Chapters 9 and 10 Lipids and Membranes Lipids- a class of biological molecules defined by low solubility in water and high solubility in nonpolar solvents. Lipids contain or are derived from fatty acids. Fatty Acids: Fatty Acids yield large amounts of energy upon oxidation and are the molecules of choice for metabolic energy storage. They are stored in adipose tissue as triacylglycerides. A) Functions: 1) Used in the formation of complex lipids for cell structures 2) Used in the formation of prostaglandins and steroids 3) Used as an energy source B) Structure: Fatty acids are composed of a large hydrocarbon chain (tail) and a terminal carboxy group (head) CH 3 -(CH 2 ) n -COO - Most have an even number of carbon atoms Can be saturated (no double bonds) or unsaturated (up to 6 double bonds in the cis conformation) Can have branched chains C) Nomenclature: Fatty acids can be named or described in three ways: by its systematic name ( octadecanoic acid), its common name (stearic acid) or its shorthand notation (18:0). Oleic acid is 18:1 (9) indicating that there are 18 carbons in the chain, and 1 double bond located at carbon 9. Triacylglycerols: Fatty acids are stored as triacylglycerols. These molecules consist of a glycerol esterified with three fatty acids. Glycerol CH 2 CH CH 2 OH OH OH + 3 fatty acids Membrane Composition: Membranes are an organized sea of lipids in a fluid state in which various components are able to move and interact. Their width is about 7-10 nm. They are semi-impermeable structures that have selective transport
systems (proteins) for moving molecules from one side of the membrane to the other. The main components of membranes are lipids and proteins, and the minor components are glycoproteins and glycolipids. A) Membrane Lipids There are three major lipids in membranes: 1) Glycerophospholipids most abundant Composed of a glycerol base unit with a phosphate group attached at C-3 and fatty acids attached at C-1 and C- 2 through either ester or ether linkages. The X group on the phosphate can be a number of different substituents Substitutions include ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also called lecithins), serine (phosphatidylserine), glycerol (phosphatidylglycerol), myo-inositol (phosphatidylinositol). See page 160 in text for figures. In most cases, a saturated fatty acid is found on C-1 (R 1 ) and an unsaturated fatty acid is found on C-2 (R 2 ). Phosphatidylcholine: 2) Sphingolipids Composed of a sphingosine base unit with a fatty acid attached off of C-3. Can have sugar molecules attached at C-1 through an O linkage (ganglioside, cerebroside). Can have a phosphate attached at C-1 with X groups on the phosphate. A ceramide is a sphingosine with a fatty acid amide linked to the amine group on C-2.
A sphingomylin: 3) Cholesterol A compact, rigid hydrophobic molecule that contains 4 fused rings and an 8 member branched hydrocarbon chain attached to the D ring at position 17. The amount of cholesterol in membranes is affected by nutritional state. The presence of cholesterol decreases the fluidity of membranes. *** The lipid composition varies from membrane type and tissue type depending on the function of the membrane. B) Membrane Proteins classified by ease of removal 1) Peripheral (extrinsic) Released from membranes by treatment with solutions of different ionic strengths (salt) or ph. Can be removed without disrupting the membrane Many are enzymes Usually soluble in water and free of lipids 2) Integral (intrinsic) Require drastic treatment (detergents or organic solvent) to be separated from the membrane Removal disrupts the entire membrane structure Usually contain tightly bound lipid Have many hydrophobic domains which interact with lipids Protein Function in membranes: 1) catalytic enzymes 2) transport membrane proteins with role in transmembrane movement 3) receptor receptors for hormones 4) structural proteins that help maintain the shape of cell 5) recognition role Vesicular Structure In a mixture of lipids and water, the hydrophobic tails interact with each other to exclude water and the polar head groups are in contact with water. Can form micelles or bilayers. Lipid bilayers - extremely stable, held together by noncovalent interactions of hydrocarbon chains and ionic interactions of the polar head groups with water. Rapid lateral diffusion lipid molecules move and trade places with neighboring molecules
Rapid rotational diffusion rotation around the C-C bonds in fatty acyl chain Rapid flexing of the hydrocarbon chains Very slow transverse movement lipids will not readily migrate from one monolayer to the other Fluid Mosaic Model of Membranes: This model accounts for the fluidity and flexibility of the membrane and permits changes of shape and form. Also includes the ability of membranes to self-seal, and the impermeability of membranes. A) Integral Proteins: These proteins span the entire membrane or are partially immersed in the lipids. Contain a stretch of nonpolar amino acids that span the nonpolar lipid portion of the membrane. An a helix consisting of hydrophobic amino acid is ideal for spanning the membrane. These can have an intracellular domain and an extracellular domain that may contain binding sites or catalytic activity. Membranes are asymmetric. B) Peripheral proteins: Loosely attached to the membrane through covalent bonds or noncovalent interactions. Can be noncovalently bound to integral proteins Can be electrostatically bound to the membrane Can have a small hydrophobic region which anchors them to the membrane Can be covalently bound to a lipid through a sugar Both integral and peripheral proteins move through the lipids in a membrane. However, different proteins can move at different rates. C) Membrane T m : Each membrane has a phase transition temperature T m. If T<T m, then the membrane is in a gel-crystalline state and not very fluid If T>T m, then the membrane is in a fluid liquid-crystalline state and components move freely The value of Tm depends on the lipids present in the membrane, the amount of protein in the membrane, and the amount of cholesterol in the membrane: cholesterol decreases the fluidity of a membrane and increases Tm saturated fatty acids decrease the fluidity of a membrane and increase Tm unsaturated fatty acids decrease Tm as protein concentration in a membrane, the fluidity and the Tm value Movement of Molecules through the Membrane: 1) Diffusion: a) solute must leave an aqueous environment and enter the nonpolar membrane b) solute must transverse the membrane c) solute must leave the nonpolar membrane and enter new aqueous environment Water and some small molecules readily diffuse through the membrane because of transitory spaces in the membrane due to fatty acyl movement O 2, N 2, CO 2, NO rapidly diffuse through the membrane very lipid soluble substances will remain dissolved in the membrane
