TheStructureandFunctionofProteinProteinsarethemostabundantmacromoleculesinlivingcellsandconstitute50percentormoreoftheirdryweight.Theyarefoundinallcellsandallpartsofcells.Proteinsalsooccuringreatvariety;hundredsofdifferentkindsmaybefoundinasinglecell.Moreover,proteinshavemanydifferentbiologicalrolessincetheyarethemolecularinstrumentsthroughwithgeneticinformationisexpressed.Itisthereforeappropriatetobeginthestudyofbiologicalmacromoleculeswiththeproteins,whosenamemeans“first”or“foremost.”Thekeytothestructureofthethousandsofdifferentproteinsisthegroupofrelativelysimplebuilding-blockmoleculesfromwhichproteinsarebuilt.Allproteinswhetherfromthemostancientlinesofbacteriaorfromthehighestformsoflife,areconstructedfromthesamebasicsetof20aminoacids,covalentlylinkedincharacteristicsequences.Becauseeachoftheseaminoacidshasadistinctivesidechainwhichlendsitchemicalindividuality,thisgroupof20building-blockmoleculesmayberegardedasthealphabetofproteinstructure.Inthispaperweshallalsoexaminepeptides,shortchainsoftwoormoreaminoacidsjoinedbycovalentbonds.Whatismostremarkableisthatcellscanjointhe20aminoacidsinmanydifferentcombinationsandsequences,yieldingpeptidesandproteinshavingstrikinglydifferentpropertiesandactivities.Fromthesebuildingblocksdifferentorganismscanmakesuchwidelydiverseproductsasenzymes,hormones,thelensproteinoftheeye,feathers,spiderwebs,tortoiseshell,nutritivemilkproteins,enkephalins(thebody’sownopiates),antibiotics,mushroompoisons,andmanyothersubstanceshavingspecificbiologicalactivity.AminoAcidsHaveCommonStructuralFeaturesWhenproteinsareboiledwithstrongacidorbase,theiraminoacidbuildingblocksarereleasedfromthecovalentlinkagesthatjointhemintochains.Thefreeaminoacidssoformedarerelativelysmallmolecules,andtheirstructuresareallknown.Thefirstaminoacidtobediscoveredwasasparagines,in1806.Thelastofthe20tobefound,threonine,wasnotidentifieduntil1938.Alltheaminoacidshavetrivialorcommonnames,sometimesderivedfromthesourcefromwhichtheywerefirstisolated.Asparagineswasfirstfoundi...
