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The malaria parasite Plasmodium falciparum possesses a functional thioredoxin system
The thioredoxin system consists of the NADPH dependent disulphide oxidoreductase thioredoxin reductase (TrxR) which catalyses the reduction of the small protein thioredoxin. This system is involved in a variety of biological reactions including the reduction of deoxyribonucleotides, transcription fa...
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Published in: | Molecular and biochemical parasitology 2001-02, Vol.112 (2), p.219-228 |
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Main Authors: | , , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | The thioredoxin system consists of the NADPH dependent disulphide oxidoreductase thioredoxin reductase (TrxR) which catalyses the reduction of the small protein thioredoxin. This system is involved in a variety of biological reactions including the reduction of deoxyribonucleotides, transcription factors and hydrogen peroxide. In recent years the TrxR of the malaria parasite
Plasmodium falciparum was isolated and characterised using model substrates like 5,5′-dithiobis (2-nitrobenzoic acid) (DTNB) and
Escherichia coli thioredoxin. Here we report on the isolation of a cDNA encoding for
P. falciparum thioredoxin (
PfTrx) and the expression and characterisation of the recombinant protein, the natural substrate of
PfTrxR. The deduced amino acid sequence of
PfTrx encodes for a polypeptide of 11 715 Da and possesses the typical thioredoxin active site motif CysGlyProCys. Both cysteine residues are essential for catalytic activity of the protein, as shown by mutational analyses. Steady state kinetic analyses with
PfTrxR and
PfTrx in several coupled assay systems resulted in
K
m-values for
PfTrx in the range of 0.8–2.1 μM which is about 250-fold lower than for the model substrate
E. coli thioredoxin. Since the turnover of both substrates is similar, the catalytic efficiency of
PfTrxR to reduce the isolated
PfTrx is at least 250-fold higher than to reduce
E. coli thioredoxin.
PfTrx contains a cysteine residue in position 43 in addition to the active-site cysteine residues, which is partially responsible for dimer formation of the protein as demonstrated by changing this amino acid into an alanine residue. Using DTNB we showed that all three cysteine residues present in
PfTrx are accessible to modification by this compound. Surprisingly the first cysteine residue of the active site motif (Cys30) is less accessible than the second cysteine (Cys33), which is highly prone to the modification. These results suggest a difference in the structure and reaction mechanism of
PfTrx compared to other known thioredoxins. |
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ISSN: | 0166-6851 1872-9428 |
DOI: | 10.1016/S0166-6851(00)00372-8 |