Protein metabolism

Protein Metabolism

1. Protein Metabolism
Dr.AzadAlamSiddiqui
Assistant Professor
Department of B.Voc(MMDT)
Km. Mayawati Govt.GirlsPG College
Badalpur, G.B. Nagar (U.P.)

2. Protein Metabolism
•Proteinmetabolismdenotesthevariousbiochemicalprocessesresponsibleforthe
synthesisofproteinsandaminoacids,andthebreakdownofproteins(andother
largemolecules)bycatabolism.
•Catabolismofaminoacidsprovidescarbonskeletonforgluconeogenesis,
ketogenesis,asalsoforenergyyieldingpathways.
•Muchofthebodyismadeofprotein,andtheseproteinstakeonamyriadofforms:
1.Catalyst
2.Transport
3.Mechanicalsupport
4.Protection
5.Regulationandcommunication
6.Bloodbuffer

3. Protein Metabolism
•Proteinmetabolismiseffectivelythemetabolismofaminoacids.
•Proteinsarealsosourcesofenergybutsupplyonlyasmallfraction ̴15%.
•Proteinundergoturnoverconstantly.
•Importantroleofdietaryproteinistosupplyessentialaminoacids.The
dietaryproteinsaredenaturedoncookingandthereforemoreeasilydigested.
•ProteinDigestion:
a.Digestioninmouth
b.Digestioninstomach
c.Digestioninintestine
•Endproductofproteindigestionisaminoacid,dipeptideandtripeptides.

4. DIGESTION OF PROTEINS
•Proteolyticenzymesaresecretedasinactivezymogenswhichareconvertedtotheiractiveforminthe
intestinallumen.Theproteolyticenzymesinclude:
➢Endopeptidases:Theyactonpeptidebondsinsidetheproteinmolecule,sothattheprotein
becomessuccessivelysmallerandsmallerunits.ThisgroupincludesPepsin,Trypsin,
ChymotrypsinandElastase.
➢Exopeptidases:Whichactonlyonthepeptidebondlocatedattheendsofthepolypeptidechain.
Thisgroupincludes:
a)Carboxypeptidase,whichactsonlyonthepeptidebondatthecarboxyterminalendofthe
chain.
b)Aminopeptidase,whichactsonlyonthepeptidebondattheaminoterminalendofthechain.
•IntracellularProteinDegradation:
1.Cathepsins 2.Ubiquitin 3.Proteasomes
•Theproteinsondegradation(proteolysis)releasesindividualaminoacids.Eachoneofthe20
naturallyoccurringaminoacidsundergoesitsownmetabolismandperformsspecificfunctions.

5. Absorption of free amino acids
Amino acids,on
absorptionfromintestine
arecarriedtoLiver
throughportalblood.
Theyaretakenupby
Livercellstosomeextent
andremainderentersthe
systemiccirculationand
diffusethroughoutthe
bodyfluidsandtakenup
bytissuecells.

6. AMINO ACID POOL
1.Proteinsarethemostabundantorganiccompounds,andconstitutea
majorpartofthebodydryweight(10-12kginadults).
2.Despitethegreatcomplexityofproteinsynthesis,proteinsaremade
atexceedinglyhighrates.Apolypeptideof100residuesis
synthesizedinanEscherichiacolicell(at37
0
C)inabout5seconds.
3.Themainroleofaminoacidsisinthesynthesisofstructuraland
functionalproteins.

7. AMINO ACID POOL
1.Thereisalsoacontinuoussynthesisofaminoacids(exceptthe
“essential”aminoacids).Aminoacidsfromallthesourcesgetmixedup
toconstitutewhatisknownas“generalaminoacidpool”ofthebody.“
2.Anadulthasabout100goffreeaminoacidswhichrepresenttheamino
acidpoolofthebody.Glutamateandglutaminetogetherconstitute
about50%,andessentialaminoacidsabout10%ofthebodypool(100
g).
3.Ifacelltakesupasmuchaminoacidsasitloses,itisinastateof
“dynamicequilibrium”,ifthelossisgreater,thecellwastes,andifthe
gainisgreaterthecellgrows.

