H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 19
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Shown are space-filling models and chemicalstructures of the ionized form of palmitic acid, a saturated fattyacid with 16 C atoms, and oleic acid, an unsaturated one withIf the acyl groups are long enough, these molecules areinsoluble in water even though they contain three polar esterbonds. Fatty acyl groups also form the hydrophobic portionof phospholipids, which we discuss next.Phospholipids Associate Noncovalently to Formthe Basic Bilayer Structure of BiomembranesBiomembranes are large flexible sheets that serve as theboundaries of cells and their intracellular organelles andform the outer surfaces of some viruses. Membranes literally define what is a cell (the outer membrane and the contents within the membrane) and what is not (the extracellularspace outside the membrane).
Unlike the proteins, nucleicacids, and polysaccharides, membranes are assembled by thenoncovalent association of their component building blocks.18 C atoms. In saturated fatty acids, the hydrocarbon chain isoften linear; the cis double bond in oleate creates a rigid kink inthe hydrocarbon chain. [After L. Stryer, 1994, Biochemistry, 4th ed.,W. H.
Freeman and Company, p. 265.]The primary building blocks of all biomembranes are phospholipids, whose physical properties are responsible for theformation of the sheetlike structure of membranes.Phospholipids consist of two long-chain, nonpolar fattyacyl groups linked (usually by an ester bond) to small, highlypolar groups, including a phosphate. In phosphoglycerides,the major class of phospholipids, fatty acyl side chains are esterified to two of the three hydroxyl groups in glycerol.
Thethird hydroxyl group is esterified to phosphate. The simplestphospholipid, phosphatidic acid, contains only these components. In most phospholipids found in membranes, the phosphate group is esterified to a hydroxyl group on anotherhydrophilic compound. In phosphatidylcholine, for example,choline is attached to the phosphate (Figure 2-19).
The negative charge on the phosphate as well as the charged or polargroups esterified to it can interact strongly with water. TheFatty acid chainsOHydrophobic tailHydrophilic headCCH2O OCCHOH2CGlycerolPHOSPHATIDYLCHOLINE▲ FIGURE 2-19 Phosphatidylcholine, a typical phosphoglyceride.
All phosphoglycerides are amphipathic, having ahydrophobic tail (yellow) and a hydrophilic head (blue) in whichglycerol is linked via a phosphate group to an alcohol. Either ofPhosphateOPOCH3H2COO−N+CH2CH3CH3Cholineor both the fatty acyl side chains in a phosphoglyceride may besaturated or unsaturated. In phosphatidic acid (red), the simplestphospholipid, the phosphate is not linked to an alcohol.2.2 • Chemical Building Blocks of Cellsphosphate and its associated esterified group, the “head”group of a phospholipid, is hydrophilic, whereas the fattyacyl chains, the “tails,” are hydrophobic.The amphipathic nature of phospholipids, which governstheir interactions, is critical to the structure of biomembranes.
When a suspension of phospholipids is mechanicallydispersed in aqueous solution, the phospholipids aggregateinto one of three forms: spherical micelles and liposomes andsheetlike, two-molecule-thick phospholipid bilayers (Figure2-20). The type of structure formed by a pure phospholipidor a mixture of phospholipids depends on several factors, including the length of the fatty acyl chains, their degree ofsaturation, and temperature. In all three structures, the hydrophobic effect causes the fatty acyl chains to aggregate andexclude water molecules from the “core.” Micelles are rarelyformed from natural phosphoglycerides, whose fatty acylchains generally are too bulky to fit into the interior of amicelle. If one of the two fatty acyl chains is removed byhydrolysis, forming a lysophospholipid, the predominanttype of aggregate that forms is the micelle.
Common detergents and soaps form micelles in aqueous solution that behave as tiny ball bearings, thus giving soap solutions theirslippery feel and lubricating properties.Under suitable conditions, phospholipids of the composition present in cells spontaneously form symmetric phospholipid bilayers. Each phospholipid layer in this lamellar45structure is called a leaflet. The fatty acyl chains in eachleaflet minimize contact with water by aligning themselvestightly together in the center of the bilayer, forming ahydrophobic core that is about 3 nm thick (see Figure 2-20).The close packing of these nonpolar tails is stabilized by thehydrophobic effect and van der Waals interactions betweenthem.
Ionic and hydrogen bonds stabilize the interaction ofthe phospholipid polar head groups with one another andwith water.A phospholipid bilayer can be of almost unlimited size—from micrometers (m) to millimeters (mm) in length orwidth—and can contain tens of millions of phospholipidmolecules. Because of their hydrophobic core, bilayers arevirtually impermeable to salts, sugars, and most other smallhydrophilic molecules. The phospholipid bilayer is the basicstructural unit of nearly all biological membranes; thus, although they contain other molecules (e.g., cholesterol, glycolipids, proteins), biomembranes have a hydrophobic corethat separates two aqueous solutions and acts as a permeability barrier. The structural organization of biomembranesand the general properties of membrane proteins are described in Chapter 5.KEY CONCEPTS OF SECTION 2.2Chemical Building Blocks of CellsThree major biopolymers are present in cells: proteins,composed of amino acids linked by peptide bonds; nucleicacids, composed of nucleotides linked by phosphodiesterbonds; and polysaccharides, composed of monosaccharides(sugars) linked by glycosidic bonds (see Figure 2-11).■Many molecules in cells contain at least one asymmetric carbon atom, which is bonded to four dissimilar atoms.Such molecules can exist as optical isomers (mirror images), designated D and L, which have different biologicalactivities.
