分子生物学1课件

Chapter1AminoAcidstoProteins

从氨基酸到蛋白质Chapter1AminoAcidstoProteins1.1ProteinComposition1.2ProteinConformations1.3ProteinStructureand

Function:AFewExamples1.4TheDynamicsofProteins1.5Experiments1.1蛋白质的组成1.2蛋白质的构象1.3蛋白质的结构与功能：几个例子1.4蛋白质动力学1.5实验研究Chapter1AminoAcidstoProteinsLifeismostdirectlytheworkofproteins.Proteinsalloworganismstogrowandreproduce.Theyprovideshape,strengthandmovement.Inthecell,proteinsareeverywhereanddoalmosteverything.生命几乎就是蛋白质的杰作。蛋白质让生物可以生长和繁殖。它们为生物提供了外形和力量，以及运动功能。在细胞中，蛋白质无处不在，行使着几乎任何功能。1.1ProteinComposition@Proteinsarecomposedofsmallmoleculescalledaminoacids.

Carbonatom

attachs

tofourdifferentchemicalgroups.

hydrogenatomRgroupDifferentaminoacidshavedifferentRgroups.Thetwentyaminoacidsand

abbreviationTheycanbedividedintofourgroups

accordingtothe

propertyoftheirRgroups.9623Structuresofhydrophobicaminoacids脂肪族和芳香族支链维持蛋白质三维结构中起着重要作用（蛋白质的疏水核心）StructuresofacidicandbasicaminoacidsNegativechargePositivecharge胍基咪唑基

acidicRgroupsbasicRgroupsAspandGlu

carrynegativecharges,Lys,ArgandHiscarrypositivechargesatphysiological

pHvalue(7.2~7.6).20种氨基酸三字符必需记住：AlaArgAspAsnCysGlu

Gln

Gly

His

Ile

丙

精

天酰氨

半

谷

酰氨

甘

组

异亮LeuLysMet

PheProSerThrTrpTyrVal

亮

赖

甲硫

苯丙脯

丝

苏

色

酪

缬TheformationofapeptidebondThecarboxylgroupofoneaminoacidcanreactwiththeaminogroup

ofanotheraminoacidtoformapeptidebond(amidebond-酰胺键).Proteinscanbecomposedofanycombinationoftwenty

aminoacids,inanynumberandinanyorder.

Anyaminoacidcanjointoanyotheraminoacid.由于C=O双键中的π电子云与N原子的未共用电子对发生“电子共振”，共振产生的结果：限制绕肽键自由旋转而产生键的平均化。(2)、一般的C-N单键>肽键C-N>一般的C=N双键（键长1.49Å)（键长1.32Å）（键长1.27Å)因此具有部分双键的性质，不能沿C-N自由旋转Rigidandflexiblebonds

Peptidebondhaspartialdouble-bondcharacter.

it

cannotrotateeasily,

soit

is

rigid

.

Butotherbondscanrotatefreely.Sotheaminoacidchainsareflexible.Actually,proteinmoleculesbend,fold,andtwisttoformcomplicatedthree-dimensionalstructures.肽键平面:由于肽键具有部分双键的性质，使参与肽键构成的六个原子被束缚在同一平面上，这一平面称为肽平面(peptideplane)但由于-碳原子与其他原子之间均形成单键，因此两相邻的平面结构可以作相对旋转。其旋转角分别为φ和ψ，可描述多肽链的所有可能构象。1.2ProteinConformations@

1.2.1DescribingProtein

Structure1.2.2ChemicalandPhysicalBasis

forProteinFoldingTheflexibilityofcertainbondsin

apolypeptide

allows

thechaintofoldinto

complicated

three-dimensionalstructures.Differentconformationsofproteinsgivethemspecializedfunctions1.2.1DescribingProteinStructure

Tosimplifydiscussionofproteinconformations,

fourlevelsofproteinstructurehavebeendefined.Primarystructure:

sequenceofaminoacids

Secondarystructures：αhelixandβsheet

arethetwomajorformsTertiarystructure：morecomplicated

conformationofthewholepolypeptide.

Itis

composedofvarioussecondarystructures.