8. Protein turnover
•300-400gofproteinperdayisconstantlydegradedandsynthesizedwhichrepresentsbody
proteinTurnover.
•Thereisawidevariationintheturnoverofindividualproteins.Forinstance,theplasma
proteinsanddigestiveenzymesarerapidlydegraded,theirhalf-livesbeinginhoursor
days.Thestructuralproteins(e.g.collagen)havelonghalf-lives,ofteninmonthsand
years.(Certainproteinswithaminoacidsequenceproline,glutamine,serineandthreonine
rapidlydegraded).

9. Tissue Amino acids
➢The amino acids are transported into tissues actively. Pyridoxal-P (B6-P) is one of the requirement for
this active transport. Tissue uptake is also favoured by hormones:
➢Insulin, growth hormone and testosterone favour the uptake of amino acids by tissues (anabolic
hormones).
➢Oestradiol stimulates selectively their uptake by uterus.
➢Epinephrine and glucocorticoids: Stimulate the uptake of amino acids by the Liver.

10. NITROGEN BALANCE
•Inanadulttomaintainconstantweight,theamountofintakeofNinfoodwill
bebalancedbyanexcretionofanequalamountofNinurineandinfaeces.The
individualisthensaidtobeinnitrogenbalanceornitrogenousequilibrium.
•AsubjectwhoseintakeofNisgreaterthantheoutput,e.g.ingrowth,issaidto
havea+venitrogenbalance.Inthegrowingperiodandalsoduring
convalescencefromillnessorwhenanabolichormonesaregiven,thebodyputs
onweight.
•AsubjectwhoseintakeofNislessthantheoutputofN,(e.g.inlosingweight),
issaidtohavea–venitrogenbalance.Inoldageandduringillnessand
starvationweightislostandresultsin–venitrogenbalance.

11. DISSIMILATION OF AMINO ACIDS
•α-NH
2groupofaminoacids,derivedeitherfromthedietorbreakdownoftissueproteins,
ultimatelyisconvertedfirsttoNH
3andthentoureaandisexcretedintheurine.
•FormationofNH
3andureacanbediscussedunderthefollowingheads:
1.Transamination
2.Deamination Oxidativedeamination
Non-oxidativedeamination
3.Transdeamination
4.NH
3transport,and
5.Formationofurea
•Ureaisthecharacteristicend-productofaminoacidcatabolisminhumanbeingsand
ureotelicorganisms.

12. TRANSAMINATION
•Transaminationisareversiblereactioninwhichα-NH
2groupofoneaminoacidis
transferredtoaα-ketoacidresultinginformationofanewaminoacidandanewketoacid.
•Importantfortheproductionofnonessentialaminoacids.
•TheprocessrepresentsonlyanintermoleculartransferofNH
2groupwithoutthesplitting
outofNH
3.Ammoniaformationdoesnottakeplacebytransaminationreaction.

14. •Therearetwotransaminasesofclinicalimportanceinthebodyinthattheyusespecific
aminoacidandspecificketoacid.Thesetwospecifictransaminasesare:
•Aspartatetransaminase(S-GOT):Asparticacidisthedonoraminoacidandα-
oxoglutarateistherecipientketoacid.Newaminoacidformedisalwaysglutamicacid.
✓Concentrationoftheenzymeishighinmyocardiumandalsoinlivercells.Helpfulin
acutemyocardialinfarction.
✓IncreasesinLiverdiseases,butitislessthanAlaninetransaminase(S-GPT).
✓Increaseinmusculardystrophies—myositis.
✓Increasedactivityseeninacutepancreatitis,leukaemia's,inacutehaemolyticanaemia.
✓Innormalpersons,afterprolongedsevereexercise.
✓Arisehasbeenseenintherapywithcertainantibiotics.

