Crystal structure of rice rubisco and implications for activation induced by positive effectors NADPH and 6-phosphogluconate

The key enzyme of plant photosynthesis, d-ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), must be activated to become catalytically competent via the carbamylation of Lys201 of the large subunit and subsequent stabilization by Mg²⁺ coordination. Many biochemical studies have reported that reduced nicotinamide adenine dinucleotide phosphate (NADPH) and 6-phosphogluconate (6PG) function as positive effectors to promote activation. However, the structural mechanism remains unknown. Here, we have determined the crystal structures of activated rice Rubisco in complex with NADPH, 6PG, or 2-carboxy-d-arabinitol 1,5-bisphosphate (2CABP). The structures of the NADPH and 6PG complexes adopt open-state conformations, in which loop 6 at the catalytic site and some other loops are disordered. The structure of the 2CABP complex is in a closed state, similar to the previous 2CABP-bound activated structures from other sources. The catalytic sites of the NADPH and 6PG complexes are fully activated, despite the fact that bicarbonate (NaHCO₃) was not added into the crystallization solution. In the catalytic site, NADPH does not interact with Mg²⁺ directly but interacts with Mg²⁺- coordinated water molecules, while 6PG interacts with Mg²⁺ directly. These observations suggest that the two effectors promote Rubisco activation by stabilizing the complex of Mg²⁺ and the carbamylated Lys201 with unique interactions and preventing its dissociation. The structure also reveals that the relaxed complex of the effectors (NADPH or 6PG), distinct from the tight-binding mode of 2CABP, would allow rapid exchange of the effectors in the catalytic sites by substrate d-ribulose 1,5-bisphosphate for catalysis in physiological conditions.