5-Aminolevulinic-acid synthetases from Rhodopseudomonas spheroides Y. Comparison of the purification and properties of enzymes extracted from bacteria grown in different iron concentrations

Characterization of the thioredoxin system in the facultative phototroph Rhodobacter sphaeroides Y

This paper reports the purification and characterization of a thioredoxin system (thioredoxin, thioredoxin reductase, NADPH) from the facultative phototroph Rhodobacter sphaeroides Y. Rhodobacter sph. Y thioredoxin was purified to homogeneity with an assay based on the reduction of 5,5'-dithiobis(2-nitrobenzoic acid) by NADPH and Escherichia coli thioredoxin reductase. Rhodobacter sph. Y thioredoxin reductase was purified with the same assay using NADPH and E. coli thioredoxin. Rhodobacter sph. Y thioredoxin contained 102 amino acid residues and had a single intrachain disulfide bond. The two half-cystine residues are part of the active site made up of the sequence -Ala-Glu-Trp-Cys-Gly-Pro-Cys-Arg- which is identical to that of E. coli thioredoxin except for the presence of an Arg instead of a Lys. Rhodobacter sph. Y thioredoxin contains two tryptophan residues. The fluorescence intensity of the tryptophan residues is quenched in oxidized thioredoxin; on reduction, a much smaller increase is observed with Rhodobacter sph. Y thioredoxin than with the E. coli protein. However, the presence of 5 M guanidine X HCl results in the complete exposure of the two tryptophan residues. Rhodobacter sph. Y thioredoxin reductase has structural and functional similarities to E. coli thioredoxin reductase: it has a molecular mass of 68 kDa, and consists of two, probably identical, subunits. Each subunit has one bound FAD molecule. The enzyme is highly specific for NADPH; it is also highly specific for Rhodobacter sph. Y thioredoxin with a Km value of 3.3 +/- 0.6 microM. A kinetic study of the two thioredoxin systems shows that they have a high degree of cross-reactivity.