The rate of diffusion is to lipid solubility and the diffusion coefficient in lipids. Therefore, uncharged lipophilic molecules (fatty acids, steroids) rapidly diffuse, while water soluble substances (sugars, inorganic ions) diffuse very slowly. The direction of movement of solute by diffusion is always from higher concentration to lower concentration. Transport Systems Can facilitate movement of solute across the membrane with a transport system. These are classified on the basis of translocation across the membrane and energetics of the system. 1) Membrane Channels: These are channels in the membrane that permit the rapid movement of specific molecules across the membrane. The tertiary and quaternary structure of integral membrane proteins create an aqueous hole in the membrane. The movement of solute through a channel is always from higher to lower concentrations. A common motif of channel proteins is hydrophobic a-helices forming domains with a central aqueous space with some of the amino acid side chains projecting into the space. Channels are specific for the substance transported. This specificity lies in the size of the aqueous opening, and also in the amino acid side chains lining the channel area. Can regulate the flow of substance through a channel by opening and closing the channel. a) voltage gated- controlled by changes in the transmembrane potential b) Binding of a specific agonist can control the opening of a channel (ex: acetylcholine opens Na + channels in nerve cells) c) Controlled by camp fast rate (ex: muscle contraction) d) Other- heat sensitive, pressure sensitive e) Can be inhibited or activated just like enzymes Clinical Cystic Fibrosis is an autosommal recessive disorder that is the commonest, fatal, inherited disease of Caucasians occurring in 1 in 2000 live births. It is characterized by a pulmonary obstruction of thick mucus that blocks airways and harbors bacteria. The CF gene encodes for a camp dependant Cl - channel expressed in epithelial tissues. CF patients have reduced Cl - permeability which impairs fluid and electrolyte secretion, leading to luminal dehydration. The most common mutation is a single nucleotide polymorphism that results in the deletion of a Phe at position 508 resulting in the incorrect folding of the protein. 2) Pores: Gap junctions- cluster of channels that create connections between two cells. Nuclear pores create an aqueous channel in the nuclear envelope. 3) Transporters Translocate substance across membrane by binding to and physically moving the substance. Can transport inorganic ions, uncharged and charged organics These show specificity for substance transported They can be regulated by competitive and noncompetitive inhibitors Slower rate than channels These are very similar to enzymes in that they show saturation kinetics, substrate specificity, can be inhibited or activated Transport Steps: 1) recognition- binding sites for a specific substrate 2) transport- movement of solute across the membrane 3) Release- of solute by the transporter. Released easily into lower concentration with no energy needed, if released into higher concentration, then must decrease the substrates affinity for the transporter by either modifying the transporter or substrate molecule (requires energy)
4) Recovery- transporter returns to original state (conformation) Transporter Types: A) Passive Transporter- simply facilitated diffusion and can only transport from high to low. No energy required B) Active transporter- Can transport against a concentration gradient from low to high, requires energy, results in a conformational change in the transport protein to decrease S affinity C) Group translocation- this is like active transport except decrease affinity for S by chemically modifying the substrate molecule. Mechanisms: Uniport, symport, and antiport. 4) ionophores: Antibiotics of bacterial origin that facilitate the movement of monovalent and divalent inorganic ions across biological and synthetic membranes (see figure 6.55 in book). Each ionophore has definite ion specificity. A) mobile carriers- ionophores that readily diffuse in a membrane and can carry an ion across the membrane. B) Channel Formers- ionophores that create a channel that transverses the membrane and through which ions can diffuse. Valinomycin transports K + by an electrogenic uniport mechanism that creates an electrochemical gradient across a membrane as it carries a positively charged K +. Energetics of Membrane Transport There is a change in free energy when an uncharged molecule moves from a substrate concentration C1 to a substrate concentration C2.?G = RTln(C2/C1) where R = 8.314 J/K-mole, T is temp in Kelvin When?G = - movement is spontaneous When?G =+ movement is not spontaneous and requires additional energy to move For a charged molecule:?g = RTln(C2/C1) + ZF? Ψ where Z=charge of ion, F= Faraday constant (96.5 kj/v-mole), and? Ψ is the electrical potential difference across the membrane If?G = -, passive transport, no energy required If?G = +, active transport, energy is required (ATP, electrochemical gradient)