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 exclusiv e to chloroplasts and is involved in photosynthetic carbon fixation. The tw o cysteine residues involved in its redox regulation have been identified b y site-directed mutagenesis. They are four residues apart in a predicted lo op between two alpha helices and probably form a disulphide bond when oxidi sed. Three-dimensional modelling of SBPase has been performed using crystal lographic data from the structurally homologous pig fructose-1,6-bisphospha tase (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 th ought to occur in chloroplast FBPase, which like SBPase, is also redox regu lated and involved in carbon fixation. From the results presented here it a ppears that the same basic mechanism for the allosteric regulation of enzym ic activity operates in the FBPases and SBPase but that the sites at which the regulatory ligands (AMP or thioredoxin) exert their effects are differe nt in each.