Classification of Lipids

Classification of Lipids Fatty Acids • long chain linear hydrocarbons carboxylic acids • Fatty acids: are long chain linear (unbranched) hydrocarbon carboxylic acids • Triglycerides: are fatty acid esters of glycerol • Phospholipids: are lipids that contain one or more phosphate groups • Glycolipids: have a carbohydrates component • Eicosanoids: are a family of derivatives of Arachidonic acid • Steroids: have a basic structure of a perhydrocyclopentanophenanthrene ring system • Lipoproteins: are complexes of lipids and proteins that circulate in the blood. ω-3 Fatty Acids • Usually have an even number of C atoms (usually 12 to 20) • The carbons are numbered starting from the carboxylic C. • They are amphiphilic; they have a polar end and rest of the molecule is nonpolar • Fatty acids may be saturated (no double bonds) or unsaturated (one or more double bonds) • All naturally occuring double bonds have a cis-configuration • 2 or more double bonds exist as non-conjugated double bonds • Longer chain and saturation increases melting point of FA • FAs are ionized at physiological pH Some Fatty Acids • the highest numbered C is called the ω-C Palmitic acid (hexadecanoic acid): 16:0 • Sometimes FA are classified according to the position of the first double bond from the ω-end Stearic acid (Octadecanoic acid) : 18:0 • Most polyunsaturated fatty acids are ω-6 fatty acids Linoleic Acid (9,12- octadecadienoic acid): 18:2 (∆9,12) • ω-3 fatty acids are found mainly in fish and fish products. Also found in flax seeds α-Linolenic Acid (9,12,15-octadecatrienoic acid): 18:3 (∆9,12,15) • ω-3 FAs inhibit formation of thromboxane A2 (an eicosanoid) required for platelet aggregation and clot formation. Thus, ω-3 FAs decrease the risk of heart disease Arachidonic Acid (5,8,11,14-eicosatetraenoic acid) 20:4 (∆ 5,8,11,14) Oleic Acid (9-octadecenoic acid): 18:1 (∆9) γ-Linolenic Acid (6,9,12-octadecatrienoic acid): 18:3 (∆ 6,9,12) EPA (5,8,11,14,17-Eicosapentaenoic acid) 20:5 (∆ 5,8,11,14,17) 1 Triglycerides • In Triacylglycerol (TG) all 3 –OH of glycerol are esterified by FAs. Monoacylglygerol and diacylglycerol have, respectively, 1 and 2 FAs • Naturally occurring glycerol is L-glycerol. • TG are the storage form of FA; most dietary fats are triglycerides • Physiologically, TG are digested in the small intestine by the enzyme pancreatic lipase • Monoacylglycerols are absorbed through the intestinal cells, re-converted to TG and assembled into lipoproteins Phospholipids Sphingolipids • These are lipids that contain one or more phosphate groups • PL are the primary components of biomembranes. Other lipids in biomembranes are glycolipids and cholesterol. Surfactants are phopsholipids, mostly phosphatidylcholine • PL are subclassified based on their parent lipid; phopshoglycerides or sphingomyelins • Phosphatidic acid: basic glycerophopholipid. 1,2diacylglycerol joined to phosphoric acid by an ester link. This phosphate can form another ester linkage with an alcohol. Serine :phosphatidylserine; Choline: phosphatidylcholine Ethanolamine: phosphatidylethanolamine; Inositol: phosphatidylinositol; Glycerol: diphosphatidylglycerol (cardiolipin) • Sphingosine is a derivative of glycerol but it has –NH2 instead of -OH at C2 and has a -OH as well as a long chain hydrocarbon on C3 • The –NH2 forms an amide bond with a long chain FA to form a ceramide. • sphigomyelin is formed when a phosphodiester bridge links the C1 -OH of ceramide to ethanolamine or choline • Sphingomyelins are found abundantly in the myelin sheath that surrounds the nerve fibers 2 Glycolipids • Glycolipids are lipids that contain carbohydrates • Cerebrosides have a monosaccharide attached to the C1 -OH of ceramide • Gangliosides have an oligosaccharide attached to the C1 -OH of ceramide • Cerebrosides and gangliosides are found in the brain and nervous tissue • In biomembranes, glycolipids are oriented asymmetrically with the sugar units always on the extracellular side of the membrane Biomembranes • Make up boundaries of cells and intracellular organelles (nucleas, golgi, mitochondria, ER, etc.) • Membranes are dynamic fluid structures • Composed of a lipid bilayer with proteins embedded within the bilayer • Lipids responsible for semipermeability of biomembranes; hydrophobic chemicals can penetrate, but most polar molecules are excluded • Membrane proteins are transporters, channels and pumps for the selective entry of specific molecules Cholesterol • Cholesterol is an essential component of biomembranes and imparts stability to the fluid structure. • Cholesterol is a steroid. All steroids have the same basic structure consisting of 4 hydrocarbon rings linked together • Cholesterol has a –OH group which provides the polarity and a hydrocarbon group at the other end which adds to its hydrophobic nature • In biomembranes, the –OH of cholesterol is aligned with the head group (phosphate) of phospholipids • Steroids are important metabolically (cholesterol), for digestion (bile salts), as hormones (human sex hormones) Lipid Bilayers • Phospholipids are amphipathic: They have both hydrophilic and hydrophobic regions • The two hydrocarbon chains are parallel to each other, the polar group is extended in the opposite direction. • In aqueous solutions, amphipathic molecules arrange themselves as micelles, bilayers or liposomes. • These structures are stabilized by hydrophobic interactions between hydrocarbon chains and hydrogen bonds between polar head groups and H2O. • Phospholipids and glycolipids favor the lipid bilayer over micelles because the interior of a micelle cannot accommodate 2 hydrocarbon chains of each molecule. Salts of fatty acids (soaps) prefer to organize as micelles. 