Часть 1 (1120999), страница 40
Текст из файла (страница 40)
zrorc-OH2CHO;C -HH- C - O HalH -sC-OHH-CH?OHIgrucosecH20HOHH'. zo,fH -C -OHmannoSe-lH-C -OHHIC -OH-CH,OHOHOHNote that eachcarbonatomhasa number.Thesesmalldifferencesmake only minor changesin thechemicalpropertiesof the sugars.But they are recognizedbyenzymesand other proteinsand thereforecan have importantbiologicaleffects.0 A N DB L T N K SSUGARDERIVATIVESThe hydroxylgroup on the carbonthat carriesthealdehydeor ketone can rapidlychangefrom onepositionto the other. Thesetwo positionsarecalleda and B.The hydroxylgroupsof a simplecan be replacedmonosaccharideby other groups.For example,glucosaminecr hydroxylB hydroxylAs soon as one sugaris linkedto another,the o orB form is frozen.qH2oHDISACCHARIDESThe carbonthat carriesthe aldehydeor the ketone can reactwith anyhydroxylgroup on a secondsugarm o l e c u l et o f o r m a d i s a c c h a r i d e .The linkageis calleda glycosidicbond.Threecommon disaccharidesaremaltose(glucose+ glucose)lactose(galactose+ glucose)sucrose(glucose+ fructose)The reactionforming sucroseisshown here.OLIGOSACCHARIDESAND POLYSACCHARIDESLargelinearand branchedmoleculescan be made from simplerepeatingsugars u b u n i t sS.
h o r tc h a i n sa r e c a l l e do l i g o s a c c h a r i d ewsh,i l e l o n g c h a i n sa r e c a l l e dpolysaccharides.Glycogen,for example,is a polysaccharidemade entirelyofglucoseunitsjoined together.glycogenCOMPLEXOLIGOSACCHARIDE5In many casesa sugarsequenceis nonrepetitive.Many differentmoleculesare possible.Suchcomplexoligosaccharidesareusuallylinkedto proteinsor to lipids,as is this oligosaccharide,which ismoleculeoart of a cell-surfacethat definesa particularblood group.COMMON FATTYACIDSFatty acidsare stored as an energy reserve(fats andoils)through an esterlinkageto glycerolto formtriacylglycerols,alsoknown astriglycerides.Theseare carboxylicacidswith long hydrocarbontails.COOHICH,ICH,ICHzICH,ICH,ICH,COOHIf",CH,ICH,l-COOHICH,II9I H ,CH,ICHrIf",Ifn,CH,fn,fn,fn'CHtCHCH"ICHrICH,ICHtICHtICH,ICH,I9H,IcH.t"CH:f*'Ii",f*,CH:CH,,r'Cotlo'CCHtICHttICHtICH,ll (oil),,cHundredsof different kindsof fatty acidsexist.Somehaveone or more double bonds in theirhydrocarbontail and are saidto be unsaturated.Fattyacidswith no double bondsare saturated.-oo\/IL-ICHIT h i sd o u b l eb o n dis rigid and creates.
