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

Abstract

Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) is involved in photosynthesis 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 photosynthesis significantly. Rubisco has therefore been studied extensively, and a challenging goal is the engineering of a more photosynthetically efficient enzyme. Hexadecameric rubiscos fall in two distinct groups, “green-like” and “red-like”. The ability to discriminate between CO2 and O2 as substrates varies significantly, and some algae have red-like rubisco with even higher specificity for CO2 than the plant enzyme. The structure of unactivated rubisco from Alcaligenes eutrophus has been determined to 2.7 Å resolution by molecular replacement and refined to R and Rfree 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 enzymes, except for the extended C-terminal domains of the small subunits which together form an eight-stranded β-barrel which sits as a plug in the entrance to the central solvent channel in the molecule. The present structure is the first which has been solved for a red-like rubisco and is likely 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.