Study on the structure and formation mechanism of 15S globulin of soybeans
The structure and formation mechanism of 15S globulin of soybeans were investigated. After purification, 15S-rich fraction (75.6%) and 11S-rich fraction (94.3%) were obtained and further characterized with analytical ultracentrifugation (AUC) and transmission electron microscope (TEM). The results show that the molar masses of 11S and 15S are 313 kDa and 651 kDa, respectively, and the weight percentages of the monomer , dimer, and trimer in the TEM images are consistent with those of 11S, 15S, and 21S fractions measured by AUC, indicating that 15S and 21S are dimers and trimers of 11S monomer, respectively. The size of the 15S molecules is 21.1 nm × 10.4 nm, suggesting that two 11S molecules interact with each other from the side position. Moreover, the contents of subunits were determined by a combination of AUC and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with the addition of 20 mM sodium dodecyl sulfate (SDS) and 10 mM 2-mercaptoethanol (2-ME). The content of the species with a sedimentation coefficient of 4.3 S in 15S-rich fraction, which is assigned as AB-SS-AB subunits, is significantly higher than that in 11S-rich fraction with the addition of SDS; this species disappears with the addition of 2-ME, demonstrating that AB-SS-AB subunits are formed by disulfide groups from two AB subunits. These disulfide groups may locate in a non-polar interior, which is not accessible to 2-ME when the protein remains its native structure.
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