The reactions between active and inactive forms of wheat ribulosebisphosphate carboxylase and effectors

The processes of activation and deactivation of ribulose-1,5-bisphosphate carboxylase purified from wheat have been investigated. Two forms of the enzyme are indistinguishable in terms of ribulose-1,5-bisphosphate carboxylation and oxidation but exhibit different rates of activation. One form is slowly activated in saturating CO2 and Mg2+ at moderate temperatures (t0.5 approximately 120 min at 25 degrees C), the other form rapidly activated (t0.5 approximately 8 s). In the presence of the effectors 6-phosphogluconate or NADPH, significantly lower concentrations of the activating co-factors can achieve full activation of both enzyme species. However, with another effector, fructose 1,6-bisphosphate, for the slowly activating species the mode of action is the same as with 6-phosphogluconate or NADPH, whereas the activation of the rapidly activating species is significantly inhibited. The substrate, ribulose 1,5-bisphosphate, also inhibits this rapid activation process. A mechanism is proposed for the reactions involving activation that accounts for the differential rates of activation and the response to effectors.