Sinh học - Chapter 5: The structure and function of large biological molecules

The Structure and Function of Large Biological MoleculesChapter 5Overview: The Molecules of LifeAll living things are made up of four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acidsMacromolecules are large molecules composed of thousands of covalently connected atomsMolecular structure and function are inseparable© 2011 Pearson Education, Inc.Concept 5.1: Macromolecules are polymers, built from monomersA polymer is a long molecule consisting of many similar building blocks These small building-block molecules are called monomersThree of the four classes of life’s organic molecules are polymersCarbohydratesProteinsNucleic acids© 2011 Pearson Education, Inc.A dehydration reaction occurs when two monomers bond together through the loss of a water moleculePolymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reactionThe Synthesis and Breakdown of Polymers© 2011 Pearson Education, Inc.Figure 5.2a(a) Dehydration reaction: synthesizing a polymerShort polymerUnlinked monomerDehydration removesa water molecule,forming a new bond.Longer polymer1234123Figure 5.2b(b) Hydrolysis: breaking down a polymerHydrolysis addsa water molecule,breaking a bond.1234123The Diversity of PolymersEach cell has thousands of different macromolecules Macromolecules vary among cells of an organism, vary more within a species, and vary even more between speciesAn immense variety of polymers can be built from a small set of monomersHO© 2011 Pearson Education, Inc.Concept 5.2: Carbohydrates serve as fuel and building materialCarbohydrates include sugars and the polymers of sugarsThe simplest carbohydrates are monosaccharides, or single sugarsCarbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks © 2011 Pearson Education, Inc.SugarsMonosaccharides have molecular formulas that are usually multiples of CH2OGlucose (C6H12O6) is the most common monosaccharideMonosaccharides are classified by The location of the carbonyl group (as aldose or ketose)The number of carbons in the carbon skeleton© 2011 Pearson Education, Inc.Figure 5.3Aldoses (Aldehyde Sugars)Ketoses (Ketone Sugars)GlyceraldehydeTrioses: 3-carbon sugars (C3H6O3)DihydroxyacetonePentoses: 5-carbon sugars (C5H10O5)Hexoses: 6-carbon sugars (C6H12O6)RiboseRibuloseGlucoseGalactoseFructoseThough often drawn as linear skeletons, in aqueous solutions many sugars form ringsMonosaccharides serve as a major fuel for cells and as raw material for building molecules © 2011 Pearson Education, Inc.Figure 5.4(a) Linear and ring forms(b) Abbreviated ring structure123456654321123456123456A disaccharide is formed when a dehydration reaction joins two monosaccharides This covalent bond is called a glycosidic linkage© 2011 Pearson Education, Inc.Figure 5.5(a) Dehydration reaction in the synthesis of maltose(b) Dehydration reaction in the synthesis of sucroseGlucoseGlucoseGlucoseMaltoseFructoseSucrose1–4glycosidiclinkage1–2glycosidiclinkage1412PolysaccharidesPolysaccharides, the polymers of sugars, have storage and structural rolesThe structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages© 2011 Pearson Education, Inc.Storage PolysaccharidesStarch, a storage polysaccharide of plants, consists entirely of glucose monomersPlants store surplus starch as granules within chloroplasts and other plastids The simplest form of starch is amylose© 2011 Pearson Education, Inc.Figure 5.6(a) Starch: a plant polysaccharide(b) Glycogen: an animal polysaccharideChloroplastStarch granulesMitochondriaGlycogen granulesAmylopectinAmyloseGlycogen1 m0.5 mGlycogen is a storage polysaccharide in animalsHumans and other vertebrates store glycogen mainly in liver and muscle cells© 2011 Pearson Education, Inc.Structural PolysaccharidesThe polysaccharide cellulose is a major component of the tough wall of plant cellsLike starch, cellulose is a polymer of glucose, but the glycosidic linkages differThe difference is based on two ring forms for glucose: alpha () and beta () © 2011 Pearson Education, Inc.Figure 5.7(a)  and  glucose ring structures(b) Starch: 1–4 linkage of  glucose monomers(c) Cellulose: 1–4 linkage of  glucose monomers Glucose