Quaternarystructure：

shapeofaproteinthathasmultiple

subunits.Eachsubunithasitsowntertiarystructure.(1)蛋白质的二级(Secondary

structure)结构指肽链主链的折叠和盘绕方式。它只涉及肽链主链的构象及链内或链间形成的氢键。

（1）α-helix(螺旋)

（2）β-sheet(折迭)（3）β-turn(转角/U形转折/回折)（4）randomcoil(无规卷曲)从N-末端出发，每个氨基酸残基的N-H与前面隔三个氨基酸残基的C=O形成氢键，这样构成的由一个氢键闭合的环，包含--??个原子肽链上所有的肽键都能参与氢键的形成，以维持螺旋结构的稳定..(2)βsheet

HydrogenbondThebindingoccursbetweenpolypeptidestretchesthatlieapproximatelyparalleltoeachother.Hydrogenbondsareformedbetweenthehydrogen

atoms

andoxygen

onseparatestretches

ofthepolypeptidechain.β-折叠是由两条或多条几乎完全伸展的肽链平行排列，通过链间的氢键交联而形成的扇面状片层构象α-碳原子总是处于折叠的角上，氨基酸的R基团交替分布在片层的两侧。侧链(Cα-C键)几乎垂直于折叠片平面。

永久性卷发（烫发）原理：

-角蛋白在湿热条件下伸展为-构象，当涂上氧化剂使很多的二硫键交联，因此当外力解除后，肽链又恢复原状。（3）蛋白质的β-转角(β-turn)蛋白质分子的多肽链上常出现180°的回折，这种陡然改变方向的肽段即----

β

转角。Pr主要二级结构超二级结构（supersecondarystructure）超二级结构是介于蛋白质二级结构和三级结构之间的空间结构,指相邻的二级结构单元组合在一起，彼此相互作用，排列形成规则的、在空间结构上能够辨认的二级结构组合体，并充当三级结构的构件（blockbuilding），其基本形式有αα、βαβ和βββ等。多数情况下只有非极性残基侧链参与这些相互作用,而亲水侧链多在分子的外表面结构域结构域（domain）结构域是在二级结构或超二级结构的基础上形成三级结构的局部折叠区，相邻的域常被一个或两个多肽片段连结。其特点是在三维空间上可以明显区分和相对独立，并且具有一定的生物功能如结合小分子。模体或基序（motif）是结构域的亚单位，通常由2～3二级结构单位组成，一般为α螺旋、β折叠和环（loop）。(3)Tertiarystructure/三级结构αhelixβsheetTurn

Itisthree-dimensionalstructureofwholepolypeptideItismostlycomposedofvarioussecondarystructures.helicalribbonsflatarrowsturnsthatconnectthehelicesandsheets.(4)Quaternarystructure

Onesubunit由两个或两个以上的具有独立三级结构的亚基（subunit）按照一定排布方式，通过非共价键缔合而形成的具有一定空间结构的蛋白质分子称为四级结构。Eachsubunithasowntertiarystructure.multiplesubunits亚基（subunit）就是指参与构成蛋白质四级结构、每条具有三级结构的多肽链。●关于亚基的几点说明:亚基是肽链而肽链不一定是亚基,如胰岛素；亚基单独存在时无活性；亚基间呈特定的三维空间排布，依赖次级键维持其结构稳定；蛋白质分子中亚基可相同，也可不同，如血红蛋白。Pleasenotehere,ifaproteinhasonlyonepolypeptide

chain,thehighestlevelitcanhaveis

structure.Onlywhenaproteinhasmorethanonepolypeptide

chain,canithavequaternarystructure.tertiary肌红蛋白let’sexaminewhatphysicalinteractionshappenintheprocessofproteinfolding.1.2.2ChemicalandPhysicalBasis

forProteinFolding1)Covalentbonding2)Hydrogenbonding3)Ionicbonding4)VanderWaalsforces5)Hydrophobicinteraction1)Covalentbond/共价键SharedelectronsH－HH+1H+1+1+1CovalentbondThestrongestbondthatjoinsatoms.

A

pairofelectronsissharedbetweentwo

atoms.

Twoelectronsfromtwohydrogenatomsaresharedbetweenthesetwoatoms.

Asinglecovalentbondis

formed.Covalentbond/共价键CovalentbondThepeptidebondsarecovalentbonds.Thebonds

withinaminoacidsarealsocovalentbonds.CovalentbonddisulfidebondCovalentbondscanformbetweenaminoacidsthatarenotadjacentinpolypeptide.Eachcysteinehasasulfhydrylgroup(─SH).

Twosulfhydrylgroupscanreacttoform

a

disulfidebond(二硫键).Itholddistantparts

oftheprotein

togetherverytightly.2)Hydrogenbond/氢键HydrogenbondHydrogenbondisresultofinteractionsbetweenpartial

negative

charges

andpartialpositivecharges.Oxygenatombringspartialnegativecharge.