15. •Alaninetransaminase(S-GPT):Alanineisthedonoraminoacidandα-
oxoglutarateistherecipientketoacid.Newaminoacidformedisagainalways
glutamicacid.
✓TheenzymeisfoundmainlyinLiver.
✓Increasesinbothtransaminasesarecommonfindinginhepaticdiseasesbutalways
S-GPT>thanS-GOT,thoughinnormalhealthypersons,S-GOTisslightlymore
thanS-GPT.
✓Itismostusefulinassessingseverityandprogressofthediseaseinacuteviral
hepatitis.
✓Highestvaluesofenzymeactivityseeninacuteviralhepatitis,peakvalues250to
1500IU/Lormoreseenatthetimeofmaximumillness.

16. DEAMINATION
•Deamination is the process by which N–of amino acid is removed as NH
3.
A. Oxidative deamination
B. Non-oxidative deamination.
(A)
•L-aminoacidoxidase,aflavoproteinisrestrictedtoliverandkidneyonlyandthusdoesnotfulfil
amajorroleinmammalianaminoacidcatabolismandformationofNH
3.

17. •D-aminoacidsarefoundinplantandmicro-organism,andnotpresentin
mammaliantissue.
•Theyareregularlytakenindiet.
•D-aminoacidoxidaseconvertD-aminoacidsintorespectiveα-ketoacids
byoxidativedeamination.
•α-ketoacidsundergotransaminationtobeconvertedtoL-aminoacids
whichparticipateinvariousmetabolism.
•D-aminoacidoxidaseisimportantasitinitiatesthestepfortheconversion
ofunnaturalD-aminoacidstoL-aminoacidsinthebody.

18. •PurposeofDeaminationistoprovideNH
3forureasynthesisandα-ketoacidsfor
varietyofreactionsincludingenergygeneration.
•L-glutamicacidisnotdeaminatedbyL-aminoacidoxidasebutbyaspecific
enzymecalledL-glutamatedehydrogenase(Zn
++
-containingmetalloenzyme).
•Itiswidelydistributedintissuesinhumansandhashighactivity,andisspecificfor
L-Glutamate.
•Glutamateservesascollectioncentreforaminogroups.
•GDHinvolvedinbothcatabolicandanabolicreactions.

19. •Reactionisreversible,andthe
equilibriumconstantfavours
glutamateformation,butthe
quickremovalofNH
3toform
ureainureacycleandα-
KetoglutaratetoTCAcycle
favoursonwardreaction,i.e.NH
3
formation

20. (B)Therearecertainaminoacids,whichcanbenonoxidativelydeaminatedby
specificenzymes,andcanformNH
3.ThesereactionsdocontributetoNH
3
formation,butagaintheydonotfulfil,amajorroleinNH
3formation.
amino acid dehydrases

21. •Transaminationtakesplaceinthecytoplasm
ofallthecellsofthebody;theaminogroup
istransportedtoliverasglutamicacid,
whichisfinallyoxidativelydeaminatedin
themitochondriaofhepatocytes.
•Thus,thetwocomponentsofthereaction
arephysicalfaraway,butphysiologically
theyarecoupled.Hencetheterm
transdeamination.
•Thismechanismseemstobethemajor
pathwayforremovalofNH
2groupfroman
L-aminoacidandformationofNH
3.
TRANSDEAMINATION

22. NH
3 TRANSPORT
•In addition to NH
3formed in the tissues, a considerable quantity of NH
3is produced in
the gut by intestinal bacterial flora, both
• From dietary proteins, and
• From urea present in fluids secreted into the GI tract.

23. •NH
3isabsorbedfromtheIntestineintoportalvenousbloodandthushasmoreNH
3
thansystemicblood.
•LiverpromptlyremovestheNH
3fromtheportalblood,sothatbloodleavingthe
liverisvirtuallyNH
3-free.ThisisessentialsinceevensmallquantitiesofNH
3are
toxictoCNS.
•NH
3isnotjustawasteproductofnitrogenmetabolism.ltisinvolved(directlyorvia
glutamine)forthesynthesisofmanycompoundsinthebody.Theseinclude
nonessentialaminoacids,purines,pyrimidines,aminosugars,asparagineetc.
Ammoniumions(NH
4
+
)areveryimportanttomaintainacid-basebalanceofthe
body.

