Biochemistry of neurotransmitters · neurotransmitters? Neurohormones:when released by neurons into...
Transcript of Biochemistry of neurotransmitters · neurotransmitters? Neurohormones:when released by neurons into...
Biochemistryofneurotransmitters
Prof.MamounAhramNeuroscience2020
References
ThislectureMark’sBasicMedicalBiochemistry,4th ed,pp.908-918http://what-when-how.com/neuroscience/neurotransmitters-the-neuron-part-1/
Definitionofaneurotransmitter
Aneurotransmitterisachemicalsubstancethatis:synthesizedinaneuron,releasedatasynapsefollowingdepolarizationofthenerveterminal(usuallydependentoninfluxofcalciumions),whichbindstoreceptorsonthepostsynapticcelland/orpresynapticterminaltoelicitaspecificresponse.
Characteristicsofaneurotransmitter
Achemicalsubstancethat:Issynthesizedandstoredinapresynapticneuron(theenzymesneededforitssynthesismustbepresentintheneuron),Isreleasedatasynapsefollowingdepolarizationofthenerveterminal(usuallydependentoninfluxofcalciumions),bindstoreceptorsonthepostsynapticcelland/orpresynapticterminal,elicitsrapid-onsetandrapidlyreversibleresponsesinthetargetcell,Isremovedorinactivatedfromthesynapticcleft.
Typesofneurotransmitters
Small-moleculeneurotransmittersBiogenicamines(epinepherine,dopamine,histmaine,serotonin)Aminoacids(GABA,glutamate,aspartate,glycine)AcetylcholinePurines(ATP)
NeuropeptidesGases(nitricoxide,carbonmonoxide)
Twoormoretransmitters(usuallyasmall-moleculetransmitterandaneuroactivepeptide)coexistinmanymatureneurons(e.g.,mostspinalmotorneuronscontainacetylcholineandcalcitoningene-relatedpeptide).
Structuresofneurotransmitters
NEUROPEPTIDES
Introduction
Morethan50neuropeptideshavebeendescribedBehaviorPainperceptionMemoryAppetiteThirstTemperatureHomeostasisSleep
Neuropeptides:neurohormonesorneurotransmitters?
Neurohormones: whenreleasedbyneuronsintothehaemolymphandexertitseffectsondistantperipheraltargets.
Peptidesactivatetheirreceptorsatlow(nMtouM)concentrationscomparedtotheconcentrationsrequiredtoactivatereceptorsforsmall-moleculeneurotransmitters.Thesepropertiesallowthepostsynaptictargetsofpeptidestobequitefarremovedfrompresynapticterminals.
Neurotransmitter:whenreleasedfromaneuronataspecializedjunctionanddiffusesacrossanarrowclefttoaffectoneortwopostsynapticneurons,amusclecell,oranothereffectorcell.
Classificationofneuropeptides
Peptidescanbegroupedbystructuralandfunctionalsimilarity.Neuropeptide Families
Tachykinins:substanceP,bombesin,substanceKInsulins:insulin,insulin-likegrowthfactorsSomatostatins:somatostatin,pancreaticpolypeptideGastrins:gastrin,cholecystokininOpioids:opiocortins,enkephalins,dynorphin
OpiateFamily
Name AminoAcidSequenceLeu-
enkephalin Tyr-Gly-Gly-Phe-Leu-OH
Met-enkephalin Tyr-Gly-Gly-Phe-Met-OH
Beta-endorphin
Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val-Thr-Leu-
Phe-Lys-Asn-Ala-Ile-Val-Lys-Asn-Ala-His-Lys-Gly-Gln-His-OH
Dynorphin Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln-OH
• Vasopressin and oxytocin share 7 of 9 amino acids, but have different functions.
• Opiate peptides share a common sequence, but are receptor-selective.
• The three glycoprotein hormones from the anterior pituitary, TSH, LH, and FSH, share a common α subunit, but have distinct β subunits.
StagesofactionSynthesis(ER(1)aspre-propeptidesthenpropeptides andtheninGolgiapparatus(2))Packagingintolarge-densecorevesicles(withmodifyingenzymes)Transport(3)(fast-axonaltransport)
Duringthetransport,proteasescleavetheprecursorneuropeptideintothefinalmatureform(4).
Release(5)Releaseisgradualovertimeinresponsetogeneralincreasesinthelevelofintracellularcalcium.
Action(prolonged)MainlyviaGPCR
Terminationbydiffusionanddegradation(6)
Diversity:alternativesplicing
AlternativesplicingofmRNAleadstotranslationofdistinctprecursors,andsubsequentprocessingleadstouniquematurepeptides.
ExampleisthesubstancePmRNAthatnormallyalsoincludesmRNAencodingsubstanceK.
