Modulation of physicochemical and conformational properties of kidney bean vicilin (phaseolin) by glycation with glucose: implications for structure-function relationships of legume vicilins

The structure-function relationships of plant oligomeric globulins are still not fully recognized. The present work investigated the influence of glycation with glucose (at 1:50 and 1:100 protein/sugar molar ratios; incubation periods of 2.5, 5.0, and 10.0 h) on the physicochemical and conformational properties of kidney bean vicilin (phaseolin), with the aim of understanding the structure-function relationships of legume vicilins. Protein solubility (PS), surface charge (isoelectric point) and hydrophobicity (H0), and secondary, tertiary, and/or quaternary conformations, as well as the emulsifying activities (emulsifying activity and emulsion stability indices, EAI and ESI) were evaluated. The 2.5 h incubation period of glycation led to least PS and highest H0, and after that, the PS and H0, on the contrary, gradually changed with increasing incubation period. The glycation increased the α-helix content and highly ordered secondary structures (α-helix+β-strand), as evidenced by far-UV circular dichroism (CD) spectroscopy. Combined analyses of differential scanning calorimetry, intrinsic emission fluorescence, and near-UV CD spectroscopy indicated that phaseolin underwent a tertiary conformation unfolding and subsequent rearrangement process (to form a new tertiary conformation), whereas the quaternary conformational flexibility progressively increased upon increasing degree of glycation. The conformation rearrangement was more distinct at the 1:100 molar ratio than at the 1:50 counterpart. The glycation at 5.0 and 10.0 h periods considerably increased the EAI, but only at the 1:50 molar ratio was the ESI progressively increased with the incubation period. These results confirmed that besides surface properties (e.g., PS and H0), the flexibility in tertiary and/or quaternary conformations played a major role in the emulsifying properties of glycated vicilins. The findings would have important implications for understanding the structure-function relationships of legume oligomeric globulins, thus providing a direction to further improve the surface-related functional properties of these proteins.