2023学年完整公开课版AminoAcidsReadingMaterial

生物技术及应用专业教学资源库EnglishForBiotechnology专业英语Chapter6Proteinengineering Unit3 AminoAcidsAminoAcids

Aminoacidsarethe"buildingblocks''ofproteins.Therefore,tounderstandthepropertiesofproteins,adiscussionofthestructuresandpropertiesofaminoacidsisrequired.氨基酸是蛋白质的“结构单元“。因此，要了解蛋白质的性质，就需要讨论氨基酸的结构和性质。WhatareAminoAcids？

AminoAcidsAminoacidsarechemicalcompounds,whichcontainbothbasicaminogroupsandaqueousgroups.Aminoacidsfoundinproteinshaveboththeaminoandcarboxylgroupsontheα-carbonatom.蛋白质的包括其碱性氨基组和酸性羧基组。蛋白质中的氨基酸在α-碳原子上同时有氨基和羧基。StructureofaminoacidsAminoAcidsAtneutralpHvaluesinaqueoussolutionsboththeaminoandthecarboxylgroupsareionized.Thecarboxylgrouplosesaprotonandobtainsanegativecharge,whiletheaminogroupgainsaprotonandhenceacquiresapositivecharge.Asaconsequence,aminoacidspossessdipolarcharacteristics.在中性pH水溶液中，氨基和羧基都呈离子状态。羧基失去一个质子而带负电荷，同时氨基得到一个质子而带正电荷。结果氨基酸便具有偶极的特性。StructureofaminoacidsAminoAcidsAminoacidsaremoresolubleinwaterthaninlesspolarsolvents.Whenpresentincrystallineform,aminoacidsmeltordecomposeatrelativelyhightemperatures(generallyabove200).Aminoacidstheyexhibitlargedipolemomentsandlargedielectricconstantsinneuralaqueoussolutions.氨基酸易溶于水而不易溶于极性很小的溶剂。当以晶体形式存在时，氨基酸要在较高温度（一般在200°C以上）下熔化或分解。氨基酸在中性溶液中显示出很大的偶极矩和介电常数。PropertiesofaminoacidsAminoAcidsTheRgroupsorsidechains,whichcanaffectthechemicalpropertyofaminoacidsandproteinssignificantly,maybeclassifiedintofourgroups.氨基酸的侧链R基团对氨基酸和蛋白质的化学性质产生重大的影响。这些侧链可以分为四类。TheRgroupsofaminoacidsAminoAcidsAminoacidswithhydrophilicgroupscanformhydrogenbondwithwaterandaregenerallysolubleinaqueoussolutions.Thehydroxylsofserine,threonine,andtyrosine;thesulfhydrylofthinlyofcysteine,andtheamidesofasparagineandglutaminearethefunctionalmoietiespresentinRgroupsoftheclassofaminoacids.带有极性非荷电的（亲水的）R基团的氨基酸能与水形成氢键，通常能溶于水溶液。丝氨酸、苏氨酸和酪氨酸的羟基，半胱氨酸的硫氢基（即硫醇）以及天冬酰胺和谷氨酰胺的酰胺基是出现在这类氨基酸R基团中的功能部分。Aminoacidswithpolar(hydrophilic)groupsAminoAcidsThetoilofcysteineandthehydroxyloftyrosine,areslightlyionizedatpH7andcanloseaprotonmuchmorereadilythanothersinthisclass.Theamidesofasparagineandglutaminearereadilyhydrolyzedbyacidorbasetoasparticandglutamicacids,respectively.其中半胱氨酸的硫氢基和酪氨酸的羟基在pH7时能轻度离子化，因而比此类中其它氨基酸更容易失去质子。天冬酰胺和谷氨酰胺的酰胺基容易被酸和碱水解，分别形成天冬氨酸和谷氨酸。Aminoacidswithpolar(hydrophilic)groupsAminoAcidsAminoacidswithnonpolar(hydrophobic)groupsarelesssolubleinaqueoussolventsthanaminoacidswithpolarunchargedgroups.Fiveaminoacidswithhydrocarbonsidechainsdecreaseinpolarityasthelengthofthesidechainisincreased.在水溶液中，带有非极性（疏水的）R基团的氨基酸溶解性比带极性非荷电的氨基酸要小得多。带有烃侧链的五种氨基酸，其侧链长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时带正电荷（碱性的）R基团的氨基酸有赖氨酸、精氨酸和组氨酸。赖氨酸带正电的原因主要在于氨基，而精氨酸则具有带正电荷的胍基。pH7时组氨酸分子中的咪唑基有10%质子化，但在pH6时则有50%以上带正电荷。Aminoacidswithpositivelycharged(basic)groupsAminoAcidsThedicarboxylicaminoacids,asparticandglutamic,possessnetnegativechargesattheneutralpHrange.Animportantartificialmeal-flavoringfood