XVI International Botanical Congess

The mechanisms of reductive activation of chloroplastic fructose-1, 6-bisphosphatase (FBPase) and NADP-malate dehydrogenase (MDH) by thiol-disulfide interchange with thioredoxin will be compared. Both enzymes exhibit specific amino-acid stretches bearing the regulatory disulfides: either one insertion (FBPase) or two extensions (MDH), one at the N-terminus, the other at the C-terminus. A common and still puzzling feature for FBP-ase and for the N-terminal extension of MDH is the involvement of 3 cysteines in the activation. This suggests a possible disulfide isomerization which could be part of the activation process. MDH exhibits an additional regulatory mechanism, with the C-terminal extension acting as an internal inhibitor.