Hydrogenatomsbringpartial

positivecharge.Interactionbetweenoxygenandhydrogen

atoms

formsthehydrogenbond.氢键（hydrogenbond）的形成常见于连接在一电负性很强的原子上的氢原子，与另一电负性很强的原子之间，如>C=O┅┅H-N<。

氢键在稳定蛋白质的空间结构上起着重要的作用。但氢键的键能较低(~12kJ/mol)，易被破坏。Hydrogenbond/氢键Hydrogenbondαhelicesandβsheetsareformedwith

hydrogen

bonds.Hydrogenbond

canalsoformbetweenindividualamino

acidswhenpartialpositivehydrogenatoms

arenearthepartial

negativeO

atoms.Na+11Cl+173)Ionicbond/离子键Theionicbondofsodiumchloride,NaCl

Ionsareatomsthathavegainedorlostoneormore

electronsandhavefullpositiveornegative

charges.

Ionswithoppositecharges

attracteachotherand

pulledtoeachother,

formingtheionicbond.GluLysnegativecharge

positivechargeInproteins,mostionicbondsformbetweentheamino

acidswithacidicRgroupandBasicRgroup.4)VanderWaalsforces/范德华力Atom1Atom2vanderWaalsforceTheseareaswithslightly

differentcharge

concentrationscanattracttoeachother.Theattractionbetweensuch

differently

charged

areasiscalledvander

Waalsforce.Itoccurs

becausecloudofelectronsareshifted,causingatom

itself

havepositivechargeinonearea,andnegativechargeinanotherarea.Vander

Waalsforces

vs.

IonicbondAtom1Atom2VanderWaalsforcesTheatomitselfhasnegativeandpositiveparts.Theinteractionistransient.IonicbondThewholeatomiseither

negativeorpositive

parts.Theinteractionisstable.Atom1Atom2VanderWaalsforces

vs.

hydrogenbondAtom1Atom2VanderWaalsforcesTheatomitselfhasnegativeandpositivepartsTheinteractionistransientHydrogenbond.Thewholeatomiseitherpartially

negativeorpositiveTheinteractionisstableAtom1Atom2δ–δ+δ–δ+5)Hydrophobicinteraction非极性物质在含水的极性环境中存在时，会产生一种相互聚集的力，这种力称为疏水键或疏水作用力（hydrophobicinteraction）Hydrophobicinteractionisthetendencyfornon-polar

moleculesto

gathernear

eachother,awayfrom

watermolecules.?

Butwhatcausesthismovement?NonpolarmoleculesareforcedtogetherWatermoleculesgetcontactswitheachother.whennon-polarmoleculesmixwithwater,

theyforcethewater

moleculesintorelativelyorderedstructuresthatsurroundthenon-polarmolecules.However,accordingtothesecondlawof

thermodynamics,systemsalwaystendtoward

increasingdisorder.Thus,thetwokindsofmoleculestosimplyavoid

eachother.Theapparentresultisthatthenon-polarmoleculesareattractedtoeachother.

Forproteinsthatexistinwater,theeffectofthishydrophobic

interactionisthatnon-polaraminoacidsareforced

intothecenteroftheprotein,awayfromthewateryenvironmentofthecell.1.3ProteinStructureandFunction

Proteinchainscanfoldintocomplexstructures.

Proteinscan

havevery

differentstructuresandverydifferentfunctions.碳酸酐酶ThereactionallowsCO2todissolveinblood.Itcancausethisreactiontooccur10,000,000timesfasterthannormal

.activesitestructureandshapesubstratemoleculescarbonicacidActin/肌动蛋白

Filamentslineinsideofcellsandareoftenusedtoprovide

support,

Actinisasmallandsimple

protein.Itcanjoinwithotheractintoformlongandthin

actinfilament.

somewhatlikepolesholdingupatent.

Actinfilamentsinacellasseen

byflourescencemicroscopyKinesin/驱动蛋白Kinesinisusedtocarrycellularcomponents,like

organelles,

fromoneendofacelltoanother.Thecargoisboundtocentralheadregion,whilethe

twolegsattachtomicrotubules.Asaresult,

theproteinwalksalongthemicrotubule

towarditsdestination.microtubuleHeadregionThestructureofkinesinincludesapairoflegs.1.4TheDynamicsofProteinsProteinsaredynamic.Thismeansthattheirstructuresarenotfrozenasfixedshapes.Theirstructuresandevenfunctions

canchangeunderdifferentconditions.Actincarbonicanhydraselock-and-keymodelsvs.Induced-fitWhensomeenzymescatalyzereactions,theiractivesite

changesstructureinordertofitwellwithsubstrate

molecules.Thiscatalysismechanism

iscalledaninduced-fitmodel(诱导契合机理).

Insomecasescatalysisoccurswithoutachangeinstructureattheactivesite;thisiscalledalock-and-key

mechanism（锁钥机理）.Substractreactioncauseschangeofactivesitestructure.33F1F0ATPsynthetase/