The crystal structure of rubisco from Alcaligenes eutrophus reveals a novel central eight-stranded beta-barrel formed by beta-strands from four subunits

Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) is involved in ph otosynthesis where it catalyzes the initial step in the fixation of carbon dioxide. The enzyme also catalyzes a competing oxygenation reaction leading to loss of fixed carbon dioxide, thus reducing the net efficiency of photo synthesis significantly. Rubisco has therefore been studied extensively, an d a challenging goal is the engineering of a more photosynthetically effici ent enzyme. Hexadecameric rubiscos fall in two distinct groups, "green-like " and "red-like". The ability to discriminate between CO2 and O-2 as substr ates varies significantly, and some algae have red-like rubisco with even h igher specificity for CO2 than the plant enzyme. The structure of unactivat ed rubisco from Alcaligenes eutrophus has been determined to 2.7 Angstrom r esolution by molecular replacement and refined to R and R-free values of 26 .6 and 32.2%, respectively. The overall fold of the protein is very similar to the rubisco structures solved previously for green-like hexadecameric e nzymes, except for the extended C-terminal domains of the small subunits wh ich together form an eight-stranded beta-barrel which sits as a plug in the entrance to the central solvent channel in the molecule. The present struc ture is the first which has been solved for a red-like rubisco and is Likel y to represent a fold which is common for this group. The small subunits in general are believed to have a stabilizing effect, and the new quaternary structure in the oligomer of the present structure is Likely to contribute even more to this stabilization of the assembled rubisco protein. (C) 1999 Academic Press.