Redox signalling in the chloroplast: structure of oxidized pea fructose-1,6-bisphosphate phosphatase

Sunlight provides the energy source for the assimilation of carbon dioxide by photosynthesis, but it also provides regulatory signals that switch on s pecific sets of enzymes involved in the alternation of light and dark metab olisms in chloroplasts. Capture of photons by chlorophyll pigments triggers redox cascades that ultimately activate target enzymes via the reduction o f regulatory disulfide bridges by thioredoxins. Here we report the structur e of the oxidized, low-activity form of chloroplastic fructose-1,6-bisphosp hate phosphatase (FBPase), one of the four enzymes of the Calvin cycle whos e activity is redox-regulated by light. The regulation is of allosteric nat ure, with a disulfide bridge promoting the disruption of the catalytic site across a distance of 20 Angstrom. Unexpectedly, regulation of plant FBPase s by thiol-disulfide interchange differs in every respect from the regulati on of mammalian gluconeogenic FBEPases by AMP, We also report a second crys tal form of oxidized FBPase whose tetrameric structure departs markedly fro m D-2 symmetry, a rare event in oligomeric structures, and the structure of a constitutively active mutant that is unable to form the regulatory: disu lfide bridge. Altogether, these structures provide a structural basis for r edox regulation in the chloroplast.