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Chemistry

Properties

The unique structures of lipids dictate their physical and chemical behaviors. This knowledge is essential for understanding how lipids are transported, stored, and used in the body

Physical and Chemical Properties

  • Lipoproteins
    • Physical: Spherical particles with a core of triglycerides and cholesterol esters, surrounded by a shell of phospholipids, cholesterol, and apolipoproteins
    • Chemical: Amphipathic nature allows transport of hydrophobic lipids in the aqueous blood
  • Phospholipids
    • Physical: Amphipathic molecules with a polar head group and nonpolar fatty acid tails. They form bilayers in aqueous solutions
    • Chemical: Hydrolyzed by phospholipases. Participate in cell signaling and membrane structure
  • Triglycerides
    • Physical: Nonpolar, hydrophobic molecules that are liquid at body temperature
    • Chemical: Ester bonds are hydrolyzed by lipases. Primary form of energy storage
  • Cholesterol
    • Physical: A rigid, hydrophobic steroid ring structure
    • Chemical: Precursor for steroid hormones and bile acids. Can form esters with fatty acids
  • Apolipoproteins
    • Physical: Proteins that bind to lipoproteins, providing structure and targeting signals
    • Chemical: Interact with cell surface receptors and enzymes. Activate or inhibit enzymes in lipid metabolism

Lipoproteins

  • Definition: Complex particles that transport lipids (triglycerides, cholesterol) in the aqueous environment of the blood. They are essential for delivering lipids to and from tissues
  • Structure
    • Core: Hydrophobic core containing triglycerides and cholesterol esters
    • Shell: Amphipathic shell containing phospholipids, free cholesterol, and apolipoproteins
  • Classes
    • Chylomicrons: Transport dietary triglycerides from the intestine to tissues
    • VLDL (Very-Low-Density Lipoproteins): Transport triglycerides from the liver to tissues
    • LDL (Low-Density Lipoproteins): Transport cholesterol from the liver to tissues
    • HDL (High-Density Lipoproteins): Transport cholesterol from tissues back to the liver (reverse cholesterol transport)
  • Physical Properties
    • Shape: Spherical
    • Size: Varies depending on the class (chylomicrons > VLDL > LDL > HDL)
    • Density: Varies depending on the lipid and protein content (HDL > LDL > VLDL > chylomicrons)
    • Solubility: Amphipathic nature allows them to be dispersed in the aqueous blood
  • Chemical Properties
    • Amphipathic Nature: The outer shell is amphipathic, allowing the hydrophobic lipids in the core to be transported in the blood
    • Apolipoprotein Interactions: Apolipoproteins on the surface interact with cell surface receptors and enzymes involved in lipid metabolism
    • Lipolysis: Triglycerides in chylomicrons and VLDL are hydrolyzed by lipoprotein lipase (LPL), releasing fatty acids for uptake by tissues
  • Apolipoproteins Functions
    • Structural Component
    • Ligands for receptors
    • Enzyme regulators

Phospholipids

  • Definition: Amphipathic lipids that are major components of cell membranes. They consist of a glycerol backbone, two fatty acid tails, and a phosphate group attached to a polar head group
  • Structure
    • Glycerol Backbone: A three-carbon molecule
    • Fatty Acid Tails: Two fatty acids attached to the glycerol backbone via ester bonds. One is saturated and the other is unsaturated
    • Phosphate Group: A phosphate group attached to the third carbon of glycerol, linked to a polar head group
    • Polar Head Group: Varies (e.g., choline, serine, ethanolamine, inositol)
  • Physical Properties
    • Amphipathic Nature: They have both hydrophobic (fatty acid tails) and hydrophilic (polar head group) regions
    • Bilayer Formation: In aqueous solutions, phospholipids spontaneously form bilayers, with the hydrophobic tails facing inward and the hydrophilic head groups facing outward
    • Fluidity: The fluidity of the bilayer depends on the saturation of the fatty acid tails and the presence of cholesterol
  • Chemical Properties
    • Hydrolysis: Phospholipids can be hydrolyzed by phospholipases, releasing fatty acids and other molecules
    • Cell Signaling: Some phospholipids (e.g., phosphatidylinositol) play a role in cell signaling pathways
    • Membrane Structure: They are essential components of cell membranes, providing structure and flexibility

