OLIGOMERIZATION ENDOWS ENORMOUS STABILITY TO SOYBEAN AGGLUTININ: A Comparison of the stability of monomer and tetramer of Soybean Agglutinin
Sharmistha Sinha 1 and Avadhesha Surolia 1*
1 Molecular Biophysics Unit, Indian Institute of Science
* To whom correspondence should be addressed. E-mail: surolia{at}mbu.iisc.ernet.in.
Submitted on February 14, 2005
Revised on March 8, 2005
Accepted on 18 March 2005
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Abstract |
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Soybean agglutinin is a tetrameric legume lectin each of whose subunits are glycosylated. This protein shows a very high degree of stability when compared to the other proteins of the same family. In a previous work, it was shown that the unusual stability of the protein is due to a high degree of subunit interactions. In this study we present the thermodynamic parameters for the stability of SBA monomer. The monomeric species is found at pH 2 and below which is most populated at pH 1.9 as evident from size exclusion chromatographic and dynamic light scattering studies. The analyses of CD and fluorescence spectroscopy suggest that the monomer is well folded and it has certain characteristic features when compared to its tetrameric counterpart. The conformational stabilities of the tetramer and the monomer at the temperature of their maximum stabilities (310K) are 59.2kcal/mol and 9.8kcal/mol, respectively, indicating that oligomerization contributes significantly to the stability of the native molecule. Also, the Tg difference for the two forms of the protein is ~40K, while the difference in DCp is only 1.6kcal/mol/K. This suggests that the major hydrophobic core is present in the monomer itself and oligomerization involves mainly ionic interactions.
Key Words:
Lectins, Oligomers, Protein Folding, Protein Stability, Two-State Folding
