Faktor XI
Faktor XI ili plazmatski tromboplastinski prethodnik jest protein koji je kod ljudi kodiran genom F11 sa hromosoma 4. To je zimogeni oblik faktora XIa, jedan od enzima u koagulacijskoj kaskadi. Kao i mnogi drugi faktori koagulacije, to je serin-proteaza.[5][6][7][8]
Aminokiselinska sekvenca
urediDužina polipeptidnog lanca je 625 aminokiselina, а molekulska težina 70 109 Da.[9]
10 | 20 | 30 | 40 | 50 | ||||
---|---|---|---|---|---|---|---|---|
MIFLYQVVHF | ILFTSVSGEC | VTQLLKDTCF | EGGDITTVFT | PSAKYCQVVC | ||||
TYHPRCLLFT | FTAESPSEDP | TRWFTCVLKD | SVTETLPRVN | RTAAISGYSF | ||||
KQCSHQISAC | NKDIYVDLDM | KGINYNSSVA | KSAQECQERC | TDDVHCHFFT | ||||
YATRQFPSLE | HRNICLLKHT | QTGTPTRITK | LDKVVSGFSL | KSCALSNLAC | ||||
IRDIFPNTVF | ADSNIDSVMA | PDAFVCGRIC | THHPGCLFFT | FFSQEWPKES | ||||
QRNLCLLKTS | ESGLPSTRIK | KSKALSGFSL | QSCRHSIPVF | CHSSFYHDTD | ||||
FLGEELDIVA | AKSHEACQKL | CTNAVRCQFF | TYTPAQASCN | EGKGKCYLKL | ||||
SSNGSPTKIL | HGRGGISGYT | LRLCKMDNEC | TTKIKPRIVG | GTASVRGEWP | ||||
WQVTLHTTSP | TQRHLCGGSI | IGNQWILTAA | HCFYGVESPK | ILRVYSGILN | ||||
QSEIKEDTSF | FGVQEIIIHD | QYKMAESGYD | IALLKLETTV | NYTDSQRPIC | ||||
LPSKGDRNVI | YTDCWVTGWG | YRKLRDKIQN | TLQKAKIPLV | TNEECQKRYR | ||||
GHKITHKMIC | AGYREGGKDA | CKGDSGGPLS | CKHNEVWHLV | GITSWGEGCA | ||||
QRERPGVYTN | VVEYVDWILE | KTQAV |
Struktura
urediIako je sintetiziran kao jedan polipeptidni lanac, FXI cirkulira kao homodimer. Svaki lanac ima relativnu molekulsku masu od približno 80.000. Tipske koncentracije FXI u plazmi su 5 μg/mL, što odgovara koncentraciji dimera FXI u plazmi od približno 30 nM. Na hromosomu 4, gen FXI je dug 23 kb, ima 15 egzona u regiji 4q32-35.[6][7]
Faktor XI se sastoji od četiri PAN-domena, koji stvaraju platformu nalik na disk oko baze 5., domena katalitske serin-proteaze. Jedan sadrži mjesto vezanja za trombin, drugi za kininogen visoke molekulske težine, treći za faktor IX, heparin i glikoprotein Ib, a četvrti je uključen u formiranje homodimera faktora XI, uključujući cisteinski ostatak koji stvara disulfidnu vezu.
U homodimeru, domeni PAN stvaraju dvije diskoidne platforme, povezane zajedno pod uglom, pri čemu katalitski domeni strše sa svake strane dimera.
Aktivacija trombinom ili faktorom XIIa se postiže cijepanjem Arg369-Ile370 peptidnih veza na obje podjedinice dimera. Ovo rezultira djelimičnim odvajanjem katalitskog domena od diskolikih domena PAN, još uvijek vezanih za četvrti domen disulfidnom vezom, ali sada dalje od trećeg domena.
Smatra se da ovo otkriva mjesto vezanja faktora IX trećeg PAN domena, omogućavajući proteazno delovanje faktora XI na njemu.[10]
Funkcija
urediFaktor XI (FXI) proizvodi jetra i cirkulira kao homodimer u svom neaktivnom obliku.[11] Poluvrijeme eliminacije FXI u plazmi je približno 52 sata. Zimogeni faktor aktivira se u faktor XIa, pomoću faktora XIIa (FXIIa), trombina i samog FXIa; zbog svoje aktivacije pomoću FXIIa, FXI je član "kontaktnog puta" (koji uključuje HMWK, prekalikrein, faktor XII, faktor XI i faktor IX).[12]
Faktor XIa aktivira faktor IX selektivnim cijepanjem arg-ala i arg-val peptidne veze . Faktor IXa, zauzvrat, formira kompleks sa faktorom VIIIa (FIXa-FVIIIa) i aktivira faktor X.
Inhibitori faktora XIa uključuju protein Z-ovisni inhibitor proteaze (ZPI, član inhibitor serin proteaze/serpin klase proteina), koji je nezavisan od proteina Z (njegovo djelovanje na faktor X, međutim, zavisi od proteina Z, otuda i njegovo ime).
Klinički značaj
urediNedostatak faktora XI uzrokuje rijetku hemofiliju C; ovo se uglavnom dešava kod Aškenazi Jevreja i vjeruje se da pogađa otprilike 8% te populacije. ]]Rjeđe, hemofilija C može se naći kod Jevreja iračkog porijekla i kod izraelskih Arapa. Stanje je opisano u drugim populacijama u oko 1% slučajeva. To je autosomno recesivni poremećaj. Spontanog krvarenja je malo, ali hirurški zahvati mogu uzrokovati prekomjeran gubitak krvi i potrebna je profilaksa.[13]
Nizak nivo faktora XI se takođe javlja u mnogim drugim bolesnim stanjima, uključujući Noonanov sindrom.
Visoki nivoi faktora XI su uključeni u trombozu, iako je neizvjesno šta određuje ove nivoe i koliko je ozbiljno prokoagulantno stanje.
Također pogledajte
uredi- Put kontaktne aktivacije
- Put tkivnog faktora (također poznat i kao vanjski put)
Reference
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Dopunska literatura
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Vanjski linkovi
uredi- The MEROPS online database for peptidases and their inhibitors: S01.213 Arhivirano 29. 5. 2020. na Wayback Machine