Pathways of assimilatory sulphate reduction in plants and microorganisms.

Assimilatory sulphate reduction, largely restricted to plants and microorganisms where it provides reduced sulphur for the formation of amino acids and proteins, nucleic acids, and various sulphur-containing coenzymes, begins with the activation of sulphate through reaction with ATP to form adenosine 5'-phosphosulphate (APS) and adenosine 3'-phosphate 5'-phosphosulphate (PAPS). Two pathways of assimilatory sulphate reduction are known. One, found in some blue-green algae (cyanobacteria) and in all oxygen-envolving eukaryotes, begins with APS where the sulpho group is transferred via APS sulphotransferase to a thiol acceptor (glutathione (G-S-) in Chlorella) to form the organic thiosulphate (G-S-SO-3). The organic thiosulphate appears to be reduced further by an organic thiosulphate reductase employing reduced ferredoxin to form G-S-S-. The terminal sulphur is then thought to be reductively transferred to O-acetylserine via O-acetylserine sulphydrase to form cysteine. A second pathway, found in bacteria and fungi, begins with PAPS where the sulpho group is transferred via PAPS sulphotransferase to an acceptor thiol to form an organic thiosulphate. Since thioredoxin is indispensable, this molecule may be the carrier or may serve to reduce the carrier. NADPH via thioredoxin reductase or glutathione and glutathione reductase reduces thioredoxin. These reactions release sulphite which is further reduced to sulphide by sulphite reductase, employing NADPH. Sulphide is then thought to react with O-acetylserine to form cysteine via O-acetylserine sulphydrase. The cellular location and evolution of these pathways is discussed.