Soy proteins: strcture, interaction and functions

Epidemiological evidence suggests that dietary factors play an important role in human health. Soy products are associated with cardiovascular protection and decreased cancer risk. The beneficial effects of soy consumption have been associated to several bioactive components including proteins, isoflavones, carbohydrates, but it is suspected that most effects on health are due to their combined and synergic action. A second issue related to the soy bioactivity/allergenicity is glycosylation of soy proteins. In soy the 7S subunit is glycosylated whereas the 11S subunit is not glycosylated. However, non-enzymatic glycosylation of both subunits can be induced by industrial processes (Maillard reaction). Another bioactive component of the soy protein fraction is the Bowman-Birk inhibitor (BBI). According to the great stability of this molecule, it is expected that the bioactivity of BBI during food processing might be modulated by interaction which other soy components like carbohydrates and isoflavones. However, there is very poor structural information available on the bioactive soy components, on their interactions at the molecular level, and still less is known about the structure-function relationship in these systems. This research project is aimed to study the interaction products of soy proteins (glycinin, beta-conglycinin and BBI) with other bioactive compounds present in soy, namely sugars and isoflavones. The research envisages the production of the interaction products in systems mimicking the industrial production, and their characterization by the combined use of a biochemical/immunological approach (Spanish research group) and of high resolution mass spectrometric techniques (to be carried out by the Italian research group). Furthermore, we plan to obtain data information related to structure modifications of these proteins due to deglycosylation and glycation and find out the link between structure and bioactivity (allergenicity and protease inhibitor properties). To this aim, β-conglycinin will be purified from commercial soy protein isolate (SPI), deglycosylated and analysed by mass spectrometry, providing information related to changes in protein structure. Similarly, to evaluate the extent of thermal treatment on soy products, powdered Maillard reaction system will be performed, aiming to enhance the antioxidant activity and other favourable properties of these vegetable proteins which are employed as functional food ingredients. The bioactivity of the compounds obtained will be evaluated by the Spanish group. In parallel, BBI will be fractionated from SPI and submitted to the heat treatment in the presence of sugar (fructose) and isoflavones in concentrations similar to those employed for industrial processing of soymilk (132ºC for 50s). Composition and conditions will be selected taking to account European consumers preferences, potentiality as natural source of BBI of the diet and processing conditions currently applied by the food industry with the intention to inactivate BBI activity. The heated mixture will be analysed before and after thermal processing employing electro-migration techniques (SDS PAGE, CZE), furosine analysis and mass spectrometry which will provide data on chemical changes induced by heating a complex matrix (matrix effect). Bioactivty of BBI will be also determined before and after heat treatment. Changes in bioactivity will be correlated with the chemical changes observed. The novel products will be also isolated from the mixture and further characterized. The structural information acquired in this research project could be ultimately used to design highly specific modifications in soy proteins to improve functionality and nutritional quality in soy-based products. Results derived from BBI studies will provide information related to the impact of other food components and processing conditions on bioactivity and to elucidate the potentially therapeutical BBI modifications.