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Unformatted text preview: Lecture 19
Overview: • fatty acids • membrane lipids • phospholipids & glycolipids • membrane processes Lipids & cell membranes: introduction lipids: edible fats & oils; esters of long-chain fatty acids and glycerol mostly non-polar there are two classes of lipids: hydrolyzable and nonhydrolyzable hydrolyzable: neutal fats (animal); oils (corn, olive, etc.) nonhydrolyzable: steroids - cholesterol, hormones, terpenes fatty acids: RCO2H where R is a long, unbranched chain (usually an even #C’s) R can be saturated (alkane) or unsaturated (alkene - cis conﬁguration) Chapter 12: Lipids and Cell Membranes HW: 12.12, 12.13 Lipids & cell membranes: introduction • membranes are the barriers that deﬁne the inside and outside of cells • membranes have selective permeability • membranes are dynamic structures • cells also have internal membranes that form the boundaries of organelles (mitochondria, etc.) • membranes also serve important roles in energy storage and information transduction** Lipids & cell membranes: introduction • membranes are sheet-like structures with thicknesses between 60 Å (6 nm) and 100 Å (10 nm) • membranes consist of lipids and proteins (1:4 to 4:1 of lipid: protein); they may also contain carbohydrates • lipids have both hydrophobic and hydrophilic properties • lipids form bilayers of structure • membranes are noncovalent assemblies • membranes are asymmetric (have two faces that differ) • membranes are ﬂuid structures Fatty acids C12 – C18 CH3(CH2)nCOOH Fatty acids CH3(CH2)16COOH C18 - stearic acid saturated CH3(CH2)7CH=CH(CH2)7COOH C18 - oleic acid (mono)unsaturated - cis conﬁguration Fatty acids saturated unsaturated Triacylglycerol
CH2 OH CHOH CH2 OH Triacylglycerol
O HO glycerol + fatty acids = fat + ester ifi cati on
O CH 2O CH OH CH 2OH hydrolysis saponi ficat ion ( alkal ine hydrol ysis ) NaOH
O C H2OH C HOH C H2OH
++ Na O mono-, di-, or triacylglycerol add 1, 2, or 3 fatty acids Triacylglycerols (triglycerides) • fatty acids commonly found in animals as triacylglycerols, which are stored mainly in adipose cell (adipocytes) • a major energy source for cells (most reduced form of carbon in nature) (more later) • efﬁcient packing (highly hydrophobic, aggregate into nonhydrated lipid droplets) • more energy per gram than carbohydrates (more later) proteins & carbohydrates ~4 kcal/g ethanol ~7 kcal/g triacylglycerol ~9 kcal/g *remember that the triacylglycerols are hydrolyzable: the products (digestion ->fatty acids and glycerol) are important for energy adipose tissue (adipocytes/fat cells) storage sites of triacylglycerols in animals low water content function is to store fatty acids in the form of triacylglycerols provide energy & insulation Fatty acid composition in food Hydrogenation of oils
O O CH2 O (Partial) hydrogenation is common for many oils in food - the hydrogenated oil becomes saturated; generally the amount of saturation is controlled such that the oil does not become too thick think about structure/function relationship; mp behavior HC O H2 C O O poor packing, l ow m p vegetabl e oil , peanut oi l hydrogena tion H2
c at a ly st unsaturated O O C H2 HC O H2 C O O O saturated
good packi ng, high mp m argar ine, peanut butte r Fatty acid + alcohol = wax a single ester bond Biological roles of lipids: membranes overview beeswax = triacontanoylpalmitate structural lipids in membranes phospholipids glycolipids cholesterol Phospholipids glycerophospholipid Membranes: glycerophospholipids O O CH2 -O P -O O O HC O CH2 O hydrophobic; "t ail " charge d; "he ad" group Phospholipids: alcohol modifying groups Phosphoglycerides Glycerophospholipids Phospholipids: sphingosines membranes of brain cells, neurons blood group determinants function depends on x; x = sugar, phosphocholine Comparison of a glycerophospholipid and sphingolipid Ether lipids hydrolysis products? more stable than esters found in archaebacteria Cerebrosides Ether lipids Glycolipids: chloroplast membranes Phospholipases hydrolyze the ester bonds of glycerophospholipids Steroids ABCD fused 6,6,6,5 ring system cholesterol is the most common steroid found in animals and is the precursor for all other steroids (i.e. hormones) in animals steroid hormones serve many functions: salt balance, metabolic function, sexual function general structure (ABCD) different enzymes are required to catalyze the hydrolysis of the two ester (oxygen) and phosphodiester bonds Biological roles of lipids: membranes Biological roles of lipids: membranes amphipathic compounds in water: fatty acids amphipathic lipid aggregates in aqueous environment Cholesterol is a component of the eukaryotic cell membrane Phospholipid bilayer membrane: idealized versus realistic views cholesterol helps stabilize cell membranes; why is it partially exposed to the aqueous exterior and interior of the cell? regular (rigid) structure (upper) versus ﬂuid model (irregular) (lower) Cholesterol why is margarine considered better than butter in terms of health?
