Bases moleculares do processamento pós-traducional na estrutura tridimensional de uma lectina de sementes de Dioclea grandiflora

Abstract

Lectins, specially the ones from plants, are widely studied and characterized by present great biotechnological potential such as antidepressant, antitumoral and anti- and pro-inflammatory activities. These properties take into account the mature form of the lectin, which is synthesized as an inactive glycoprotein precursor that is post-translationally modified by deglycosylation, proteolytic breakdown and polypeptide chain inversion. Deglycosylation makes the precursor able to bind carbohydrates. The deglycosylated precursor from Dioclea grandiflora lectin (pDGL) do not solely binds to monosaccharides, presenting hemagglutinating activity, but also presents higher affinity to mannose in relation to its mature counterpart. Although the post-translational processing is well understood, the structural basis for this different affinity pattern is less clear. In this way, the present work aimed to elucidate the crystallographic structure of pDGL in complex to mannose to identify the structural characteristics that determine its greater affinity to this hexose. Recombinant pDGL was cocrystallized with mannose and crystals were submitted to X-ray diffraction and the resulting data were computationally analyzed. In silico studies were carried out to compare the interaction pattern of mannose and more complex carbohydrates between pDGL and their mature form. The crystallographic structure of pDGL presented as a tetramer, conserving all structural characteristics of its mature counterpart, including the presence of non-repetitive secondary structures that fold the Carbohydrate Recognition Domain (CRD). However, pDGL seems unable to form stable tetramers, once it lacks two important interdimeric contacts orchestrated by histidine residues His22 and His194. Mannose bound at all four polypeptide chains being stabilized by several interactions, including a CH—π ligation. In silico analyses showed that pDGL, in relation to its mature form, binds with higher affinity to mannose and the trimannoside 3,6-di-O-(ɑ-D-manopyranosyl)-ɑ-D-mannopiyranose due its CRD presents lower depthness. Together, these results show that post-translational processing (i) allows the formation of stable tetramers and (ii) produces subtle changes in the disposal of CRD residues that decreases the affinity of the mature protein for carbohydrate molecules.