Glucose41414141© 2011 Pearson Education, Inc.Polymers with  glucose are helicalPolymers with  glucose are straightIn straight structures, H atoms on one strand can bond with OH groups on other strandsParallel cellulose molecules held together this way are grouped into microfibrils, which form strong building materials for plantsCell wallMicrofibrilCellulosemicrofibrils in aplant cell wallCellulosemolecules Glucosemonomer10 m0.5 mFigure 5.8Enzymes that digest starch by hydrolyzing  linkages can’t hydrolyze  linkages in celluloseCellulose in human food passes through the digestive tract as insoluble fiberSome microbes use enzymes to digest celluloseMany herbivores, from cows to termites, have symbiotic relationships with these microbes© 2011 Pearson Education, Inc.Chitin, another structural polysaccharide, is found in the exoskeleton of arthropodsChitin also provides structural support for the cell walls of many fungi© 2011 Pearson Education, Inc.Figure 5.9Chitin forms the exoskeletonof arthropods.The structureof the chitinmonomerChitin is used to make a strong and flexiblesurgical thread that decomposes after thewound or incision heals.Concept 5.3: Lipids are a diverse group of hydrophobic moleculesLipids are the one class of large biological molecules that do not form polymersThe unifying feature of lipids is having little or no affinity for waterLipids are hydrophobic becausethey consist mostly of hydrocarbons, which form nonpolar covalent bondsThe most biologically important lipids are fats, phospholipids, and steroids© 2011 Pearson Education, Inc.FatsFats are constructed from two types of smaller molecules: glycerol and fatty acidsGlycerol is a three-carbon alcohol with a hydroxyl group attached to each carbonA fatty acid consists of a carboxyl group attached to a long carbon skeleton© 2011 Pearson Education, Inc.Figure 5.10(a) One of three dehydration reactions in the synthesis of a fat(b) Fat molecule (triacylglycerol)Fatty acid(in this case, palmitic acid)GlycerolEster linkage© 2011 Pearson Education, Inc.Fats separate from water because water molecules form hydrogen bonds with each other and exclude the fatsIn a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglycerideFatty acids vary in length (number of carbons) and in the number and locations of double bondsSaturated fatty acids have the maximum number of hydrogen atoms possible and no double bondsUnsaturated fatty acids have one or more double bonds© 2011 Pearson Education, Inc.Figure 5.11(a) Saturated fat(b) Unsaturated fatStructuralformula of asaturated fatmoleculeSpace-fillingmodel of stearicacid, a saturatedfatty acidStructuralformula of anunsaturated fatmoleculeSpace-filling modelof oleic acid, anunsaturated fattyacidCis double bondcauses bending.Fats made from saturated fatty acids are called saturated fats, and are solid at room temperatureMost animal fats are saturatedFats made from unsaturated fatty acids are called unsaturated fats or oils, and are liquid at room temperaturePlant fats and fish fats are usually unsaturated© 2011 Pearson Education, Inc.A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits Hydrogenation is the process of converting unsaturated fats to saturated fats by adding hydrogenHydrogenating vegetable oils also creates unsaturated fats with trans double bondsThese trans fats may contribute more than saturated fats to cardiovascular disease© 2011 Pearson Education, Inc.Certain unsaturated fatty acids are not synthesized in the human body These must be supplied in the dietThese essential fatty acids include the omega-3 fatty acids, required for normal growth, and thought to provide protection against cardiovascular disease© 2011 Pearson Education, Inc.The major function of fats is energy storageHumans and other mammals store their fat in adipose cellsAdipose tissue also cushions vital organs and insulates the body© 2011 Pearson Education, Inc.PhospholipidsIn a phospholipid, two fatty acids and a phosphate group are attached to glycerol The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head© 2011 Pearson Education, Inc.Figure 5.12CholinePhosphateGlycerolFatty acidsHydrophilicheadHydrophobictails(c) Phospholipid symbol(b) Space-filling