Structure of the core region of the soybean β-conglycinin α′ subunit

The crystal structure of the core region of the α′ subunit (α) of soybean β-conglycinin has been determined at 2.3 Å resolution. α was superimposed on the known crystal structure of the β-conglycinin β subunit with a small root-mean square deviation of 0.77 Å, which is consistent with the high sequence identity of 75.5% between α and the β subunit. It is known that the thermal stability of the β subunit is higher than that of the α′ subunit and that their thermal stabilities are conferred by highly homologous core regions. Comparisons of the three-dimensional structures and primary sequences between α and the β subunit suggest that five factors account for this difference between subunits as regards the difference in thermal stability: (i) the total cavity volume is larger in α, (ii) the cluster of charged residues at the intermonomer interface is smaller in α and α lacks the intermonomer salt bridge of the β subunit, (iii) the solvent-accessible surface is more hydrophobic in α, (iv) there are fewer proline residues in α and (v) a loop region between helix 3 and strand J′ in α is more flexible owing to the insertion of five additional residues. Although more hydrogen bonds were found in α, this difference should be more than compensated for by the combined contributions of these other factors.