Improvement of nutritional value and functional properties of soybean glycinin by protein engineering

Glycinin is one of the predominant storage proteins of soybean. To improve its functional properties (heat-induced gelation and emulsification) and/or nutritional value, the A1aB1b proglycinin subunit was modified on the basis of genetically variable domains suggested from the comparison of amino acid sequences of glycinin-type globulins from various legumes and nonlegumes and the relationships between the structure and the functional properties of glycinin. Thus, nucleotide sequences corresponding to each of the variable domains were deleted from the cDNA encoding the A1aB1b proglycinin, and a synthetic DNA encoding four continuous methionines was inserted into the cDNA region corresponding to each of the variable domains. Expression plasmids carrying the modified cDNAs were constructed and expressed in Escherichia coli strain JM105. Some of the modified proteins were accumulated as soluble proteins in the cells at a high level and self-assembled. They exhibited functional properties superior to those of the native glycinin from soybean, which establishes the possibility of creating theoretically designed novel glycinins with high food qualities.