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Lipid

Core properties and roles

  • lipids are hydrophobic

  • major roles:

  • membranes

  • energy storage (triacylglycerol)
  • signaling (inter- and intracellular)

Fatty acids

  • carboxylic acids with long hydrocarbon chains (often even-numbered, ~16–20 carbons)
  • hydrophobic tail + hydrophilic head
  • amphipathic

Saturated vs unsaturated fatty acids

Saturated

  • no double bonds between carbons
  • examples: palmitic acid, stearic acid

Unsaturated

  • at least 1 double bond
  • monounsaturated vs polyunsaturated

Melting point (T\(_m\))

  • T\(_m\) increases with chain length
  • T\(_m\) decreases with number of double bonds

Cis vs trans

  • unsaturated double bonds can be:

  • cis (same side)

  • trans (opposing side)
  • saturated chains are more flexible and pack tightly
  • trans fats pack more like saturated fats

Health note:

  • trans fat increases LDL and increases risk of heart issues and type 2 diabetes

Nomenclature

Numbering systems

  • carboxyl (hydrophilic) end carbon is C1 and increases toward the other end (or \(\alpha\), \(\beta\), …)
  • omega system counts from the methyl end

Examples

  • 18:0 = 18 carbons, 0 double bonds
  • 18:1 \(\Delta^9\) = 18 carbons, 1 double bond between C9 and C10
  • 18:1 \(\omega\)-9 = first double bond at omega position 9
  • 18:3 \(\Delta^{9,12,15}\)
  • 18:3 \(\omega\)-3 = first double bond at omega position 3

Essential fatty acids

  • linolenate (\(\omega\)-3)
  • linoleic acid (\(\omega\)-6)

Why essential:

  • needed for lipid signaling molecules (eicosanoids)
  • dietary fatty acid composition changes membrane composition and eicosanoid production

Triacylglycerols (triglycerides)

  • glycerol backbone with fatty acids (positions 1–3 on glycerol)
  • stored in adipocytes as fat

Lipid droplets

  • store TAG in cells