A Selenocysteine-Containing Peroxiredoxin from the Strictly Anaerobic Organism Eubacterium acidaminophilum

A strongly [75]Selabeled 22 kDa protein detected previously showed in its Nterminal sequence the highest similarity to the family of thioldependent peroxidases, now called peroxiredoxins. The respective gene prxU was cloned and analyzed. prxU encodes a protein of 203 amino acids (22470 Da) and contains an inframe UGA codon (selenocysteine) at the position of the so far strictly conserved and catalytically active Cys47. The second conserved cysteine present in 2-Cys peroxiredoxins was replaced by alanine. Heterologous expression of the Eubacterium acidaminophilum PrxU as a recombinant selenoprotein in Escherichia coli was not possible. A cysteineencoding mutant gene, prxU47C, containing UGC instead of UGA was strongly expressed. This recombinant PrxU47C mutant protein was purified to homogeneity by its affinity tag, but was not active as a thioldependent peroxidase. The identification of prxU reveals that the limited class of natural selenoproteins may in certain organisms also include isoenzymes of peroxiredoxins, previously only known as nonselenoproteins containing catalytic cysteine residues.