Location of the redox-active cysteines in chloroplast sedoheptulose-1,7-bisphosphatase indicates that its allosteric regulation is similar but not identical to that of fructose-1,6-bisphosphatase

Sedoheptulose-1,7-bisphosphatase (SBPase) is a Calvin Cycle enzyme exclusive to chloroplasts and is involved in photosynthetic carbon fixation. The two cysteine residues involved in its redox regulation have been identified by site-directed mutagenesis. They are four residues apart in a predicted loop between two alpha helices and probably form a disulphide bond when oxidised. Three-dimensional modelling of SBPase has been performed using crystallographic data from the structurally homologous pig fructose-1,6-bisphosphatase (FBPase). The results suggest that formation of the disulphide bridge in SBPase is directly analogous to the allosteric regulation of pig FBPase by AMP in terms of the resulting structural changes. Similar changes are thought to occur in chloroplast FBPase, which like SBPase, is also redox regulated and involved in carbon fixation. From the results presented here it appears that the same basic mechanism for the allosteric regulation of enzymic activity operates in the FBPases and SBPase but that the sites at which the regulatory ligands (AMP or thioredoxin) exert their effects are different in each.