EFFECT OF ALKALINE PH ON THE ACTIVITY AND THE STRUCTURE OF CHLOROPLAST FRUCTOSE-1,6-BISPHOSPHATASE

The effect of alkaline pH on the stability of chloroplast fructose-1,6-bisphosphatase was examined. The incubation of the native enzyme (tetramer) at pH 8.9 lowered the specific activity (t0.5 = 6 min). The presence of fructose 1,6-bisphosphatase and Ca2+, two enzyme modulators, delayed the inactivation process (t0.5 = 50 min) whereas sodium trichloroacetate, a chaotropic anion, accelerated it (t0.5 < 1 min). On the other hand, the conversion of 60% of the enzyme to dimers required an incubation of 3 h at pH 8.9. On this basis, it appeared that an inactive tetramer form preceded the dissociation of chloroplast fructose-1,6-bisphosphatase at pH 8.9; both fructose 1,6-bisphosphate and Ca2+ prevented enzyme inactivation by stabilizing the tetramer form. These results indicated that, in addition to their roles as enzyme modulators, sugar biphosphates and bivalent cations contribute to stabilizing the active conformation of chloroplast fructose-1,6-bisphosphatase. © 1990.