3 Membrane Proteins Properties of Lipid Bilayers • They form spontaneously. The bilayer structure is inherent in the structures of the constituent lipids. • They are cooperative structures stabilized by hydrophobic interactions and hydrogen bonding. • They are extensive / continuous without beginning or end. • Membrane proteins include pumps, channels, transporters, receptors and enzymes. Their function include transport and cellular communication. • Membrane proteins are classified as peripheral or integral. • Peripheral proteins are associated to polar head groups of the lipid bilayer by electrostatic interactions. They can be easily dissociated by high salt or altering pH. • They are enclosed: do not have edges with exposed hydrocarbons. Thus, they form compartments • Integral proteins are embedded within the bilayer by hydrophobic interactions with hydrocarbon chains. Their embedded domains contain aa with hydrophobic side chains. • They are non-leaky and self-sealing. Accounts for their permeability barrier. • Proteins that form channels and transporters have multiple transmembrane domains • Certain proteins may be anchored to the bilayer by covalent bonds between aa of proteins and head groups on lipids The Fluid-Mosaic Model of Membrane Structure • The Fluid-Mosaic model for membrane structure was proposed by Singer and Nicolson in 1972 • States that biomembranes are fluid and dynamic. The membrane lipids are a solvent for the proteins. Thus, membranes are a solution of oriented lipids and proteins. They are constantly flowing or in motion. • Lateral diffusion or movement of membrane components is rapid and constant. Transverse diffusion (flip-flop) between the two layers of the bilayer is slow and occasional. Fluid-Mosaic Model (Continued) • The FM model also states that membranes are like a mosaic because they are assembled from various components which includes lipids, proteins and carbohydrates. • Oligosaccharide chains usually float outward toward the extracellular matrix rather than being exposed to the cytoplasm. • The orientation of proteins and carbohydrates and the slow flip-flop rate causes the membrane bilayer to be assymetrical 4 Membrane Fluidity • The degree of fluidity is determined by the fatty acid composition and cholesterol content. • Unsaturation increases fluidity. The cis-configuration introduces a bend in hydrocarbon structure which interferes with close packing and hydrophobic interactions • The fluidity is inversely related to fatty acid chain length. Longer the hydrocarbon chains, stronger the interaction between them. • Cholesterol decreases membrane fluidity because of its bulky irregular structure. Cholesterol also restricts the dynamic movement of membrane lipids by specifically interacting with certain phospholipids Membrane Receptors • Entry of large molecules and proteins into cells is mediated by membrane proteins called receptors • Receptors are highly specific and have a high affinity for their ligands. Ligands may be hormones, growth factors etc • Entry into cells is by the process of endocytosis. • In endocytosis, a region of the bilayer containing the receptor-ligand complex invaginates, fuses and buds out as an enclosed vesicle containing the ligand. • The vesicle continues to fuse with other internal vesicles till it is transported to its target • Receptors control key biological processes and rely on membrane fluidity. Double Membranes • Certain cells and organelles are enclosed by double membranes. Eg: Bacteria, Mitochondria. • The outer membrane is permeable to small molecules due to the presence of membrane channels made up membrane proteins called porins. • The inner membrane is highly impermeable and contains pumps and transporters that regulate movement of small molecules across the membrane Protein Targeting: Example of Membrane Function • The protein distribution amongst different cellular organelles is highly variable. • Proteins are synthesized with specific signal sequences that target them to the appropriate location • Eg: peroxisomal proteins contain a C-terminal SKL sequence, nuclear proteins contain an internal stretch of 5 basic aa, mitochondrial proteins have an N-terminal amphipathic helix. • The signal sequences are recognized by specific receptors present on target membranes 5