Lipids (Biochemistry Unit 4) Notes

Classification of Lipids

  • Lipids are a diverse group of hydrophobic biomolecules.
  • Essential for energy storage, cellular structures, and signaling.
  • (Transcript notes an extraneous word “Love” which is not a key concept; ignore for content.)

Simple Lipids

  • Definition: Esters of fatty acids with alcohols.
  • Examples:
    • Triglycerides (fats)
    • Waxes

Compound Lipids

  • Definition: Lipids containing an additional functional group beyond the basic ester linkage.
  • Examples:
    • Phospholipids
    • Glycolipids

Derived Lipids

  • Definition: Hydrolysis products or derivatives of simple/compound lipids.
  • Examples:
    • Fatty acids
    • Cholesterol
    • Steroids

Lipid Metabolism

  • Refers to the synthesis and degradation of lipids in the body.
  • Primary purposes:
    • Energy production
    • Structural needs of cells

Major Processes in Lipid Metabolism

  • Lipogenesis: fatty acid and triglyceride synthesis from acetyl-CoA.
  • Lipolysis: breakdown of triglycerides into glycerol and fatty acids.
  • Beta-oxidation: mitochondrial oxidation of fatty acids for ATP production.
  • Ketogenesis: ketone body production during fasting or low carbohydrate intake.
  • These constitute the major processes by which lipids are managed in the body.

Cholesterol Metabolism

  • Cholesterol is a steroid lipid.
  • Vital roles include:
    • Cell membranes
    • Vitamin D synthesis
    • Steroid hormone production

Metabolic Pathway of Cholesterol

  • Cholesterol is synthesized mainly in the liver from acetyl-CoA via the HMG-CoA reductase pathway.
  • Regulation occurs through feedback inhibition and dietary intake.
  • Transported in blood via lipoproteins: LDL and HDL.
  • Excreted from the body as bile acids.

Lipoproteins

  • Lipoproteins are complexes of lipids and proteins that transport cholesterol and triglycerides in the bloodstream.
  • They are commonly referred to as Good Cholesterol and Bad Cholesterol based on their roles.

HDL, LDL, VLDL/IDL

  • HDL ( High-Density Lipoproteins )
    • Function: Transport excess cholesterol to the liver.
    • Descriptor: “Good Cholesterol, Protective.”
  • LDL ( Low-Density Lipoprotein )
    • Function: Delivers cholesterol to peripheral tissues.
    • Descriptor: “Bad cholesterol, Atherogenic.”
  • VLDL / IDL ( Very-Low-Density Lipoproteins / Intermediate-Density Lipoprotein )
    • Function: Carry triglycerides; precursors to LDL.
    • Descriptor: “May contribute to heart disease risk.”

L-Carnitine

  • L-Carnitine is a vitamin-like compound.
  • Function: Facilitates transport of long-chain fatty acids into the mitochondria for β-oxidation.

Beta-Oxidation

  • Definition: Mitochondrial catabolic process that breaks down fatty acids into acetyl-CoA, which then enters the TCA cycle.

Beta-Oxidation Cycle Steps

  • Step 1: Dehydrogenation (FADH2 formed).
  • Step 2: Hydration.
  • Step 3: Second Dehydrogenation (NADH formed).
  • Step 4: Thiolytic cleavage.
  • Acetyl-CoA production per 2-carbon unit cleaved: 1 Acetyl-CoA1\ \text{Acetyl-CoA}.
  • Note: Each cycle shortens the fatty acyl chain by 2 carbon atoms and yields one acetyl-CoA (per cycle for the 2-carbon unit).

Acetyl-CoA and Its Role

  • Acetyl-CoA is the product that enters the TCA cycle for energy production.
  • Serves as a central metabolic intermediate linking carbohydrate, lipid, and protein metabolism.

Connections to Broader Concepts

  • Lipid metabolism intersects with carbohydrate metabolism via acetyl-CoA as a common substrate.
  • Energy balance and fasting states influence whether lipids are stored (lipogenesis) or mobilized (lipolysis and beta-oxidation).
  • Lipoprotein transport links hepatic synthesis to peripheral tissue needs and cardiovascular risk.

Key Terms to Remember

  • Lipogenesis, Lipolysis, Beta-oxidation, Ketogenesis
  • Cholesterol, Mevalonate pathway (HMG-CoA reductase pathway)
  • Lipoproteins: HDL, LDL, VLDL, IDL
  • L-Carnitine
  • Acetyl-CoA
  • Per 2-carbon unit cleaved: 1 Acetyl-CoA