pH-Dependent oligomerisation of γ-conglutin: A key element to understand its molecular mechanism of action

Despite extensive research carried out on lupin seed γ-conglutin neither its mechanism of action as a hypoglycaemic nutraceutical compound nor its physiological role for the plant has been unveil. This article revealed a pH-dependent reversible association/dissociation equilibrium involving monomer, dimer and hexamer of Lupinus angustifolius γ-conglutin. The interaction between different oligomeric forms of this protein is reversible, and spectroscopic studies showed that the intact structure of γ-conglutin was preserved under the tested environmental conditions tested (pH 4.5–7.5). The obtained results prove that the hexameric form was preferred under basic conditions and was stabilised by a number of bonds formed upon association of individual protomers. The simultaneous occurrence of several oligomeric forms at a given pH value was shown, and their share was strongly driven by protein concentration. The main changes in oligomerisation of γ-conglutin take place in a pH range of 4.5–6.0, correlating with the pKaR values of the amino acid residues of His (6.0), Glu (4.1), and Asp (3.9). Moreover, a structural model of the protein in hexamer assembly was obtained based on small-angle X-ray scattering (SAXS) and negative staining cryo-electron microscopy (cryo-EM) analyses. The presented study provides essential knowledge about the colloidal dynamics and stability of γ-conglutin in solution, improving our understanding of fundamental environmental factors that could affect the health-promoting activity of this lupin seed protein.