Gain of function conferred by selenocysteine: catalytic enhancement of one-electron transfer reactions by thioredoxin reductase

被引:4
作者
Barber, Drew R. [1 ]
Hondal, Robert J. [1 ]
机构
[1] Dept Biochem, 89 Beaumont Ave,Given Bldg Room B413, Burlington, VT 05405 USA
关键词
selenocysteine; cysteine; one-electron reaction; thioredoxin reductase; ascorbyl radical; CYTOCHROME-C; SELENIUM; DEHYDROASCORBATE; ASCORBATE; SEMISYNTHESIS; GLUTAREDOXIN; RESISTANCE; MECHANISM; KINETICS; COMPLEX;
D O I
10.1002/pro.3480
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
Selenocysteine (Sec) is the 21st amino acid in the genetic code and it is present in a small number of proteins where it replaces the much more commonly used amino acid cysteine (Cys). It is both more complicated and bioenergetically costly to insert Sec into a protein in comparison to Cys, and this cost is most likely compensated by a gain of function to the enzyme/protein in which it is incorporated. Here we investigate one such gain of function, the enhancement of one-electron transfer catalysis. We compared the ability of Sec-containing mouse mitochondrial thioredoxin reductase (mTrxR2) to catalyze the reduction of bovine cytochrome c, ascorbyl radical, and dehydroascorbate in comparison to Cys-containing thioredoxin reductases from D. melanogaster (DmTrxR) and P. falciparum (PfTrxR). The Sec-containing mTrxR2 was able to reduce all three substrates, while the Cys-containing enzymes had little or no activity. In addition, we constructed Cys -> Sec mutants of DmTrxR and PfTrxR and found that this substitution resulted in a gain of function, as these mutant enzymes were now able to catalyze the reduction of these substrates. We also found that in the case of PfTrxR, reduction of cytochrome c was enhanced five-fold in a truncated PfTrxR in which the C-terminal redox center was removed. This shows that some of the ability of thioredoxin reductase to reduce this substrate comes from the flavin coenzyme. We also discuss a possible mechanism by which Sec-containing thioredoxin reductase reduces dehydroascorbate to ascorbate by two sequential, one-electron reductions, in part catalyzed by Sec.
引用
收藏
页码:79 / 89
页数:11
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