生物技術及應用專業(yè)教學資源庫EnglishForBiotechnology專業(yè)英語Chapter6Proteinengineering Unit3 AminoAcidsAminoAcids

Aminoacidsarethe"buildingblocks''ofproteins.Therefore,tounderstandthepropertiesofproteins,adiscussionofthestructuresandpropertiesofaminoacidsisrequired.氨基酸是蛋白質(zhì)的“結構單元“。因此，要了解蛋白質(zhì)的性質(zhì)，就需要討論氨基酸的結構和性質(zhì)。WhatareAminoAcids？

AminoAcidsAminoacidsarechemicalcompounds,whichcontainbothbasicaminogroupsandaqueousgroups.Aminoacidsfoundinproteinshaveboththeaminoandcarboxylgroupsontheα-carbonatom.蛋白質(zhì)的包括其堿性氨基組和酸性羧基組。蛋白質(zhì)中的氨基酸在α-碳原子上同時有氨基和羧基。StructureofaminoacidsAminoAcidsAtneutralpHvaluesinaqueoussolutionsboththeaminoandthecarboxylgroupsareionized.Thecarboxylgrouplosesaprotonandobtainsanegativecharge,whiletheaminogroupgainsaprotonandhenceacquiresapositivecharge.Asaconsequence,aminoacidspossessdipolarcharacteristics.在中性pH水溶液中，氨基和羧基都呈離子狀態(tài)。羧基失去一個質(zhì)子而帶負電荷，同時氨基得到一個質(zhì)子而帶正電荷。結果氨基酸便具有偶極的特性。StructureofaminoacidsAminoAcidsAminoacidsaremoresolubleinwaterthaninlesspolarsolvents.Whenpresentincrystallineform,aminoacidsmeltordecomposeatrelativelyhightemperatures(generallyabove200).Aminoacidstheyexhibitlargedipolemomentsandlargedielectricconstantsinneuralaqueoussolutions.氨基酸易溶于水而不易溶于極性很小的溶劑。當以晶體形式存在時，氨基酸要在較高溫度（一般在200°C以上）下熔化或分解。氨基酸在中性溶液中顯示出很大的偶極矩和介電常數(shù)。PropertiesofaminoacidsAminoAcidsTheRgroupsorsidechains,whichcanaffectthechemicalpropertyofaminoacidsandproteinssignificantly,maybeclassifiedintofourgroups.氨基酸的側鏈R基團對氨基酸和蛋白質(zhì)的化學性質(zhì)產(chǎn)生重大的影響。這些側鏈可以分為四類。TheRgroupsofaminoacidsAminoAcidsAminoacidswithhydrophilicgroupscanformhydrogenbondwithwaterandaregenerallysolubleinaqueoussolutions.Thehydroxylsofserine,threonine,andtyrosine;thesulfhydrylofthinlyofcysteine,andtheamidesofasparagineandglutaminearethefunctionalmoietiespresentinRgroupsoftheclassofaminoacids.帶有極性非荷電的（親水的）R基團的氨基酸能與水形成氫鍵，通常能溶于水溶液。絲氨酸、蘇氨酸和酪氨酸的羥基，半胱氨酸的硫氫基（即硫醇）以及天冬酰胺和谷氨酰胺的酰胺基是出現(xiàn)在這類氨基酸R基團中的功能部分。Aminoacidswithpolar(hydrophilic)groupsAminoAcidsThetoilofcysteineandthehydroxyloftyrosine,areslightlyionizedatpH7andcanloseaprotonmuchmorereadilythanothersinthisclass.Theamidesofasparagineandglutaminearereadilyhydrolyzedbyacidorbasetoasparticandglutamicacids,respectively.其中半胱氨酸的硫氫基和酪氨酸的羥基在pH7時能輕度離子化，因而比此類中其它氨基酸更容易失去質(zhì)子。天冬酰胺和谷氨酰胺的酰胺基容易被酸和堿水解，分別形成天冬氨酸和谷氨酸。Aminoacidswithpolar(hydrophilic)groupsAminoAcidsAminoacidswithnonpolar(hydrophobic)groupsarelesssolubleinaqueoussolventsthanaminoacidswithpolarunchargedgroups.Fiveaminoacidswithhydrocarbonsidechainsdecreaseinpolarityasthelengthofthesidechainisincreased.在水溶液中，帶有非極性（疏水的）R基團的氨基酸溶解性比帶極性非荷電的氨基酸要小得多。帶有烴側鏈的五種氨基酸，其側鏈長度增加而在水溶液中的溶解性降低。Aminoacidswithnonpolar(hydrophobic)groupsAminoAcidsTheaminoacidswithpositivelycharged(basic)groupsatpH6-7arelysine;argininehasapositivelychargedguanidinegroup.AtpH7.010%oftheimidazolegroupsofhistidinemoleculesareprotonated,butmorethan50%carrypositivechangesatpH6.0.pH6~7時帶正電荷（堿性的）R基團的氨基酸有賴氨酸、精氨酸和組氨酸。賴氨酸帶正電的原因主要在于氨基，而精氨酸則具有帶正電荷的胍基。pH7時組氨酸分子中的咪唑基有10%質(zhì)子化，但在pH6時則有50%以上帶正電荷。Aminoacidswithpositivelycharged(basic)groupsAminoAcidsThedicarboxylicaminoacids,asparticandglutamic,possessnetnegativechargesattheneutralpHrange.Animportantartificialmeal-flavoringfood