Modulation of the secondary and tertiary structures of African yam bean (Sphenostylis stenocarpa)seed globulins, albumins and protein concentrate by pH and NaCl

Abstract

African yam bean (Sphenostylis stenocarpa) seed flour was extracted with 0.5 M NaCl and the supernatant dialyzed against water to obtain two protein products, the water‐soluble albumin and water‐insoluble globulin. The flour was also extracted separately with alkaline water followed by acid‐induced protein precipitation at pH 5.0 to produce a protein concentrate. Amino acid composition was similar for the three protein products and showed a low Arg/Lys ratio. The three protein products had higher contents of the α‐helix fraction than the β‐strand but the level of unordered secondary structure was highest at pH 8.0. The near‐UV chromatograms showed phenylalanine and tyrosine transitions in the albumin, whereas only the phenylalanine transition was seen in globulin and protein concentrate. Addition of increasing levels of NaCl led to increased structural rigidity in the albumin and protein concentrate but not so much in the globulin. Practical applications Plant proteins continue to be highly desirable as functional ingredients in foods, especially low fat products. However, functionality of seed proteins from under‐utilized crops has not been properly studied. It is well known that food matrices vary in ionic strength and pH, which then determine protein functionality during processing and storage. Since protein functionality is directly dependent on structural conformation, knowledge of the structural properties of seed proteins at different pH and salt concentrations could facilitate formulation of food products with desirable or novel attributes.