COMPARISON OF THE CRYSTAL-STRUCTURES OF L2 AND L8S8 RUBISCO SUGGESTS A FUNCTIONAL-ROLE FOR THE SMALL SUBUNIT

Comparison of the crystal structures of the L2 and L8S8 forms of ribulose-1,5-bisphosphate carboxylase from Rhodospirillum rubrum and spinach respectively, reveals a remarkable similarity in the overall architecture of the L2 building blocks in the two enzymes. Within the L subunits, no large conformationl differences such as domain - domain rotations were found. In spite of a somewhat different packing of the L subunits in the L2 dimer, the active sites of the two enzymes are highly conserved. Significant local conformational differences are, however, observed for the C-terminal part of the polypeptide chains as well as for loop 7, helix α7, loop 8 and helix α8 in the barrel domain. The small subunit forms extensive interactions with one of these α helices, α8, in the spinach L8S8 enzyme. The loops are at the active site and one of them forms a phosphate binding site for the substrate. We suggest that the small subunit modulates substrate binding and, possibly, the carboxylation/oxygenation ratio by inducing conformational changes in the active site through interactions distant from this site.