SM_c06Enzymes

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1、c06Enzymes.qxd12/6/124:19PMPageS-63chapterEnzymes61.KeepingtheSweetTasteofCornThesweettasteoffreshlypickedcorn(maize)isduetothehighlevelofsugarinthekernels.Store-boughtcorn(severaldaysafterpicking)isnotassweet,becauseabout50%ofthefreesugarisconvertedtostarchwithinonedayofpic

2、king.Topreservethesweetnessoffreshcorn,thehuskedearscanbeimmersedinboilingwaterforafewminutes(“blanched”)thencooledincoldwater.Cornprocessedinthiswayandstoredinafreezermaintainsitssweetness.Whatisthebiochemicalbasisforthisprocedure?AnswerAfteranearofcornhasbeenremovedfromthe

3、plant,theenzyme-catalyzedconver-sionofsugartostarchcontinues.Inactivationoftheseenzymesslowsdowntheconversiontoanimperceptiblerate.Oneofthesimplesttechniquesforinactivatingenzymesisheatdenatura-tion.Freezingthecornlowersanyremainingenzymeactivitytoaninsignificantlevel.2.Intr

4、acellularConcentrationofEnzymesToapproximatetheactualconcentrationofenzymesinabacterialcell,assumethatthecellcontainsequalconcentrationsof1,000differentenzymesinsolutioninthecytosolandthateachproteinhasamolecularweightof100,000.Assumealsothatthebacterialcellisacylinder(diame

5、ter1.0mm,height2.0mm),thatthecytosol(specificgravity1.20)is20%solu-bleproteinbyweight,andthatthesolubleproteinconsistsentirelyofenzymes.Calculatetheaveragemolarconcentrationofeachenzymeinthishypotheticalcell.AnswerTherearethreedifferentwaystoapproachthisproblem.(i)Theconcent

6、rationoftotalproteininthecytosolis(1.2g/mL)(0.20)530.2410mol/mL2.410M100,000g/molThus,for1enzymein1,000,theenzymeconcentrationis32.410M62.410M1000molesofeachenzymeincell(ii)Theaveragemolarconcentrationvolumeofcellinliters223Volumeofbacterialcytosolprh(3

7、.14)(0.50mm)(2.0mm)1.6mm123121.610cm1.610mL151.610LAmount(inmoles)ofeachenzymeincellis331233(0.20)(1.2g/cm)(1.6mm)(10cm/mm)213.810mol(100,000g/mol)(1000)213.810molAveragemolarconcentration151.610L662.410mol/L2.410MS-63c06Enzymes.qxd12/6/124:1

8、9PMPageS-64S-64Chapter6Enzymes2(iii)Volumeofbacterialcytosolprh2312(3.14