Chloroplast thioredoxin-f functions efficiently in the light-dependent activation of chloroplast fructose-1, 6-bisphosphatase by reducing a specific disulfide bond located at the negatively charged domain of the enzyme. Around the nucleophile cysteine of the active site (-W-C-G-P-C-), chloroplast thioredoxin-f shows lower density of negative charges than the inefficient modulator Escherichia coli thioredoxin. To examine the contribution of long range electrostatic interactions to the thiol/disulfide exchange between protein-disulfide oxidoreductases and target proteins, we constructed three variants of E. coli thioredoxin in which an acidic (Glu-30) and a neutral residue (Leu-94) were replaced by lysines. After purification to homogeneity, the reduction of the unique disulfide bond by NADPH via NADP-thioredoxin reductase proceeded at similar rates for all variants. However, the conversion of cysteine residues back to cystine depended on the target protein. Insulin and difluoresceinthiocarbamyl-insulin oxidized the sulfhydryl groups of E30K and E30K/L94K mutants more effectively than those of wild type and L94K counterparts. Moreover, the affinity of E30K, L94K, and E30K/L94K E. coli thioredoxin for chloroplast fructose-1,6-bisphosphatase (A0.5 = 9, 7, and 3 microM, respectively) increased with the number of positive charges, and was higher than wild type thioredoxin (A0.5 = 33 microM), though still lower than that of thioredoxin-f (A0.5 = 0.9 microM). We also demonstrated that shielding of electrostatic interactions with high salt concentrations not only brings the A0.5 for all bacterial variants to a limiting value of approximately 9 microM but also increases the A0.5 of chloroplast thioredoxin-f. While negatively charged chloroplast fructose-1,6-bisphosphatase (pI = 4.9) readily interacted with mutant thioredoxins, the reduction rate of rapeseed napin (pI = 11.2) diminished with the number of novel lysine residues. These findings suggest that the electrostatic interactions between thioredoxin and (some of) its target proteins controls the formation of the binary noncovalent complex needed for the subsequent thiol/disulfide exchange.
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