za kinkin the chain.'ll rh" restof the chainis free to rotateabout the other C-Cbonds.CHtI9H,ICHttICH.ICH,ICH,ICH.3i,'J''''t'(Crs)stear|ca c i d( C r e )CH:space-fillingmodeloleicacid (Cre)carbonskeletonUNSATURATEDCARBOXYLGROUPSATURATEDP H O S P H O L I P I D S P h o s p h o l i pai dr set h e m a j o rc o n s t i t u e n t sof cellmembranes.lf free, the carboxylgroup of afatty acidwill be ionized.,a/&c..B u t m o r e u s u a l l yi t i s l i n k e dt oother groupsto form either estersWCor amideso,1/C\s p a c e - f i l l i nmg o d e lo fthe phospholipidp h o s p h a t i d y l c hionleIn phospholipidstwo of the -OH groupsin glycerolarelinkedto fatty acids,while the third -OH group is linkedg e n e r a ls t r u c t u r e to phosphoricacid.The phosphateis further linkedtoo f a p h o s p h o l i p i d one of a varietyof smallpolar groups(alcohols).LIPIDAGGREGATESPOLYISOPRENOIDSlong-chainpolymersof isopreneln water they can form a surfacefilmo r f o r m s m a l lm i c e l l e s .Their derivativescan form largeraggregatesheld together by hydrophobicforces:Triglyceridescan form largesphericalfat dropletsin the cell cytoplasm.O T H E RL I P I D SipidP h o s p h o l i p i dasn d g l y c o l i p i dfso r m s e l f - s e a l i nl gbilayersthat are the basisfor all cell membranes.Lioidsare defined asthe water-insolublem o l e c u l e isn c e l l st h a t a r e s o l u b l ei n o r g a n i csolvents.Two other commontypesof lipidsare steroidsand polyisoprenoids.Both aremade from isopreneunits.CH.\ -CH:CHr.C,//Lr 12isopreneSteroidshavea common multiple-ringstructure.cholesterol-found in manv membranesGLYCOLIPIDSLikephospholipids,thesecompoundsare composedof a hydrophobic- C - NI Hdolicholphosphate-usedto carryactivatedsugarsin the membrane-associatedsynthesisof glycoproteinsand somepolysaccharidesNUCLEOTIDESPHOSPHATESA nucleotideconsistsof a nitrogen-containingbase,a five-carbonsugar,and one or morephosphategroups.The phosphatesare normallyjoined tothe C5 hydroxylof the riboseordeoxyribosesugar(designated5').
Mono-,di-, and triphosphatesare common.BASICSUGARLINKAGEBASE-o--ililtlP- o -"-P- o -IIo-o-oP- . ) - c H ^l-"a sI nl"'ottThe phosphatemakesa nucleotidenegativelycharged.The baseis linkedtothe samecarbon(C1)usedin sugar-sugarbonds.NHl2IiHclC\NH\HuIll UHCli\x"t-'.,-t\llClcvtosinent-r-t\oluracilThe basesare nitrogen-containingringc o m p o u n d se, i t h e rp y r i m i d i n eosr p u r i n e s .Hol1" n,.I'\./-\**3ll r Ithylrl\ - t \PENTOSEa five-carbonsugarEachnumberedcarbonon the sugarof a nucleotideisfollowed by a prime mark;therefore,one speaksof the" 5 - p r i m ec a r b o n , "e t c .two kindsare usedboseB-o-2-deoxyriusedin deoxvribonucleicacidNOMENCLATUREA nucleosideor nucleotideis named accordingto its nitrogenousbase.Singleletter abbreviationsare usedvariouslyasshorthandfor (1) the basealone,(2) thenucleoside,or (3) the whole nucleotidethe contextwill usuallymake clearwhich ofthe three entitiesis meant.When the contextis not sufficient,we will add the terms "base", B A S E+ S U G A R= N U C L E O S I D E"nucleoside","nucleotide",or-as in theexamolesbelow-use the full 3-letternucleotidecooe.AMPdAMPUDPATPN U C L E IACC I D S= adenosinemonophosphate= deoxyadenosinemonophosphate= u r i d i n ed i p h o s p h a t e= adenosinetriphosphate= NUCLEOTIDEBASE+ SUGAR+ PHOSPHATENUCLEOTIDESHAVEMANY OTHERFUNCTIONSNucleotidesare joined together by aphosphodiesterlinkagebetween 5' and3' carbonatomsto form nucleicacids.The linearsequenceof nucleotidesin anucleicacid chain is commonlyabbreviated by a one-letter code,A-G-C-T-T-A-C-A,with the 5'end of the chain at the left.bonds.Theycarrychemicalenergy in their easilyhydrolyzedphosphoanhydridep h o s p h o a n h y d r i dbeo n d s-looo- o- i l i l t l-l- fH,l- o- P- o- P- oo-o-o-example:ATP(orOH)OHTheycombinewith other groupsto form coenzymes.HOHHOHHSttttlllllllHtHHHHHOH CH.Hll l l'lil r rt t_C _C _N _C _C -C _N_C _C _C -Cll-O-HO CH3HOllP_O-P-OO-- CH2O-example:coenzymeA (CoA)ooH5' endof chainIO : P- O -5',oTheyare usedas specificsignalingmoleculesin the cell'l)Hzexample:cyclicAMP (cAMP)ooIIIP_Io-ooHT H EI M P O R T A N COEF F R E EE N E R GFYO RC E L L SLife is possiblebecauseof the complexnetwork of interactingchemicalreactionsoccurringin everycell.In viewingthemetabolicpathwaysthat comprisethis network, one mightsuspectthat the cell has had the abilityto evolvean enzymetocarryout any reactionthat it needs.But this is not so.Althoughenzymesare powerful catalysts,they can speedup only thosereactionsthat are thermodynamicallypossible;other reactionsproceedin cellsonly becausethey are coupledto very favorablereactionsthat drive them.
