CHLOROPLAST FRUCTOSE-1,6-BISPHOSPHATASE - MODIFICATION OF NONCOVALENTINTERACTIONS PROMOTE THE ACTIVATION BY CHIMERIC ESCHERICHIA-COLI THIOREDOXINS

Although all thioredoxins contain a highly conserved amino acid sequen ce responsible for thiol/disulfide exchanges, only chloroplast thiored oxin-f is effective in the reductive stimulation of chloroplast fructo se-1,6-bisphosphatase. We set out to determine whether Escherichia col i thioredoxin becomes functional when selected modulators alter the co nformation of the target enzyme. Wild type and chimeric Escherichia co li thioredoxins match the chloroplast counterpart when the activation of chloroplast fructose-1,6-bisphosphatase is performed in the presenc e of fructose 1,6-bisphosphate, Ca2+, and either trichloroacetate or 2 -propanol. These modulators of enzyme activity do change the conformat ion of chloroplast fructose-1,6-bisphosphatase whereas bacterial thior edoxins remain unaltered, Given that fructose 1,6-bisphosphate, Ca2+, and non-physiological perturbants modify non-covalent interactions of the protein but do not participate in redox reactions, these results s trongly suggest that the conformation of the target enzyme regulates t he rate of thio/disulfide exchanges catalyzed by protein disulfide oxi doreductases.