BMB 462 Lecture 2 Outline of Last Lecture I Definition of a Lipid II Structure and function of fatty acids III Structure and function of triacylglycerols Outline of Current Lecture I Introduction to Membrane lipids a Basic Structure b Basic properties II Classes of Membrane lipids a Glycerophospholipids b Galactolipids Sulfolipids c Sphingolipids d Sterols e Isoprenoids III Lipase activity IV Lipid Signaling Current Lecture Concepts to remembers from previous courses lectures Organic chemistry structures specifically glycerol fatty acid esters Charged groups vs Net charge Entropy Membrane lipids form a bilayer containing a hydrophobic interior contained by 2 hydrophilic barriers separating the hydrophobic interior of the membrane from the aqueous environment and cell organelle interior The shape of a single membrane phospholipid is more cylindrical than a fatty acid free phospholipids in solution can fold back along themselves to form a double layer liposome as opposed to a micelle These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute The bilayer forms because hydrophobic areas in water force the water molecules to make a structured cage around the molecules water does not like structured arrangements it prefers to be disorganized and to have higher entropy Separating the hydrophobic side of phospholipids from water allows an increase in entropy To be a membrane lipid there needs to be a polar and a nonpolar end I Introduction to Membrane Lipids a Basic structure includes i Amphipathic molecules that contain both hydrophobic nonpolar and hydrophilic polar portions 1 The polar head group can be Phosphates phosphodiesters sugars hydroxyl groups etc 2 Hydrophobic portion fatty acid tails of Carbons and Hydrogens Isoprene derivatives etc ii Variety of combinations from diverse head groups and tails available see Classes of Membranes iii Backbone made of either glycerol or sphingosine b Basic Properties The amphipathic nature and shape of phospholipids allows the formation a lipid bilayer i The hydrophilic head is exposed to the aqueous environment ii The hydrophobic tails are buried iii Lipids in a membrane are held together via Van der Waals interactions and similar loosely connected forces not covalent bonding II Classes of Membranes a Glycerophospholipids i Contain a glycerol backbone ii Fatty acids are attached via ester bonds to Carbon 1 and Carbon 2 1 Carbon 1 can be bond with an ether linkage there is a lot more diversity with this bond in plants than in mammals prokaryotes iii The polar head group is attached to Carbon 3 with a phosphodiester bond iv Different fatty acids and head groups change the properties of the lipids i e solubility in organics or aqueous solvents polarity etc v Carbon 1 is often saturated while Carbon 2 is typically the one with one or more double bonds mono or poly unsaturated vi Glycerophospholipids are a major component in the membrane and can be used as a precursor for signal molecules 1 Refer to chart on pg 8 in the class notes for names and structures of different types of glycerophospholipids Key derivatives include phosphatidylethanolamine PE phosphatidylcholine PC phosphatidylserine PS and Phosphatidylinosiol 4 5 bisphosphate PI b Galactolipids Sulfolipids i These structures are mostly found in plants 1 Plants have trouble making phosphate but have the ability to make plenty of galactose Plant cells would have a shortage of phosphates if they used phospholipids for all membranes i e the membranes of grana in chloroplasts ii Structure 1 Glycerol backbone 2 Fatty acids linked to Carbons 1 and 2 via ester bonds 3 Head group attached to Carbon 3 via glycosidic linkages iii Properties 1 Different fatty acids and head groups change the properties of the lipids i e solubility in organics or aqueous solvents polarity etc 2 Carbon 1 is often saturated while Carbon 2 is typically the one with one or more double bonds mono or poly unsaturated iv Function 1 Abundant in chloroplasts galactolipids and sulfolipids are the most abundant membrane lipids 2 Compensate for the phosphate limitation in plants c Sphingolipids i Structure 1 Built on the amino alcohol sphingosine a Amino group on Carbon 2 b Hydroxyl group and a trans double bond in the sphingosine structure on Carbon 3 c A fatty acid is attached to the nitrogen on Carbon 2 via an amide bond d Polar head group attached to Carbon 1 ii Properties 1 Different fatty acids and head groups change the properties of the lipids i e solubility in organics or aqueous solvents polarity etc a Look at table on pg 9 in the class notes for the structures of different polar head groups and the names of different sphingolipids iii Functions 1 Though sphingolipids like ganglioside GM2 are small portion of the components in membranes they help identify cells for cell to cell communication 2 Key for cell recognition i e determining ABO blood type 3 Sphingolipids are prevalent in the myelin sheath and other neuronal membranes 4 Though sphingolipids are very low in concentration problems with turn over break down are serious a i e an inability to break down gangliosides results in Tay Sachs disease d Cholesterol an Isoprenoid i Structure 1 A steroid nucleus formed of 4 rigid fused rings that are planar in structure 2 Hydrocarbon tail 3 Hydroxyl group on Carbon 3 4 Root structure is Isoprene there are no fatty acids ii Properties 1 Amphipathic the hydrocarbon rings and the tail are hydrophobic the OH is polar 2 Rigid due to the rings a Rigidity has implications on how cholesterol behaves in membranes by impacting membrane fluidity i Membranes without cholesterol have a sharp transfer to liquid state when temperature increases ii Cholesterol maintains fluidity over a higher range of temperatures the rings keep the phospholipids from packing in as close so the membrane stays more fluid at low temps the rigidity of the rings adds stability to the membrane at higher temps because it cannot rotate like fatty acids do b Cholesterol is a hormone signaling precursor i e testosterone estradiol c Cholesterol is a bile acid precursor i e taurocholic acid i Bile acid cells form complex micells that work to break up lipids so they can be digested e Isoprenoids i Like cholesterol other molecules are formed from an isoprene base 1 Isopentenyl pyrophosphate IPP activated isoprene b c it is phosphorylated it s a key intermediate in the
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