24. •ThetransportofNH
3betweenvarioustissuesandthelivermostlyoccursinthe
formofglutamineoralanineandnotasfreeNH
3.
•NH
3isalwaysproducedbyalmostallcells,includingneurons.The
intracellularNH
3isimmediatelytrappedbyglutamicacidtoformglutamine,
especiallyinbraincells.Theglutamineisthentransportedtoliver,wherethe
reactionisreversedbytheenzymeglutaminase.TheNH
3thusgeneratedis
immediatelydetoxifiedintourea.

25. Why NH
3is Toxic?
•NormalrangeofNH
3inbloodis10-80µg/dl.
•IncreasedNH
3concentrationdepressingtheTCAcycle,affectingthecellular
respiration.
•IncreasedNH
3concentrationenhancesglutamineformationfromGlutamateand
thusreduces‘braincell’poolofglutamicacid.Hencethereisdecreasedformation
ofinhibitoryneurotransmitterGABA.
•Riseinbrainglutaminelevelenhancestheoutflowofglutaminefrombraincells.
Glutamineiscarried‘out’bythesame“transporter”whichallowstheentryof
‘tryptophan’intobraincells.Hence‘tryptophan’concentrationinbraincells
increaseswhichleadstoabnormalincreasesinsynthesisof“serotonin”,a
neurotransmitter.

26. UREA FORMATION (KREBS-HENSELEIT CYCLE)
•StepsofureasynthesishavebeenelucidatedbyKrebsandHenseleit(1932).
•Itisacyclicprocess,fivereactionswhichinvolvesornithine,citrulline,arginineand
asparticacid.
•Kidneys:Ureacycleoperatesinalimitedextent.Kidneycanformuptoarginine
butcannotformurea,asenzymearginaseisabsentinkidneytissues.
•Brain:Braincansynthesiseureafromcitrulline,butlackstheenzymeforforming
citrullinefromornithine.Thus,neitherthekidneysnorthebraincanformureain
significantamounts.
•EnzymeforUreacycleispartlymitochondrialandpartlycytosolic.
•1mol.ofNH
3andonemol.ofCO
2areconvertedto1mol.ofureaforeachturnof
thecycleandornithineisregeneratedattheend,whichactsasacatalyticagent.
•Theoverallprocessineachturnofcyclerequires3molesofATP.

27. Biosynthesis of urea or ornithine—urea cycle

28. Energetics of Urea Cycle
•Theoverallreactionmaybesummarizedas:
NH
3+CO
2+Aspartate→Urea+Fumarate
•2ATPsareusedinthefirstreaction,anotherATPisconvertedtoAMPandPPi.
Sotheureacycleconsumes4highenergyphosphatebonds.
•Fumarateformedinthe4thstepmaybeconvertedtomalate.Malatewhen
oxidizedtooxaloacetateproduces1NADHequivalentto2.5ATP.
•So,netenergyexpenditureisonly1.5highenergyphosphates.Theureacycle
andTCAcycleareinterlinked,andso,itiscalledas"ureabicycle".

29. Significance of Urea Cycle
1.DetoxificationofNH
3:Majorbiologicalroleofthispathwayisthe
detoxificationofNH
3.Toxicammoniaisconvertedintoanontoxicsubstance
ureaandexcretedinurine.
2.Biosynthesisofarginine:Theureacyclealsoservesforthebiosynthesisof
argininefromornithineinliver,kidneyandintestinalmucosa.Kidneyand
intestinalmucosaprobablycontributemostofthebodyargininebecausethey
possessalltheureacycleenzymesexceptarginase.Hencetheycanformupto
arginineandcannotformurea.Thearginineisusedforproteinsynthesis.
3.TheureacycleislinkedtotheTCAcyclethroughtheproductionoffumarate.
Aminoacidcatabolismisthereforedirectlycoupledtoenergyproduction.

