Structure-physicochemical function relationships of soybean glycinin at subunit levels assessed by using mutant lines

Glycinin is a hexameric protein composed of five kinds of subunits. The subunits are classified into two groups, group I (A1aB1b, A1b132, and A2B1a) and group II (A3B4 and A5A4B3). We purified four mutant glycinins composed of only group I subunits (group I-glycinin), only group II subunits (group II-glycinin), only A3B4 (A3B4-glycinin), and only A5A4B3 (A5A4B3-glycinin) from mutant soybean lines. The physicochemical properties of these glycinin samples were compared with those of the normal glycinin (11 S) composed of five kinds of subunits. The thermal stabilities (as measured by thermal denaturation midpoint temperatures) of 11 S, group I-glycinin, and group II-glycinin were similar to each other, although that of A3B4-glycinin was significantly lower than those of the others. The orders of aromatic and aliphatic surface hydrophobicities were the same: A3B4-glycinin > group II-glycinin > A5A4B3-glycinin > 11 S > group I-glycinin. The solubility of 11 S as a function of pH at mu = 0.5 was governed by that of group I-glycinin and followed this order at acidic pH: 11S = group I-glycinin > A3B4-glycinin > group II-glycinin = A5A4B3-glycinin. The order of emulsifying abilities was A5A4B3-glycinin > group II-glycinin > A3B4-glycinin > 11 S > group 1-glycinin. This order was consistent with that of the length of their hypervariable regions. Except for this relationship, there was no significant relationship among the other physicochemical properties of the mutant glycinins.