Diversity:proteolytic,differential,sequentialprocessingNeuropeptidesareproducedfromalongerprecursorproteinby
ProteolyticprocessingVesicularpackagingofdifferentproteasesthatrecognizedifferentcleavagesequencesHidingaproteolyticsitebypost-translationalmodifications(example:additionofacarbohydratesidechain).Tissue-specific
Processing of the pro-opiomelanocortin(POMC) precursor proceeds in an ordered, stepwise fashion. Some of the reactions are tissue specific. ACTH, adrenocorticotropichormone; CLIP, corticotropin-like intermediate lobe peptide; JP, joining peptide; LPH, lipotropin; MSH, melanocyte-stimulating hormone; PC, prohormoneconvertase.
RoleofCa2+ions
• Vesicles are located further away from the presynaptic membrane and away from area of Ca2+ ions influx
• Ca2+ ion influx can be from external of internal sources and at higher concentrations than required for small-molecule neurotrasmitters.
Thelevelsofregulationofneuropeptide expression
Diffusion
Proteolysis
Proteases may or may not be released
SMALL-MOLECULENEUROTRANSMITTERS
StagesofactionSynthesisofenzymesinER(1)andGolgiapparatus(2)wheretheyaremodified(3).Transportofsoluble enzymesviaslowaxonaltransport(4)Nurotransmittersprecursorsaretakenupviatransporterproteinslocatedintheplasmamembraneofthenerveterminal(5),andtheneurotransmitterissynthesizedinthepresynapticnerveterminalthenpackagedinsmallsynapticvesicles(6).Releaseisstimulatedbybriefpulseseachtimeanactionpotentialtriggerstheinfluxofcalcium.Action(short)Terminationbydiffusion,re-uptake,orinactivation
[Ca+] = 2 mM
[Ca+] = 0.1 uM
100 uM-[Ca+] = 50
Proteinsandexocytosis
TheinfluxofCa2+ionsthemtointeractwithsynaptotagminleadingtofusionofthevesicularandpresynapticmembranes.
http://www.sumanasinc.com/webcontent/animations/content/synaptictransmission.html
TYROSINE-DERIVEDNEUROTRANSMITTERS
Dopamine,norepinephrine,andepinephrine
Notes
RoleofcofactorsS-adenosylmethionine(methyltransfer)Pyrodoxalphosphate(vitaminB6):transamination,decarboxylationTetrahydrobiopterin(BH4)
Rate-limitingstep
Pyridoxal phosphate vesicular
Vitamin B12 or folate
Diet/liver
phenylalanine hydroxylase
cytoplasm
Tyrosine (hydroxy-phenylalanine) is present in all food products and synthesized from phenylalanine. It enters the neuron by active transport.
50%
10%
Na+-dependent dopamine co-transporter (DAT)
CocaineAmphetamine
Most
Some
LDCV
Na+-dependent norepinepherine co-transporter (DAT)
Amphetamine
Leaking
Packagingintovesicles
Thecatecholamines(dopamineandepinepherine)aretransportedintovesiclesbyanATP-dependentprocesslinkedtoaprotonpump.ProtonsarepumpedintothevesiclesbyavesicularATPase(V-ATPase).Theprotonsthenexchangeforthepositively-chargedcatecholamineviathetransporterVMAT(vesicularmonoaminetransporter).
COMTandMAO
Parkinson’s disease
Inactivation is dependent on SAM,
vitamin B12 and folate
Regulation
• Tyrosinehydroxylase– Shortterm:
• Inhibitionbyfreecytosoliccatecholamines,whichcompetewithBH4bindingtoenzyme
• Activationbydepolarization,whichactivatesseveralproteinkinasesincludingPKC,PKA,Ca2-calmodulin–dependentkinasesthatphosphorylatetyrosinehydroxylase.ThismakestheenzymebindmoretightlyBH4and,consequently,lesssensitivetoendproductinhibition.
– Long-term(plusdopamineb-hyroxylase)Prolongedsympatheticneuronalactivityincreasestheamountsoftyrosinehydroxylaseanddopamine–hydroxylasemRNAs.
TRYPTOPHAN-DERIVEDNEUROTRANSMITTERS
Serotoninandmelatonin
requiresBH4
Serotonin
5-hydroxyindoleacetic acid
urine
MAO
Antidepressants , called selective serotonin re-uptake inhibitors (SSRIs) like Prozac® inhibit the reuptake process resulting in prolonged serotonin presence in the synaptic cleft.
VMAT
Melatonin
Serotoninissynthesizedinthepinealglandandservesasaprecursorforthesynthesisofmelatonin,whichisaneurohormoneinvolvedinregulating:
sleeppatternsSeasonalandcircadian(daily)rythymsDark-lightcycle
VMAT vesicles
GLUTAMATEANDASPARTATE
Glutamateandaspartate
NonessentialaminoacidsDonotcrossBBB
mustbesynthesizedinneuronsMainsyntheticcompartments
neuronsglialcells
Bothareexcitatoryneurotransmitters.
