Disulfide bond oxidoreductase D .28DsbD.29 Disulfide oxidoreductase D

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1 disulfide bond oxidoreductase d (dsbd)

1.1 structure
1.2 system reduction pathway
1.3 reverse pathway





disulfide bond oxidoreductase d (dsbd)

the best characterized member of dsbd family dsbd of e. coli (tc# 5.a.1.1.1). dsbd protein membrane-embedded putative n-terminal transmembrane segment (tms) plus 8 additionaltmss. smallest homologues (190 aas 6 putative tmss) found in archaea, while largest found in both gram-negative bacteria (758 aas 9 putative tmss) , gram-positive bacteria (695 aas 6 putative tmss).


the overall vectorial electron transfer reaction catalyzed dsbd is:


2 e−

cytoplasm → 2 e−

periplasm


structure

dsbb contains 4 essential cysteine residues, reversibly forming 2 disulfide bonds. although dsba displays no proofreading activity repair of wrongly paired disulfides, dsbc, dsbe , dsbg have been found demonstrate proofreading activity. therefore, 2 transmembrane pathways involving dsbd , dsbb catalyze extracellular disulfide reduction (dsbd) , oxidation (dsbb) in superficially reversible process allows dithiol/disulfide exchange.


system reduction pathway

in e. coli dsbd system, electrons transferred nadph in cytoplasm periplasmic dithiol/disulfide-containing proteins via electron transfer chain sequentially involves nadph, thioredoxin reductase (trxb; present in cytoplasm), thioredoxin (trxa; in cytoplasm), dsbd (the integral membrane constituent of system), , periplasmic electron acceptors (dsbc, dsbe (ccmg) , dsbg).


all of these last 3 proteins (dsbc, dsbe (ccmg) , dsbg) can donate electrons oxidized disulfide-containing proteins in periplasm of gram-negative bacterium or presumably in external milieu of gram-positive bacterium or archaeon.


thus, pathway is:


nadph → trxb → trxa → dsbd → (dsbc, dsbe, or dsbg) → proteins.


dsbd contains 3 cysteine pairs undergo reversible disulfide rearrangements. trxa donates electrons transmembrane cysteines c163 (c3) , c285 (c5) in putative tmss 1 , 4 in dsbd model proposed katzen , beckwith (2000). dithiol donates electrons periplasmic c-terminal thioredoxin motif (cxxc) of dsbd, thereby reducing c461 , c464 (c6 , c7, respectively). dithiol pair attacks periplasmic n-terminal disulfide bridge @ c103 , c109 (c1 , c2, respectively) transfers electrons dsbc , other protein electron acceptors noted above.


reverse pathway

dsbd catalyses irreversible reaction due fact electrons flow down electrochemical gradient inside cell (negative inside) outside cell (positive outside). in order reverse reaction, electrons transferred dithiol proteins in periplasm electron acceptor in cytoplasm follows:


reduced proteinperiplasm → dsbaperiplasm → dsbbmembrane → quinonesmembrane → reductasemembrane→ terminal electron acceptorcytoplasm (e.g., o2, no−

3 or fumarate).








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