In biological systems, nearly all sugars are D isomers, while nearly all amino acids are L isomers.■MicelleLiposomeDifferences in the size, shape, charge, hydrophobicity,and reactivity of the side chains of amino acids determinethe chemical and structural properties of proteins (see Figure 2-13).■Amino acids with hydrophobic side chains tend to cluster in the interior of proteins away from the surroundingaqueous environment; those with hydrophilic side chainsusually are toward the surface.■Phospholipid bilayer▲ FIGURE 2-20 Cross-sectional views of the three structures formed by phospholipids in aqueous solutions.
Thewhite spheres depict the hydrophilic heads of the phospholipids,and the squiggly black lines (in the yellow regions) representthe hydrophobic tails. Shown are a spherical micelle with ahydrophobic interior composed entirely of fatty acyl chains; aspherical liposome, which has two phospholipid layers and anaqueous center; and a two-molecule-thick sheet of phospholipids,or bilayer, the basic structural unit of biomembranes.The bases in the nucleotides composing DNA and RNAare heterocyclic rings attached to a pentose sugar. Theyform two groups: the purines—adenine (A) and guanine(G)—and the pyrimidines—cytosine (C), thymine (T), anduracil (U) (see Figure 2-15). A, G, T, and C are in DNA,and A, G, U, and C are in RNA.■■ Glucose and other hexoses can exist in three forms: anopen-chain linear structure, a six-member (pyranose) ring, and46CHAPTER 2 • Chemical Foundationsa five-member (furanose) ring (see Figure 2-16).
In biologicalsystems, the pyranose form of D-glucose predominates.next section, we examine energy changes during reactionsand their relationship to equilibria.Glycosidic bonds are formed between either the or anomer of one sugar and a hydroxyl group on anothersugar, leading to formation of disaccharides and other polysaccharides (see Figure 2-17).Equilibrium Constants Reflectthe Extent of a Chemical Reaction■The long hydrocarbon chain of a fatty acid may containno carbon-carbon double bond (saturated) or one or moredouble bonds (unsaturated), which bends the chain.■Phospholipids are amphipathic molecules with a hydrophobic tail (often two fatty acyl chains) and a hydrophilichead (see Figure 2-19).■In aqueous solution, the hydrophobic effect and van derWaals interactions organize and stabilize phospholipidsinto one of three structures: a micelle, liposome, or sheetlike bilayer (see Figure 2-20).■In a phospholipid bilayer, which constitutes the basicstructure of all biomembranes, fatty acyl chains in eachleaflet are oriented toward one another, forming a hydrophobic core, and the polar head groups line both surfaces and directly interact with the aqueous solution.The equilibrium constant Keq depends on the nature of thereactants and products, the temperature, and the pressure(particularly in reactions involving gases).
Under standardphysical conditions (25 ºC and 1 atm pressure, for biological systems), the Keq is always the same for a given reaction,whether or not a catalyst is present.For the general reactionaA bB cC zZ yY xX (2-1)where capital letters represent particular molecules or atomsand lowercase letters represent the number of each in the reaction formula, the equilibrium constant is given byx■Keq yz[X] [Y] [Z]ac[A] [B]b [C](2-2)where brackets denote the concentrations of the molecules.The rate of the forward reaction (left to right in Equation2-1) isRateforward kf[A]a[B]b[C]c2.3Chemical EquilibriumWe now shift our discussion to chemical reactions in whichbonds, primarily covalent bonds in reactant chemicals, arebroken and new bonds are formed to generate reaction products.
At any one time several hundred different kinds ofchemical reactions are occurring simultaneously in every cell,and many chemicals can, in principle, undergo multiplechemical reactions. Both the extent to which reactions canproceed and the rate at which they take place determine thechemical composition of cells.When reactants first mix together—before any productshave been formed—their rate of reaction is determined inpart by their initial concentrations. As the reaction productsaccumulate, the concentration of each reactant decreases andso does the reaction rate.
Meanwhile, some of the productmolecules begin to participate in the reverse reaction, whichre-forms the reactants (microscopic reversibility). This reverse reaction is slow at first but speeds up as the concentration of product increases. Eventually, the rates of the forwardand reverse reactions become equal, so that the concentrations of reactants and products stop changing. The systemis then said to be in chemical equilibrium.At equilibrium the ratio of products to reactants, calledthe equilibrium constant, is a fixed value that is independent of the rate at which the reaction occurs. The rate of achemical reaction can be increased by a catalyst, whichbrings reactants together and accelerates their interactions,but is not permanently changed during a reaction.
In this section, we discuss several aspects of chemical equilibria; in thewhere kf is the rate constant for the forward reaction. Similarly, the rate of the reverse reaction (right to left in Equation2-1) isRatereverse kr[X]x[Y]y[Z]zwhere kr is the rate constant for the reverse reaction. Atequilibrium the forward and reverse rates are equal, soRateforward/Ratereverse 1. By rearranging these equations,we can express the equilibrium constant as the ratio of therate constantsKeq kfkr(2-3)Chemical Reactions in Cells Are at Steady StateUnder appropriate conditions and given sufficient time, individual biochemical reactions carried out in a test tube eventually will reach equilibrium.
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