The ouestionof whether a reactioncan occurspontaneously,or insteadneedsto be coupledtoanother reaction,is centralto cell biology. The answerisobtained by referenceto a quantity calledthe free energy: thetotal changein free energyduring a set of reactionsdetermineswhether or not the entire reactionseouencecan occur.ln thispanel we shallexplainsomeof the fundamentalideas-derivedfrom a specialbranchof chemistryand physicscalledthermodynamics-that are requiredfor understandingwhat freeenergy is and why it is so important to cells.E N E R GRY E L E A S EBDY C H A N G EISN C H E M I C ABLO N D I N GI SC O N V E R T EI ND T OH E A Tof molecularcollisionsthat heat uo first the walls of the boxand then the outsideworld (representedby the sea inour example).In the end, the systemreturnsto its initialtemperature,by which time all the chemicalbond energyreleasedin the box has been convertedinto heat energyandtransferredout of the box to the surroundings.Accordingtothe first law, the changein the energyin the box (AE6o",which5EAwe shalldenote as AE) must be equal and oppositeto theamount of heat energytransferred,which we shalldesignateas h: that is,AE = -h.
Thus,the energy in the box (E) decreaseswhen heat leavesthe system.Ealso can changeduring a reactionas a resultof work beingdone on the outsideworld. For example,supposethat there isUNIVERSEa smallincreasein the volume (Ay) of the box during a reaction.Sincethe walls of the box must pushagainstthe constantpressure(P) in the surroundingsin order to expand,this doesAn enclosedsystemis defined as a collectionof moleculesthatwork on the outsideworld and requiresenergy.The energydoesnot exchangematter with the restof the universe(forusedis P(AV),which accordingto the first law must decreaseexample,the "cell in a box" shown above).Any suchsystemwillthe energy in the box (E) by the sameamount. ln most reactionscontainmoleculeswith a total energyE. Thisenergywill bechemicalbond energy is convertedinto both work and heat.distributedin a varietyof ways:someasthe translationalenergyEnthalpy (H) is a compositefunction that includesboth of theseof the molecules,someas their vibrationaland rotationalenergies, (H = E + PV).To be rigorous,it is the changein enthalpybut most asthe bonding energiesbetweenthe individualatoms(AH) in an enclosedsystem,and not the changein energy,thatthat make up the molecules.Supposethat a reactionoccursinis equal to the heat transferredto the outsideworld during athe system.The f irst law of thermodynamicsplacesa constraintreaction.Reactionsin which H decreasesreleaseheat to theon what typesof reactionsare possible:it statesthat "in anysurroundingsand are saidto be "exothermic,"while reactionsprocess,the total energyof the universeremainsconstant."in which H increasesabsorbheat from the surroundingsandForexample,supposethat reactionA - B occurssomewhereinare saidto be "endothermic."Thus,-h = AH.
