The primary structure of Escherichia coli glutaredoxin. Distant homology with thioredoxins in a superfamily of small proteins with a redox-active cystine disulfide/cysteine dithiol.

An immunosorbent method using antiglutaredoxin-Sepharose was developed for purification of glutaredoxin in high yield from a mutant strain of Escherichia coli K 12 lacking thioredoxin reductase (C 10-17). The primary structure of the protein was determined by analyses of [14C]carboxymethylated glutaredoxin and its proteolytic fragments obtained by digestions with trypsin, clostripain, chymotrypsin and staphylococcal Glu-specific extracellular protease. The single active-center disulfide has the structure-Cys-Pro-Tyr-Cys-, with the half-cystine residues located at positions 11 and 14 in the polypeptide chain. In total the protein was deduced to have 85 residues corresponding to a molecular weight of 9674 for the reduced form of glutaredoxin, making it one of the smallest known enzymes (a glutathione-disulfide transhydrogenase). The half-cystines are identically spaced and similarly positioned in the N-terminal part of the protein when compared with a corresponding functionally active disulfide/dithiol in thioredoxins. Glutaredoxin is also distantly homologous with thioredoxins from phage T4 and E. coli, but extensive differences, even around the redox-active disulfide, distinguish glutaredoxin from the thioredoxins. Allowing for deletions in the glutaredoxin sequence (or insertions in the T4 thioredoxin sequence) at four places, there are identical residues at 25 positions of the 77 compared (= 32% identity). The results establish that glutaredoxin belongs to the same superfamily of small redox proteins as the thioredoxins. The structures are, however, subject to large changes, only four positions have residues identical among all presently analyzed forms. The fluorescence of reduced and oxidized glutaredoxin demonstrates an increase in the quantum yield of the tyrosine emission upon reduction with dithiothreitol. Differences in the spectra support the presence of tyrosine adjacent to the redox-active disulfide bridge. They also confirm that glutaredoxin lacks the disulfide-adjacent tryptophan residues of E. coli thioredoxin. There are known to be great differences between the bacterial E. coli and phage T4 forms of thioredoxin. The glutaredoxin structure is most similar to the phage type, both with respect to size of the polypeptide chain and to actual sequence. From the structural results and the previously known functional similarities it appears possible that the phage thioredoxin may have evolved from an early glutaredoxin gene. The mixed properties are compatible with this conclusion, the superfamily assignment, and the differences in biological activity.

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