Triglycerides

  • Definition: Neutral lipids consisting of a glycerol molecule esterified with three fatty acids. They are the primary form of energy storage in the body
  • Structure
    • Glycerol Backbone: A three-carbon molecule
    • Fatty Acids: Three fatty acids attached to the glycerol backbone via ester bonds. The fatty acids can be saturated, unsaturated, or a combination thereof
  • Physical Properties
    • Hydrophobic Nature: Nonpolar and insoluble in water
    • Liquid at Body Temperature: Most triglycerides are liquid at body temperature (fats that are solid at room temperature have a higher proportion of saturated fatty acids)
    • Energy Density: High energy density (9 kcal/gram)
  • Chemical Properties
    • Ester Bonds: The ester bonds linking the fatty acids to glycerol can be hydrolyzed by lipases, releasing fatty acids and glycerol
    • Saponification: Triglycerides can be saponified (hydrolyzed with a strong base) to produce glycerol and soap (fatty acid salts)
    • Oxidation: Fatty acids can be oxidized to produce energy (beta-oxidation)

Cholesterol

  • Definition: A sterol lipid that is an essential component of cell membranes and a precursor for steroid hormones and bile acids
  • Structure
    • Steroid Ring Structure: Four fused hydrocarbon rings (A, B, C, and D)
    • Hydroxyl Group: A hydroxyl (-OH) group at carbon 3, making it weakly amphipathic
    • Alkyl Side Chain: An alkyl side chain at carbon 17
  • Physical Properties
    • Hydrophobic Nature: Mostly nonpolar and insoluble in water
    • Rigid Structure: The steroid ring structure is rigid and provides stability to cell membranes
    • Crystalline Solid: Cholesterol is a crystalline solid at room temperature
  • Chemical Properties
    • Precursor for Steroid Hormones: Cholesterol is the precursor for steroid hormones such as cortisol, aldosterone, testosterone, estrogen, and progesterone
    • Precursor for Bile Acids: Cholesterol is converted to bile acids in the liver, which aid in the digestion and absorption of fats
    • Esterification: The hydroxyl group at carbon 3 can be esterified with a fatty acid, forming cholesterol esters, which are more hydrophobic and are stored in cells and lipoproteins
    • Hydroxylation: Cholesterol can be hydroxylated to form various oxysterols, which have regulatory functions

Apolipoproteins

  • Definition: Proteins that bind to lipoproteins, providing structural support, targeting signals, and enzyme regulation
  • Types
    • ApoA-I: Major protein of HDL; activates LCAT (lecithin-cholesterol acyltransferase)
    • ApoB-100: Major protein of VLDL and LDL; ligand for LDL receptor
    • ApoC-II: Activates lipoprotein lipase (LPL)
    • ApoE: Ligand for LDL receptor and LDL receptor-related protein (LRP)
  • Physical Properties
    • Soluble Proteins: Soluble in aqueous solutions
    • Amphipathic Regions: Contain amphipathic regions that allow them to interact with both lipids and water
  • Chemical Properties
    • Lipid Binding: Bind to lipids in lipoproteins, stabilizing the structure
    • Receptor Binding: Interact with cell surface receptors, mediating lipoprotein uptake
    • Enzyme Activation: Activate or inhibit enzymes involved in lipid metabolism
  • Functions
    • Structural Component
    • Ligands for receptors
    • Enzyme regulators

Key Terms

  • Lipoproteins: Particles that transport lipids in the blood
  • Phospholipids: Amphipathic lipids that are major components of cell membranes
  • Triglycerides: Neutral lipids consisting of a glycerol molecule esterified with three fatty acids
  • Cholesterol: A sterol lipid that is an essential component of cell membranes and a precursor for steroid hormones and bile acids
  • Apolipoproteins: Proteins that bind to lipoproteins, providing structural support, targeting signals, and enzyme regulation
  • Amphipathic: Having both hydrophobic and hydrophilic regions
  • Hydrophobic: Repelling water
  • Hydrophilic: Attracted to water
  • Ester Bond: A chemical bond formed between an alcohol and a carboxylic acid
  • Saponification: The hydrolysis of a triglyceride with a strong base to produce glycerol and soap
  • Steroid Ring Structure: The four fused hydrocarbon rings that form the basic structure of cholesterol and other steroid hormones
  • Lipoprotein Lipase (LPL): An enzyme that hydrolyzes triglycerides in lipoproteins
  • LCAT (Lecithin-Cholesterol Acyltransferase): An enzyme that esterifies cholesterol in HDL