Lipids Chapter 10 (Fall 2023) PDF

10| Lipids © 2017 W. H. Freeman and Company Lipids: Structurally Diverse Class Lipids are diverse but relatively hydrophobic All biomolecules, even lipids, are amphipathic  Classification of Lipids Lipids that contain fatty acids (complex lipids) – storage lipids –...

10| Lipids © 2017 W. H. Freeman and Company Lipids: Structurally Diverse Class Lipids are diverse but relatively hydrophobic All biomolecules, even lipids, are amphipathic  Classification of Lipids Lipids that contain fatty acids (complex lipids) – storage lipids – membrane lipids Lipids that to not contain fatty acids – cholesterol, vitamins, pigments, etc.  Fatty Acids Carboxylic acids with hydrocarbon chains containing between 4 to 36 carbons – Almost all natural fatty acids have an even number of carbons. – Most natural fatty acids are unbranched. Saturated: no double bonds between carbons in the chain Unsaturated – One or more C-C double bonds in the alkyl chain – Monounsaturated or polyunsaturated – Cis configuration; introduces an approximate 30◦ kink in the alkyl chain  Fatty Acid Nomenclature Multiple designations – Systematic name: cis-9-octadecanoic acid – Common name: oleic acid – Numbering Systems delta numbering of carbon skeleton: 18:1Δ9 – Counting from the carbonyl carbon omega numbering of carbon skeleton: 18:1ω9 – Counting from terminal methyl carbon – Sometimes n used instead of ω  Double Bonds in PUFAs Not usually conjugated! – Spaced by 3 C atoms Linoleic acid – cis-,cis-9,12- Octadecadienoic acid – 18:2 (Δ9,12) – 18:2 (n-6) or 18:2 (ω-6)  Trans Fatty Acids Trans fatty acids form in the presence of metal catalysts used for partial hydrogenation – Shelf life – Stability at cooking temps – Solid consistency of A trans double bond allows an extended conformation – Mimic the Van der Waals interactions of saturated fats Strongly linked to heart disease. Why? PUFAs are Susceptible to Lipid Peroxidation Triacylglycerols  Glycerophospholipids Primary constituents of cell membranes Two fatty acids form ester linkages with the first and second hydroxyl groups of L-glycerol-3-phosphate. The phosphoryl group is charged at physiological pH.  Glycerophospholipids Unsaturated fatty acids are commonly found connected to C2 of glycerol-3-phosphate. The highly polar phosphate group may be further substituted. Such substituent groups are called the head groups. From where does glycerol come?  Sphingolipids The backbone of sphingolipids is the long- chain amino alcohol sphingosine instead of glycerol A fatty acid is linked by an amide linkage rather than an ester linkage A polar head group is connected to sphingosine. The sugar-containing glycosphingolipids are in the outer leaflet of plasma membranes. Sphingomyelin Is Structurally Similar to Phosphatidylcholine Lipids can be hydrolyzed by Lipases Catalytic Mechanism of Secretory PLA2  Bilayer Fluidity Opposite of viscosity Inversely related to lipid packing – Strength of Van der Waals interactions – Additive: Points of contact! – Maximized by, long, fully extended and saturated fatty acyl chains – Decreased by increased unsaturation, size of head groups and decreased # C atoms – Overcome by increased temperature  Hydrocarbon Conformations Eclipsed not relevant Staggered – Anti Fully extended Maximal packing Favored at low T – Gauche Chain “bending” Decreased packing Favored at higher T – Above TM Two States of Bilayer Lipids Gel to liquid crystalline transition (melting) occurs at TM TM of most biological membranes = 10-40 oC Determined by differential scanning calorimetry (DSC) Depends on lipid composition of bilayer – #C atoms, degree of unsaturation, size of head groups Tm increases with # of carbons in fatty acyl chains of phospholipids  Sterols and Cholesterol Sterols – steroid nucleus: four fused rings – hydroxyl group (polar head) in the A-ring – various nonpolar side chains