CRYSTAL-STRUCTURE OF ACTIVATED TOBACCO RUBISCO COMPLEXED WITH THE REACTION-INTERMEDIATE ANALOG 2-CARBOXY-ARABINITOL 1,5-BISPHOSPHATE

The crystal structure of activated tobacco rubisco, complexed with the reaction-intermediate analogue 2-carboxy-arabinitol 1,5-bisphosphate (CABP) has been determined by molecular replacement, using the structu re of activated spinach rubisco (Knight, S., Andersson, I., & Branden, C.-I., 1990, J. Mol. Biol. 215, 113-160) as a model. The R-factor aft er refinement is 21.0% for 57,855 reflections between 9.0 and 2.7 angs trom resolution. The local fourfold axis of the rubisco hexadecamer co incides with a crystallographic twofold axis. The result is that the a symmetric unit of the crystals contains half of the L8S8 complex (mole cular mass 280 kDa in the asymmetric unit). The activated form of toba cco rubisco is very similar to the activated form of spinach rubisco. The root mean square difference is 0.4 angstrom for 587 equivalent C(a lpha) atoms. Analysis of mutations between tobacco and spinach rubisco revealed that the vast majority of mutations concerned exposed residu es. Only 7 buried residues were found to be mutated versus 54 residues at or near the surface of the protein. The crystal structure suggests that the Cys 247-Cys 247 and Cys 449-Cys 459 pairs are linked via dis ulfide bridges. This pattern of disulfide links differs from the patte rn of disulfide links observed in crystals of unactivated tobacco rubi sco (Curmi, P.M.G., et al., 1992, J. Biol. Chem. 267, 16980-16989) and is similar to the pattern observed for activated spinach tobacco.