Synthesisofglutamate
Sources:1. Glycolysis® Krebscycle®dehydrogenationofa-ketoglutarate
2. Glutamine(deamination)3. Aspartate(transamination)Removal
excitatoryaminoacidcarrier-1(EAAC1)glutamatetransporter-1(GLT-1)andglutamate—aspartatetransporter(GLAST)
GABA
glutaminaseGlutaminesynthetase
transaminase 1
2
3
a-KG
GluDehydro
Sourcesofglutamate(supplementary)
Aspartate
Avesicularuptakemechanismforaspartatehasnotyetbeendemonstrated,somewhatweakeningthecaseforconsideringaspartatetobeaneurotransmitterPrecursor:oxaloacetate(transamination)
Glycine
AmajorinhibitoryneurotransmitterItissynthesizedfromserinebyserinehydroxymethyltransferasethrough3-phosphoglycerateRemoval:high-affinitytransporter
GABA
GABAispresentinhighconcentrations(millimolar)inmanybrainregions.
Theseconcentrationsareabout1,000timeshigherthanconcentrationsoftheclassicalmonoamineneurotransmittersinthesameregions.
TheGABAshuntisaclosed-loopprocesswiththedualpurposeofproducingandconservingthesupplyofGABA.
GABAshunt
Gln ® Glubyglutaminase.Glu® GABAbyglutamatedecarboxylase(GAD),whichrequirespyridoxalphosphate(vitaminB6).GABAisstoredinvesiclesuntilreleased.GABAiseither
takenupintopresynapticterminalandrepackagedgoesintotheGABAShuntwhereitistakenupintothegliaandconvertedtoGlu.
GluisconvertedintoGln,whichistransportedintotheneighboringnerveterminalstosynthesizeGlu.
Diet
MembranePL
Synthesisofacetylcholine
Choline+acetylcoenzyme-AbycholineacetyltransferaseincytoplasmTransportedintoandstoredinvesicles.Removal:hydrolysisbyacetylcholinesterase
Histamine
itdoesnotpenetratetheblood-brainbarrierand,hence,mustbesynthesizedinthebrain.Histamineisinactivatedbytwoenzymes—histaminemethyltransferaseanddiamineoxidase(histaminase).
Pyridoxal phosphate
Astrocytes(MAO) Neuron
XNo recycling
Histamine is packaged into vesicles by VMAT
Nitricoxide(NO)
Glutamateisreleased(1)andactsonNMDAreceptorslocatedonthepost-synapticneuron(2)Ca2+entersthepostsynapticneuronactivatingNOS(3),whichformsNOfromsarginine(4).NOstimulatesguanylatecyclaseformingcGMP(5),whichresultsinaphysiologicalresponse(6)NOcandiffuseout:a)tothepresynapticterminal(retrogrademessenger)(7)prolongingeffectandb)intoadjacentneurons(8)andglialcells(9)stimulatingguanylatecyclase. Half-life: 2-4 seconds
NO is inhibited by hemoglobin and other heme proteins which bind it tightly
IsNOaneurotransmitter?
Yes,but:ItisnotstoredinvesiclesItisnotreleasedbycalcium-dependentexocytosis(itdiffuses)Itsinactivationispassive(thereisnoactiveprocessthatterminatesitsaction)
ItdecaysspontaneouslyItdoesnotinteractwithreceptorsontargetcells
Itssphereofactiondependsontheextenttowhichitdiffuses,anditsactionisnotconfinedtotheconventionalpresynaptic-postsynapticdirection.
NOactsasaretrogrademessengerandregulatesthefunctionofaxonterminalspresynaptictotheneuroninwhichitissynthesized.
NOsynthase
IsoformI(nNOSorcNOS)Neuronsandepithelialcellsactivatedbytheinfluxofextracellularcalcium
isoformII(iNOS)Macrophagesandsmoothmusclecellsinducedbycytokines
andisoformIII(eNOS)Endothelialcellsliningbloodvesselsactivatedbytheinfluxofextracellularcalcium
AllthreeisoformsrequireBH2asacofactorandnicotinamideadeninedinucleotidephosphate(NADPH)asacoenzyme.
NotethedifferencesbetweenneuropeptidesandneurotransmittersActivity(slowvs.fast),response(slowvs.fast),anddurationofaction(longvs.short)Receptortargets(multiplevs.single)Geneexpression(yesvs.no)Synthesis,transport,andpackagingConcentrationforaction(lowvs.high)Speedofrelease(slowvs.fast)Concentrationof[Ca+2]forreleaseSiteofsynthesis,modification,actionFate
Neuropeptidesvs.Neurotransmitters
Neuropeptides Neurotransmitters
Size Largermolecules(3to36aminoacids)
Smallermoleculesofdifferentprecursors
Re-uptake Not–re-takenupagain Possible re-uptake
Post-release modification Yesby extracellularpeptidases NomodificationsStorage Large densecorevesicles SmallsynapsevesiclesLocation Anywhereintheneuron Inthepresynapticaxon
terminalSecretions Co-releasedAction Slow-acting Fast-actingSynthesis Ribosomes, ER,Golgibodies,etc. Cytoplasm ofpresynapticendEfficiencyofsignaling Moreefficient LessefficientConcentrations Lower Higher
DiffusionatReleaseSite Diffuseaway Donotdiffusefar