside Mitochondria:
    • Unsaturated fatty acids are present as fatty acyl-CoA and undergo β-oxidation in the mitochondrial matrix.

β-Oxidation of Polyunsaturated Fatty Acids

  • Mechanism:
    • Linolenic acid undergoes three cycles of β-oxidation, yielding three acetyl-CoA and a 12-carbon chain fatty acyl-CoA with cis double bonds at positions 3 and 6.
  • Mitochondrial enzymes convert cis bonds into trans bonds for effective breakdown.
  • The reductase and isomerase facilitate further modification leading to additional cycles of β-oxidation, producing more acetyl-CoA.
  • All final acetyl-CoA molecules enter the Krebs cycle.

Oxidation of Odd Chain Fatty Acids

  • Process:
    • Odd-length fatty acids are oxidized similarly, producing acetyl-CoA and ultimately yielding propionyl-CoA (3 carbons).
  • Conversion:
    • Propionyl-CoA is transformed into succinyl-CoA (TCA cycle component) through multiple enzymatic actions (Biotin and Vitamin B12 dependent).
  • Significance of Propionate:
    • Important precursor for gluconeogenesis, especially in ruminants.
  • Methylmalonic aciduria: A metabolic disorder due to enzyme deficiencies leading to toxic accumulation of methylmalonic acid.

Ketogenesis

  • Pathway Overview:
    • The process converts fatty acids into ketone bodies (acetone, acetoacetate, β-hydroxybutyrate) in the liver.
  • Mechanism and Regulation:
    • Activated acetyl-CoA enters the pathway when glucose is scarce, producing ketones as energy sources during fasting or starvation.
    • Significant for organs primarily relying on ketone bodies (brain, muscles) for ATP synthesis.

Cholesterol Synthesis

  • Mevalonate Pathway Overview:
    • Key transformations include activating isoprene units from mevalonate through multiple enzymes and reactions, leading to farnesyl diphosphate and further applications (Ubiquinone, Dolichol).
  • Cholesterol Biosynthesis:
    1. Acetyl-CoA → HMG-CoA → Mevalonate (rate-limiting step via HMG-CoA reductase).
    2. Sequential transformations via various enzymes produce cholesterol from the lipid precursors.
  • Regulation:
    • Insulin and glucagon modulate HMG-CoA reductase activity impacting cholesterol levels.

Bile Acid Biosynthesis

  • Pathway:
    • Cholesterol is hydroxylated via multiple enzymes forming bile acids, with critical reactions being rate-limiting and branched based on enzyme presence or absence.
    • Conjugation with glycine or taurine increases solubility, preparing bile acids for intestinal health.

Summary of Key Concepts

  • Fatty Acid Metabolism:
    • Encompasses the breakdown (β-oxidation), synthesis, transport, and conversion processes of fatty acids.
  • Ketogenesis Role:
    • Offers alternative energy substrates when carbohydrates are insufficient.
  • Cholesterol and Bile Acid Significance:
    • Central for cellular functions and metabolic homeostasis, largely influencing heart and general health.
  • Clinical Relevance:
    • Disorders related to lipid metabolism can have substantial health implications and are linked with metabolic diseases.