model(a) Structural formulaHydrophilic headHydrophobic tailsWhen phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interiorThe structure of phospholipids results in a bilayer arrangement found in cell membranesPhospholipids are the major component of all cell membranes© 2011 Pearson Education, Inc.Figure 5.13HydrophilicheadHydrophobictailWATERWATERSteroidsSteroids are lipids characterized by a carbon skeleton consisting of four fused ringsCholesterol, an important steroid, is a component in animal cell membranesAlthough cholesterol is essential in animals, high levels in the blood may contribute to cardiovascular disease© 2011 Pearson Education, Inc.Figure 5.14Concept 5.4: Proteins include a diversity of structures, resulting in a wide range of functionsProteins account for more than 50% of the dry mass of most cellsProtein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances© 2011 Pearson Education, Inc.Figure 5.15-aEnzymatic proteinsDefensive proteinsStorage proteinsTransport proteinsEnzymeVirusAntibodiesBacteriumOvalbuminAmino acidsfor embryoTransportproteinCell membraneFunction: Selective acceleration of chemical reactionsExample: Digestive enzymes catalyze the hydrolysisof bonds in food molecules.Function: Protection against diseaseExample: Antibodies inactivate and help destroyviruses and bacteria.Function: Storage of amino acidsFunction: Transport of substancesExamples: Casein, the protein of milk, is the majorsource of amino acids for baby mammals. Plants havestorage proteins in their seeds. Ovalbumin is theprotein of egg white, used as an amino acid sourcefor the developing embryo.Examples: Hemoglobin, the iron-containing protein ofvertebrate blood, transports oxygen from the lungs toother parts of the body. Other proteins transportmolecules across cell membranes.Figure 5.15-bHormonal proteinsFunction: Coordination of an organism’s activitiesExample: Insulin, a hormone secreted by thepancreas, causes other tissues to take up glucose,thus regulating blood sugar concentrationHighblood sugarNormalblood sugarInsulinsecretedSignalingmoleculesReceptorproteinMuscle tissueActinMyosin100 m60 mCollagenConnectivetissueReceptor proteinsFunction: Response of cell to chemical stimuliExample: Receptors built into the membrane of anerve cell detect signaling molecules released byother nerve cells.Contractile and motor proteinsFunction: MovementExamples: Motor proteins are responsible for theundulations of cilia and flagella. Actin and myosinproteins are responsible for the contraction ofmuscles.Structural proteinsFunction: SupportExamples: Keratin is the protein of hair, horns,feathers, and other skin appendages. Insects andspiders use silk fibers to make their cocoons and webs,respectively. Collagen and elastin proteins provide afibrous framework in animal connective tissues.Enzymes are a type of protein that acts as a catalyst to speed up chemical reactionsEnzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life© 2011 Pearson Education, Inc.PolypeptidesPolypeptides are unbranched polymers built from the same set of 20 amino acidsA protein is a biologically functional molecule that consists of one or more polypeptides© 2011 Pearson Education, Inc.Amino Acid MonomersAmino acids are organic molecules with carboxyl and amino groupsAmino acids differ in their properties due to differing side chains, called R groups© 2011 Pearson Education, Inc.Figure 5.UN01Side chain (R group)AminogroupCarboxylgroup carbonFigure 5.16Nonpolar side chains; hydrophobicSide chain(R group)Glycine(Gly or G)Alanine(Ala or A)Valine(Val or V)Leucine(Leu or L)Isoleucine (Ile or I)Methionine(Met or M)Phenylalanine(Phe or F)Tryptophan(Trp or W)Proline(Pro or P)Polar side chains; hydrophilicSerine(Ser or S)Threonine(Thr or T)Cysteine(Cys or C)Tyrosine(Tyr or Y)Asparagine(Asn or N)Glutamine(Gln or Q)Electrically charged side chains; hydrophilicAcidic (negatively charged)Basic (positively charged)Aspartic acid(Asp or D)Glutamic acid(Glu or E)Lysine(Lys or K)Arginine(Arg or R)Histidine(His or H)Amino Acid PolymersAmino acids are linked by peptide bondsA polypeptide is a polymer of amino acidsPolypeptides range in length from a few to more than