Assessing the Biomolecular Interactions of Pea Protein Fractions and Curcumin: Impact on In Vitro Protein Digestibility

Abstract

Leguminous plant proteins, such as pea proteins, are good alternatives to animal proteins due to their biological and techno-functional properties, which greatly benefit health. Unlike other legumes, they are high in lysine, which improves immunological function. Polyphenols, such as curcumin, have bioactive properties such as anti-hypertensive, antioxidant, and anti-diabetic properties, hereby promoting their effective use in food and nutraceuticals. Despite its significant biological value, the use of curcumin is limited due to water insolubility, rapid cell metabolism, degradation, and instability. Thus, the complexation of proteins and curcumin improves all its limitations and enhances high bioavailability, favoring the development of delivery systems. Furthermore, the protein-polyphenolic complex could facilitate high thermal stability and better techno-functional properties of protein such as foaming, emulsification, gelation, water and oil holding capacity. This study examined the complexation of pea protein fractions (albumin, globulin and glutelin) with curcumin and the influence of this interaction on protein digestibility. Results showed that the complexation was facilitated mostly by non-covalent interactions i.e. hydrophobic interactions. Curcumin tends to form compact structures with proteins which reduces the accessibility of digestive enzymes, thereby resulting in decreased protein digestibility in albumin and globulin and no protein hydrolysis in glutelin - mainly due to the tight sheet-like structures formed. The effect of ionic strength on the water-soluble fraction (pea albumin) and curcumin complexes and the extent of this influence on protein digestibility was investigated. The influence of the physiological condition (ionic strength) of the gastrointestinal tract impacted the characteristics of pea protein albumin-curcumin complexes, however, there were negligible changes in protein digestibility. This was mainly attributed to the formation of large aggregates or unstable complexes with increasing NaCl salt concentration which influenced protein digestibility revealed by dynamic light scattering and fluorescence microscopy. The mode and nature of binding of the curcumin to protein in the presence and absence of ionic strength revealed by fluorescence quenching showed more salt-induced effects despite no significant progress in protein digestion. Binding was more favorable in the absence of ionic strength (0 mM NaCl). These findings offer a novel understanding of the mechanisms underlying the interactions between pea protein and curcumin, their impact on protein digestibility, and provide valuable insights for optimizing curcumin loading in the development of delivery systems, achieving both maximum encapsulation efficiency and curcumin bioavailability, without compromising protein digestibility.