1 2 3 4 5 6 Omega-3 fatty acids
O C OH omega-6 fatty acids (vegetable oils) think about how it is made - hydrogenation of vegetable oil; it is lacking cholesterol
1 C18 l ino leic acid
O C 3 what about artiﬁcial fats (olestra)? sucrose (in place of glycerol) + carboxylic acids (8-10 C)9 esters cannot be digested by human enzymes approved in 1996 by FDA (olean) side effects?
1 2 OH C18 li nol en ic acid
O C 3 omega-3 fatty acids
OH 2 (ﬁsh oils) C20
1 3 eicosap en taenoi c acid (EPA) OH 2 C C22 do co sahexaeno ic acid (DHA) O abundant in ﬁsh oil - associated with lower blood pressure Biological roles of lipids: membranes and hormone precursors • the polar lipids, with polar heads and nonpolar tails, are major components of membranes (most abundant are glycerophospholipids) • glycerophospholipids have different head groups - the most common are phosphatidylethanolamine and phosphatidylcholine • chloroplast membranes are rich in galactolipids • archaebacteria have unique membranes - their ether-linked lipids are more stable under harsh conditions • sphingolipids contain sphingosine in place of glycerol - examples are cerebrosides, globosides, and gangliosides, which contain sugar components • sterols are fused rings and a hydroxyl group - they serve as structural components of membranes and are precursors to a wide variety of steroids Liposomes (lipid vesicles): useful for drug delivery? Preparation of glycine-containing liposomes Integral and peripheral membrane proteins Step 2 Step 1 Membrane anchors Membrane-spanning alpha helices of bacteriorhodopsin Membrane-spanning beta strands of bacterial porin Phosphatidylinositols play roles in intracellular signaling
hydolysis by phospholipase C is regulated by a hormone response products of hydrolysis act as intracellular messengers Arachidonic acid and eicosanoid derivatives Arachidonic acid and eicosanoid derivatives
• prostaglandins are products of reactions of arachidonic acid • prostaglandins regulate production of cAMP, an important chemical messenger • prostaglandins regulate blood ﬂow to speciﬁc organs • prostaglandins elevate body temperature and cause inﬂammation & pain • anti-inﬂammatory drugs (aspirin, ibuprofen, etc.) block the formation of prostaglandins by inhibiting the enzyme cyclooxygenase (prostaglandin H2 synthase) in response to a hormone signal, phospholipase A2 cleaves the phosphodiester bond to release arachidonic acid (arachidonate at pH 7.0), the precursor to eicosanoids can you draw the products? Prostaglandin H2 synthase 1 Fluid mosaic model & lipid movement in membranes lateral diffusion of lipids is much more rapid than transverse diffusion (ﬂip-ﬂop) Cell membranes of eukaryotes vs. prokaryotes The cells of prokaryotes can have one or two membranes. ...
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