a thousand monomers Each polypeptide has a unique linear sequence of amino acids, with a carboxyl end (C-terminus) and an amino end (N-terminus)© 2011 Pearson Education, Inc.Figure 5.17Peptide bondNew peptidebond formingSidechainsBack-boneAmino end(N-terminus)PeptidebondCarboxyl end(C-terminus)Protein Structure and FunctionA functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape© 2011 Pearson Education, Inc.The sequence of amino acids determines a protein’s three-dimensional structureA protein’s structure determines its function© 2011 Pearson Education, Inc.Figure 5.19Antibody proteinProtein from flu virusFour Levels of Protein StructureThe primary structure of a protein is its unique sequence of amino acidsSecondary structure, found in most proteins, consists of coils and folds in the polypeptide chainTertiary structure is determined by interactions among various side chains (R groups)Quaternary structure results when a protein consists of multiple polypeptide chains© 2011 Pearson Education, Inc.Figure 5.20aPrimary structureAminoacidsAmino endCarboxyl endPrimary structure of transthyretinPrimary structure, the sequence of amino acids in a protein, is like the order of letters in a long word Primary structure is determined by inherited genetic information© 2011 Pearson Education, Inc.Figure 5.20bSecondarystructureTertiarystructureQuaternarystructureHydrogen bond helix pleated sheet strandHydrogenbondTransthyretinpolypeptideTransthyretinproteinThe coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backboneTypical secondary structures are a coil called an  helix and a folded structure called a  pleated sheet© 2011 Pearson Education, Inc.Secondary structureHydrogen bond helix pleated sheet strand, shown as a flatarrow pointing towardthe carboxyl endHydrogen bondFigure 5.20cTertiary structure is determined by interactions between R groups, rather than interactions between backbone constituentsThese interactions between R groups include hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactionsStrong covalent bonds called disulfide bridges may reinforce the protein’s structure© 2011 Pearson Education, Inc.Figure 5.20fHydrogenbondDisulfidebridgePolypeptidebackboneIonic bondHydrophobicinteractions andvan der WaalsinteractionsQuaternary structure results when two or more polypeptide chains form one macromoleculeCollagen is a fibrous protein consisting of three polypeptides coiled like a ropeHemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains© 2011 Pearson Education, Inc.HemoglobinHemeIron subunit subunit subunit subunitFigure 5.20iSickle-Cell Disease: A Change in Primary StructureA slight change in primary structure can affect a protein’s structure and ability to function Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin© 2011 Pearson Education, Inc.Figure 5.21PrimaryStructureSecondaryand TertiaryStructuresQuaternaryStructureFunctionRed BloodCell Shape subunit subunitExposedhydrophobicregionMolecules do notassociate with oneanother; each carriesoxygen.Molecules crystallizeinto a fiber; capacityto carry oxygen isreduced.Sickle-cellhemoglobinNormalhemoglobin10 m10 mSickle-cell hemoglobinNormal hemoglobin12345671234567What Determines Protein Structure?In addition to primary structure, physical and chemical conditions can affect structureAlterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravelThis loss of a protein’s native structure is called denaturationA denatured protein is biologically inactive© 2011 Pearson Education, Inc.Figure 5.22Normal proteinDenatured proteinDenturtonRentrtonaaiauaiProtein Folding in the CellIt is hard to predict a protein’s structure from its primary structureMost proteins probably go through several stages on their way to a stable structureChaperonins are protein molecules that assist the proper folding of other proteinsDiseases such as Alzheimer’s, Parkinson’s, and mad cow disease are associated with misfolded proteins© 2011 Pearson Education, Inc.Figure 5.23The cap attaches, causingthe cylinder to changeshape in such a way thatit creates a hydrophilicenvironment for thefolding of the polypeptide.CapPolypeptideCorrectlyfoldedproteinChaperonin(fully assembled)Steps of