Determinação da estrutura tridimensional de uma lectina de sementes de Canavalia boliviana por cristalografia de raios X

Abstract

Lectins are proteins of non immune origin with non-catalytic site that bind reversibly and specific carbohydrates, containing or not a catalytic-site. Plant lectins are the most studied group of carbohydrate binding proteins. Despite the high similarity between the members of the Diocleinae sub tribe (Leguminosae) group, they present different biological activities. Canavalia boliviana lectin (Cbol) was purified using a Sephadex G-50 column and crystallized in the presence of X-Man by hanging-drop vapor diffusion at 293K. After optimizations crystals suitable for diffraction were obtained under the condition 0.1 M HEPES pH 7.5 and 3.0 M sodium formate. The crystal belongs to the monoclinic space group C2, with unit-cell parameters a = 126.70 Å, b = 66.64 Å, c = 64.99 Å and the angles α = 90.0° β = 120.8° γ = 90.0°. A complete data set was collected at 1.5 Å resolution. Assuming the presence of a dimmer in the asymmetric unit, the solvent content was estimated to be about 46%. The structure was solved at 1.6 Å using molecular replacement to solve the phase problem and CGL coordinates was used as model. The refinement was satisfactory Rfactor and Rfree were respectively 17.98 and 20.71 and all amino acids residues were found in allowed regions. The primary structure showed 98% of identity to ConA and others ConA like lectins. The tridimensional structure observed showed high similarity to others already described structures, but some differences could be observed mainly on loop regions. These differences could be responsible for the distinct biological effects of these proteins