REDOX AND SITE-DIRECTED MUTAGENESIS STUDIES OF 5'-ADENYLYLSULFATE (APS) REDUCTASES

摘要

The enzyme APS reductase (EC 1.8.4.9), which catalyzes the two-electron reductive conversion of 5'-adenylylsulfate (APS) to AMP and sulfite, is a key enzyme in the assimilation of sulfate in eubacteria, and plants. The assirniiatory APS reductases aredistinct from the dissimilatory APS reductase. APS reductases from different organisms utilize either reduced thioredoxin or glutathi-one, but not both, as the electron donor. Some APS reductases contain two types of cysteine-containing motifs, CCXXRKXXPL and SXGCXXCT. In APS reductases found in the chloroplasts of Arabidopsis thaliana and Lemna minor, three of the four-cysteine residues found in these domains appear to be ligands to a [4Fe-4S] cluster (Kopriva et al 2002). Similar cysteine-containingmotifs are present in APS reductases from the bacterium Pseudomonas aemginosa and the marine alga Enteromorpha intestinalis (Bick et al 2000, Gao et al 2000). The P. aemginosa APS reductase (PaAPR), which uses reduced thioredoxin as an electron donor (Bick et al 2000), exhibits significant homology to the N-terminal domain of the E. intestinalis APS reductase (EiAPR), an enzyme that uses glutathione as an electron donor (Gao et al 2000) (Fig. 1).