ChaperoninAction:An unfolded poly-peptide enters thecylinder fromone end.HollowcylinderThe cap comesoff, and theproperly foldedprotein isreleased.123Scientists use X-ray crystallography to determine a protein’s structureAnother method is nuclear magnetic resonance (NMR) spectroscopy, which does not require protein crystallizationBioinformatics uses computer programs to predict protein structure from amino acid sequences© 2011 Pearson Education, Inc.Figure 5.24DiffractedX-raysX-raysourceX-raybeamCrystalDigital detectorX-ray diffractionpatternRNADNARNApolymerase IIEXPERIMENTRESULTSConcept 5.5: Nucleic acids store, transmit, and help express hereditary informationThe amino acid sequence of a polypeptide is programmed by a unit of inheritance called a geneGenes are made of DNA, a nucleic acid made of monomers called nucleotides© 2011 Pearson Education, Inc.The Roles of Nucleic AcidsThere are two types of nucleic acidsDeoxyribonucleic acid (DNA)Ribonucleic acid (RNA)DNA provides directions for its own replicationDNA directs synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesisProtein synthesis occurs on ribosomes© 2011 Pearson Education, Inc.Figure 5.25-3Synthesis ofmRNAmRNADNANUCLEUSCYTOPLASMmRNARibosomeAminoacidsPolypeptideMovement ofmRNA intocytoplasmSynthesisof protein123The Components of Nucleic AcidsNucleic acids are polymers called polynucleotidesEach polynucleotide is made of monomers called nucleotidesEach nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groupsThe portion of a nucleotide without the phosphate group is called a nucleoside© 2011 Pearson Education, Inc.Figure 5.26Sugar-phosphate backbone5 end5C3C5C3C3 end(a) Polynucleotide, or nucleic acid(b) NucleotidePhosphategroupSugar(pentose)NucleosideNitrogenousbase5C3C1CNitrogenous basesCytosine (C)Thymine (T, in DNA)Uracil (U, in RNA)Adenine (A)Guanine (G)SugarsDeoxyribose (in DNA)Ribose (in RNA)(c) Nucleoside componentsPyrimidinesPurinesNucleoside = nitrogenous base + sugarThere are two families of nitrogenous basesPyrimidines (cytosine, thymine, and uracil) have a single six-membered ringPurines (adenine and guanine) have a six-membered ring fused to a five-membered ringIn DNA, the sugar is deoxyribose; in RNA, the sugar is riboseNucleotide = nucleoside + phosphate group© 2011 Pearson Education, Inc.Nucleotide PolymersNucleotide polymers are linked together to build a polynucleotideAdjacent nucleotides are joined by covalent bonds that form between the —OH group on the 3 carbon of one nucleotide and the phosphate on the 5 carbon on the nextThese links create a backbone of sugar-phosphate units with nitrogenous bases as appendagesThe sequence of bases along a DNA or mRNA polymer is unique for each gene© 2011 Pearson Education, Inc.The Structures of DNA and RNA MoleculesRNA molecules usually exist as single polypeptide chains DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helixIn the DNA double helix, the two backbones run in opposite 5→ 3 directions from each other, an arrangement referred to as antiparallelOne DNA molecule includes many genes© 2011 Pearson Education, Inc.The nitrogenous bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)Called complementary base pairingComplementary pairing can also occur between two RNA molecules or between parts of the same moleculeIn RNA, thymine is replaced by uracil (U) so A and U pair© 2011 Pearson Education, Inc.Figure 5.27Sugar-phosphatebackbonesHydrogen bondsBase pair joinedby hydrogen bondingBase pair joinedby hydrogenbonding(b) Transfer RNA(a) DNA5353DNA and Proteins as Tape Measures of EvolutionThe linear sequences of nucleotides in DNA molecules are passed from parents to offspringTwo closely related species are more similar in DNA than are more distantly related speciesMolecular biology can be used to assess evolutionary kinship© 2011 Pearson Education, Inc.The Theme of Emergent Properties in the Chemistry of Life: A ReviewHigher levels of organization result in the emergence of new propertiesOrganization is the key to the chemistry of life© 2011 Pearson Education, Inc.Figure 5.UN02Figure 5.UN02aFigure 5.UN02bFigure 5. UN07Figure 5. UN08Figure 5. UN09Figure 5. UN10Figure 5. UN12Figure 5. UN13