Spectroscopic studies on the pH-dependent structural dynamics of γ-conglutin, the blood glucose-lowering protein of lupin seeds.

γ-Conglutin is a blood glucose-lowering protein purified from lupin (Lupinus albus, L.) seed. Despite various features of this protein have already been studied, no function in the seed nor any mechanism of action as a hypoglycemic nutraceutical compound have been identified so far. The lupin protein was shown to exist both in monomeric and multimeric forms as a function of pH. However, a detailed description of the pH-dependent structural dynamics of this protein, as the basis to investigate the reason/s of its functional behaviour, is not available yet. In this study, multiple and independent spectroscopic approaches, including light scattering associated to size exclusion chromatography of both untreated and covalently cross-linked protein, near and far UV circular dichroism, intrinsic and extrinsic fluorescence measurements, have been used to monitor oligomeric and conformational modifications caused by pH changes. Altogether, the results revealed a tetramer-dimer-monomer transition between neutral to slightly acidic pH and a dramatic and abrupt conformational change below pH 3.5. According to these findings, a model depicting γ-conglutin structural dynamics was drawn. This model highlights the primary role of amino acid side group electrostatic interactions in the oligomer association/dissociation equilibria and in the pH-driven collapse of the native conformation.