However,thethe box and releasesa great deal of chemicalbond energy.Thisvolume changeis negligiblein most biologicalreactions,so toenergywill initiallyincreasethe intensityof molecularmotionsa good approximation(translational,vibrational,and rotational)in the system,whichis equivalentto raisingits temperature.However,these increasedmotionswill soon be transferredout of the svstembv a seriesTHESECONDLAW OFTHERMODYNAMICSConsidera containerin which 1000coinsare all lying headsup.lf the containeris shakenvigorously,subjectingthe coinstothe typesof random motionsthat all moleculesexperiencedueto their frequent collisionswith other molecules,one will endup with about half the coinsoriented headsdown.
Thereasonfor this reorientationis that there is only a singleway inwhich the originalorderlystateof the coinscan be reinstated(everycoin mu{lie headsup), whereasthere are many differentways (about 102s8)to achievea disorderlystate in which there isan equal mixtureof headsand tails;in fact, there are more wavsto achievea 50-50statethan to achieveany other state.Eachstate has a probabilityof occurrencethat is proportionalto thenumber of ways it can be realized.The secondlaw of thermodynamicsstatesthat "systemswill changespontaneouslyfromstatesof lower probabilityto statesof higher probability."Sincestatesof lower probabilityare more "ordered" thanstatesof high probability,the secondlaw can be restated:"the universeconstantlychangesso as to becomemoredisordered."THEENTROPY,5The secondlaw (but not the first law) allowsone to predictthedirectionof a particularreaction.But to make it usefulfor thispurpose,one needsa convenientmeasureof the probabilityor,equivalently,the degreeof disorderof a state.The entropy (5)i s s u c ha m e a s u r el.t i s a l o g a r i t h m i fcu n c t i o no f t h e p r o b a b i l i t ysuchthat the changein entropy (45) that occurswhen thereactionA - B convertsone mole of A into one mole of B isA5=RInpB/pAwhere p4 and p, are the probabilitiesof the two statesA and B,R is the gas constant(2 cal deg 1 mole 1),and A5 is measuredi n e n t r o p yu n i t s( e u ) .I n o u r i n i t i a le x a m p l eo f 1 0 0 0c o i n s t, h erelativeprobabilityof all heads(stateA) versushalf headsandhalf tails (stateB) is equal to the ratio of the number of differentwaysthat the two resultscan be obtained.One can calculatet h a t p A = 1 a n d p , = 1 0 0 0 1 ( 5 0 x0 15 0 0 1-) 1 0 2 s eT.h e r e f o r e ,the entropy changefor the reorientationof the coinswhen theircontaineris vigorouslyshakenand an equal mixtureof headsor about 1370eu per mole ofand tails is obtained is R In (102e8),We seethat, becauseA5suchcontainers(6 x 1023containers).defined above is positivefor the transitionfrom stateA tostate B (ps/p4 > 1), reactionswith a large increasein 5 (that is,for which A5 > 0) are favoredand will occurspontaneously.in Chapter2, heat energycausesthe randomAs discussedthe transferof heat from ancommotionof molecules.Becausethe number ofenclosedsystemto its surroundingsincreasesdifferent arrangementsthat the moleculesin the outsideworldtheir entropy.lt can be shown that thecan have,it increasesreleaseof a fixed quantity of heat energyhasa greaterdisordering effect at low temperaturethan at high temperature,andthat the value of A5 for the surroundings.as defined above(ASr"u),is preciselyequalto h, the amount of heattransferredtothe surroundingsfrom the system,dividedby the absolutetemperature(f ):T H EG I B B SFREEE N E R G YG,When dealingwith an enclosedbiologicalsystem,one wouldlike to have a simpleway of predictingwhether a given reactionwill or will not occurspontaneouslyin the system.We haveseenthat the crucialquestionis whether the entropy changeforthe universeis positiveor negativewhen that reactionoccurs.In our idealizedsystem,the cell in a box,there are two separatecomponentsto the entropy changeof the universe-the entropychangefor the systemenclosedin the box and the entropychangefor the surrounding"sea"-and both must be addedtogether before any predictioncan be made.For example,it ispossiblefor a reactionto absorbheat and therebydecreasetheentropy of the sea (A5r""< 0) and at the sametime to causesucha large degreeof disorderinginsidethe box (A56o*> 0)= A5r"" + A56o,is greater than 0.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
