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 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 A 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 beta-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.

Structural studies on the binding of 4-methylumbelliferone glycosides of chitin to rainbow trout lysozyme.

Two complexes between rainbow trout lysozyme (RBTL) and 4-methylumbelliferyl chitobioside, 4MeU-(GlcNAc)2, and chitotrioside, 4MeU-(GlcNAc)3, were produced by co-crystallization and soaking, respectively, and the crystal structures were solved at 2.0 A resolution. The results show that 4-MeU-(GlcNAc)3 binds in subsites A-D and that 4-MeU-(GlcNAc)2 binds in subsites B-D in the active-site cleft of RBTL. This agrees well with earlier crystallographic studies on the binding of oligosaccharides of chitin to RBTL, which showed that (GlcNAc)3 binds to sites B-D in RBTL and not to A-C as seen in the human and turkey egg-white lysozymes. For both complexes the 4-MeU moiety in site D has diffuse electron density and is flexible, as it is only bound to water molecules and not to the protein. Since no electron density was observed in site E, the solved structures give views of nonproductive enzyme-substrate complexes.