南京农业大学生物化学课件29.ppt

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1、第12章氨基酸代谢第一节ThenitrogencycleNitrogenexistspredominantlyinanoxidizedstateintheenvironment,occurringprincipallyasN2intheatmosphereorasnitrateion(NO3-)inthesoilsandoceans.Itsacquisitionbybiologicalsystemsisaccompaniedbyitsreductiontoammoniumion(NH4+)andtheincorporation

2、ofNH4+intoorganiclinkageasaminogroup.ThereductionofNO3-toNH4+occursingreenplants,variousfungi,andcertainbacteriainatwo-stepmetabolicpathwayknownasnitrateassimilation.第一节ThenitrogencycleTheformationofNH4+fromN2gasistermednitrogenfixation.N2fixationisanexclusivelyprok

3、aryoticprocess.Noanimalsarecapableofeithernitrogenfixationornitrateassimilation.Animalsreleaseexcessnitrogeninareducedform,eitherasNH4+orasorganicnitrogenouscompoundssuchasurea.ThereleaseofNoccursbothduringlifeandasaconsequenceofmicrobialdecompositionfollowingdeath.

4、第一节ThenitrogencycleVariousbacteriareturnthereducedformsofnitrogenbacktotheenvironmentbyoxidizingthem.TheoxidationofNH4+toNO3-isperformedbynitrifyingbacteria.NitratenitrogenalsoreturnstotheatmosphereasN2asresultofthemetabolicactivityofdenitrifyingbacteria.第一节Thenitro

5、gencycleDietaryproteinsaredigestedintoaminoacidsinthegastrointestinal(胃肠)tractviatheactionofpepsin,trypsin,chymotrypsin,carboxypeptidasesandaminopeptidases.SourcesofaminoacidsforanimalsProteins(butnotpepsin)unfoldedAbsorbedastri-&dipeptides,andaminoacidsDegradation&

6、absorptionofdietaryproteinsPepsin:thefirstenzymediscovered(18thcentury).proteasesEssentialaminoacidsAminoacidscannotbestoredinanimals:excessbeingcompletelyoxidizedtoreleaseenergyorconvertedtostorablefuels(fattyacidsorcarbohydrates).Overallfateofexcessaminoacids第二节Am

7、inoaciddegradation1.氧化脱氨基氨基酸在酶的作用下脱去氨基生成相应酮酸的过程，叫氧化脱氨基作用。一.氨的去路Glu+NAD(P)+H2Oa-KG+NH4++NADH(P)+H+2.脱氢酶作用-GDH一.氨的去路3.转氨基作用一.氨的去路转氨基作用是α-氨基酸和α-酮酸之间氨基的转移作用。一种α-氨基酸的α-氨基借助转氨酶（transaminase）的催化作用转移到α-酮酸的羰基上，结果生成新的酮酸，而原来的α-酮酸则形成相应的氨基酸。3.转氨基作用谷丙转氨酶催化的转氨基作用机理一.氨的去路4.联合脱氨作用(

8、转氨酶-谷氨酸脱氢酶)谷氨酸α-酮戊二酸丙氨酸丙酮酸转氨酶谷氨酸脱氢酶NAD(P)+H+NAD(P)+联合脱氨基作用PLP一.氨的去路在氨基酸脱羧酶催化下进行脱羧作用，生成一个伯胺类化合物和CO2，其反应可以用下式表示二.脱羧基作用PLPactsasatemporarycar