30. DECARBOXYLATION REACTION AND BIOGENIC AMINES
•DecarboxylationisthereactionbywhichCO
2isremovedfromtheCOOHgroupofan
aminoacidasaresultanamineisformed.
•Thereactioniscatalysedbytheenzymedecarboxylase,whichrequirespyridoxal-P(B6-
PO
4)ascoenzyme.
•Tissueslikeliver,kidney,brainpossesstheenzymedecarboxylaseandalsoby
microorganismsofintestinaltract.

31. •Histamine:
•Actsasneurotransmitterinhypothalamusandisamediatorofanaphylacticshockand
inflammation.
•ActionsaremediatedbybothhistamineH1andH2typeofreceptors.
•Drugsthatblocksbothreceptorsareknownasantihistamines.(Promethazineand
mepyramineareH1blockerandcimetidineisH2blocker.)
•GABA:
•Decarboxylationofglutamicacidbyglutamate-a-decarboxylaseproducesGABA.
(coenzymeB
6-PinpresenceofMg++)
•ItisinhibitoryneurotransmitterofCNSandretina.
•GABAbindsto2distinctreceptorsGABA-AandGABA-B.

32. METABOLISM OF INDIVIDUAL AMINO ACIDS
•GLYCINE:
•Simplestaminoacid.non-essential.Thoughitisnon-essentialbutitisan
importantaminoacidasitformsmanybiologicallyimportantcompoundsinthe
body.
•Neutral,aliphatic,Opticallyinactiveandglucogenic.
•Mostabundantaminoacidnormallyexcretedintourine.

33. Glycine is formed from:
1.Serine:
2.Threonine:

34. Aromatic Amino Acids
➢PHENYLALANINE AND TYROSINE:
•Phenylalanineandtyrosinearestructurallyrelatedaromaticaminoacids.
•Phenylalanineisanessentialaminoacidwhiletyrosineisnon-essentialamino
acid.
•Onlyfunctionofphenylalanineisitsconversiontotyrosine.
•Phenylalaninewillberequiredinminimal,ifadequatetyrosineissuppliedinthe
food.Thisiscalledthesparingactionoftyrosineonphenylalanine.
•Tyrosineisincorporatedintoproteinsandisinvolvedinthesynthesisofavariety
ofbiologicallyimportantcompounds-epinephrine,norepinephrine,dopamine,
thyroidhormonesandthepigmentmelanin.
•Duringthecourseofdegradation,phenylalanineandtyrosineareconvertedto
metaboliteswhichcanserveasprecursorsforthesynthesisofglucoseandfat.
Henceitispartlyglucogenicandpartlyketogenic.

35. Phenylalanine to Tyrosine
•Thereactioninvolvesadditionofa
hydroxylgrouptothearomaticring,by
phenylalaninehydroxylase.
•NeedsNADPH, NADH and
tetrahydrobiopterinasco-enzymes.
•Irreversiblereaction,tyrosinecannot
replenishphenylalanine.Hence,
phenylalanineisessentialinfood.
•OnemoleculeofO
2isneededinthis
reaction;outofwhichoneatomis
incorporatedintheOHgroupandthe
otherisreducedtowater.

36. •Duetoadefectinphenylalaninehydroxylase,theconversionof
phenylalaninetotyrosineisblockedresultinginthedisorder
phenylketonuria(PKU)
•Accumulatedphenylalaninemakesphenylpyruvatebytransaminationand
phenyllactatebyreductionwhichareexcretedinurine.
•Phenylpyruvateisaphenylketone,hencethenamephenylketonuria.
•Phenylpyruvateandphenyllactateareexcretedintracesinnormalperson.
•Tyrosineisnotformedandsoitbecomesanessentialaminoacid.
•Iftyrosineintakeisnotadequate,thereisdecreaseformationof
catecholamines,melaninetc.
PHENYLKETONURIA (PKU)

37. •Duetogeneticmutationeithertheenzymeisnotsynthesized,oranon-functional
enzymeissynthesized.
•FrequencyofPKUwasconsideredtobe1in10,000births;butrecentintroduction
ofbetterdiagnosticfacilitiesshowedthattheincidenceisashighas1in1,500
births.
•Thereare5typesofPKUdescribed.TypeIistheclassicalone.Itisdueto
phenylalaninehydroxylasedeficiency.TypesIIandIIIareduetodeficiencyof
dihydrobiopterinreductase.TypeIVandVareduetothedeficiencyoftheenzyme
synthesizingbiopterin.
•TheclassicalPKUchildismentallyretardedwithanIQof50.
•Agitation,hyperactivity,tremorsandconvulsionsareoftenmanifested.
•Thechildoftenhashypopigmentation.

