ABSTRACT

Soybean seeds contain 40-50% protein on a dry matter basis. Soybean proteins possess unique physiochemical properties suitable for various

1US Department of Agriculture, Agricultural Research Service, Soybean Genomics and Improvement Laboratory, PSI, Beltsville, MD 20705, USA. aMention of trade name, proprietary product or vendor does not constitute a guarantee or warranty of the product by the US Department of Agriculture or imply its approval to the exclusion of other products or vendors that also may be suitable. 2 US Department of Agriculture, Agricultural Research Service, Animal Biosciences and Biotechnology Laboratory, Beltsville, MD 20705, USA. 3US Department of Agriculture, Agricultural Research Service, Food Composition and Methods Development Laboratory, Beltsville, MD 20705, USA. *Corresponding author: savi.natarajan@ars.usda.gov

human and animal food uses. Soybean seed proteins are used in baby formula, flour, protein supplements, concentrates, and textured fibers. Several reviews (Mityko et al. 1990; Warner and Wemmer 1991) describe soybean seed proteins and the comprehensive studies that have been conducted to identify these proteins and understand their functions (Beachy et al. 1983b; Nielsen et al. 1995). Soybean storage proteins are grouped into two types based on their sedimentation coefficients, -conglycinin and glycinin (Thanh and Shibasaki 1978). -conglycinin, a 7S globulin, is a trimeric glycoprotein consisting of three types of non-identical but homologous polypeptide subunits. Glycinin is an 11S globulin and its subunits are composed of acidic and basic polypeptides linked by a disulfide bond. These two major groups account for about 70-80% of the total seed protein. Soybean seeds also contain less abundant proteins including -amylase, cytochrome c, lectin, lipoxygenase, urease, Kunitz Trypsin Inhibitor (KTI) and the Bowman Birk Inhibitor (BBI) of chymotrypsin and trypsin. Soybeans are also sources of several secondary metabolites including isoflavones, saponins, phytic acid, flatus-producing oligosaccharides, and goitrogens (Liener 1994; Friedman and Brandon 2001). In soybean, storage proteins are deposited in specialized protein storage vacuoles (PSV) and endoplasmic reticulum (ER)-derived protein bodies (Herman and Larkins 1999; Chrispeels and Herman 2000; Kinney et al. 2001).