Thioredoxin, sulfate reduction

In bacteria PAPS is a substrate for sulfate reduction. In plants, adenosine-5 -phosphosulfate is the substrate. Thioredoxin, a small thiol-containing protein, reduces the sulfate in PAPS to sulfite (SOS "). Sulfite is reduced by sulfite reductase in a six electron transfer through the intermediates NADPH, FAD, FMN, an iron-sulfur center, and the porphyrin siroheme. The end product is H2S. 

In eukaryotic plants the former reaction is presumed to be involved in the synthesis of sulfolipids and sulfate esters. In cyanobacteria, the two reactions are thought to be involved in assimilatory sulfate reduction. The APS kinase of the green alga Chlamydomonas (Schwenn and Schriek, 1984) and the PAPS sulfotransferase of the cyanobacterium Synechococcus (Schmidt and Christen, 1978) are subject to light modulation via a thioredoxin-mediated system similar to that reported for several other light-mediated enzymes (Buchanan, 1980). However, the significance of these r ulatory mechanisms with respect to APS metabolism is not currently understood. 

The thioredoxin system, consisting of thioredoxin and thioredoxin reductase, was originally discovered as the hydrogen carrier system, which provides, with NADPH, the reducing potential for the reduction of ribonucleotides (5, 35). Since then considerable evidence has been accumulated to indicate that this or a closely related system also participates in a variety of other enzymatic reductions. For instance thioredoxin can function as an electron carrier between NADPH and several disulfides, such as insulin, lipoate and oxidized glutathione. Furthermore Porque et al. (114) have shown that thioredoxin and thioredoxin reductase from yeast can function as hydrogen carriers in the reduction of methionine sulfoxide and sulfate. 

Thioredoxin (1,2) are small (M. 11-12,000) ubiquitous redox proteins with two half-cystine residues in the conserved active site structure Trp-Cys-Gly-Pro-Cys. The oxidized form Trx-S2 is reduced by NADPH and thioredoxin-reductase the reduced rorm Trx(SH)2 is a powerful protein disulfide oxido-reductase which regulatesthe activity of enzymes by thiol redox control it serves as hydrogen donor for various reductive enzymes such as ribonucleotide reductase or enzymes reducing sulfate or methionine sulfoxide. Also, Trx(SH)2 is essential for phage T7 DNA replication as a subunit of T7 DNA polymerase and assembly of filamentous phages (fl and M13), at least in E, ooti. 

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