38. Dopamine and Parkinson's disease
•Parkinson'sdiseaseisacommondisorderinmanyelderlypeople,withabout1%of
thepopulationabove60yearsbeingaffected.ltischaracterizedbymuscular
rigidity,tremors,expressionlessface,lethargy,involuntarymovementsetc.
•Theexactbiochemicalcausehasnotbeenidentified.however,Iinkedwitha
decreasedproductionofdopamine.Thediseaseisduetodegenerationofcertain
partsofthebrain(substantianigraandlocuscoeruleus),leadingtotheimpairment
inthesynthesisofdopamine.
•Dopaminecannotenterthebrain,henceitsadministrationisofnouse.DOPA
(levodopaorL-dopa)isusedinthetreatmentofParkinson'sdisease.Inthebrain,
DOPAisdecarboxylatedtodopaminewhichalleviatesthesymptomsofthis
disorder.
•Carbidopaandγ-methyl-dopa(dopaanalogs)areadministeredalongwithdopafor
thetreatmentofParkinson'sdisease.

39. ALKAPTONURIA
•Thisisbasedontheobservationthattheurinebecomesblackonstanding.
Homogentisateinurineisoxidizedandpolymerizedtobrownishblackcolor.
•Thediseaseisinheritedanditisduetothedeficiencyoftheenzymerequiredfor
furthermetabolismofhomogentisicacid.
•Alkaptonuriaisanautosomalrecessiveconditionwithanincidenceof1in250,000
births.
•Themetabolicdefectisthedeficiencyofhomogentisicacidoxidase.Thisresults
inexcretionofhomogentisicacidinurine.
•Thehomogentisicacidisoxidizedbypolyphenoloxidasetobenzoquinoneacetate.
Itisthenpolymerizedtoblackcolouredalkaptonebodies.
•Nospecifictreatmentisrequired.Butminimalproteinintakewithphenylalanine
lessthan500mg/dayisrecommended.
•Ferricchloridetestwillbepositiveforurine.
•Benedict’stestisstronglypositive.

40. One Carbon Metabolism
•Thereisagroupofbiochemicalreactionsthathaveaspecialsetofenzymesand
coenzymes.Theyareinvolvedinaminoacidmetabolism.Thisgroupofreactionsis
referredtoasone-carbonmetabolism.
•Aminoacidmetabolismisparticularlyimportantforthetransferorexchangeofone
carbonunits.
•Methyl (-CH
3)
•Hydroxymethyl (-CH
2OH)
•Methylene (=CH
2)
•Methenyl (-CH=)
•Formyl (-CH=O)
•Formimino (-CH=NH)
•Tetrahydrofolicacid(THF)isaversatilecoenzymethatactivelyparticipatesinone
carbonmetabolism.VitaminB
12isalsoinvolvedbesidesTHF.

41. Generation of one carbon units
•TheformylreleasedfromglycineandtryptophanmetabolismcombineswithTHF
toformN
10
-formylTHF.
•HistidinecontributesformiminofragmenttoproduceN
5
-formiminoTHF.
•Whenserineisconvertedtoglycine,N
5
,N
10
-methyleneTHFisformed.
•ThedifferentderivativesofTHFcarryingonecarbonunitsareinterconvertible,
andthisismetabolicallysignificantforthecontinuityofone-carbonpool.
•One-carbonfragmentsfromTHFareusedforthesynthesisofawidevarietyof
compounds.Theseincludepurines,formylmethioninetRNA(requiredfor
initiationofproteinsynthesis),glycine,pyrimidinenucleotideetc.