Thermal and chemical denaturation of Colocasia esculenta tuber agglutinin from 22 to unfolded state

The major tuber storage protein of Colocasia esculenta, is a monocot mannose-binding, widely used dietary lectin, containing two polypeptides of 12.0 and 12.4kDa. By both gel filtration and dynamic light scattering at pH 7.2, the lectin has a (22) form of apparent molecular mass of 48.2kDa and a hydrodynamic radius of 6.1 +/-.2nm; however, at pH 3, it migrates as and has a reduced hydrodynamic radius of 4.6 +/-.3nm. Our circular dichroism spectroscopy studies show that the lectin retains approximately 100% of its secondary structure between pH 2-8, going down to similar to 90% for extreme acidic/alkaline conditions. The fluorescence emission maxima of 346 to 350nm for pH 4 to 10 show that the tryptophan residues are relatively exposed. The unfolding is a simple two-state process, N-4 4U, as seen in our denaturation scan profiles. These denaturation profiles, monitored separately by fluorescence, far-UV CD, and near-UV CD, are completely super imposable. Analyses of these profiles provide an estimate of several thermodynamic parameters at each guanidinium chloride concentration, including the melting temperature T-g, which is 346.9K in 0M, but lowers to 321.8K in 3.6M. Dimeric and tetrameric interfaces observed in the crystal structure for the same protein are used to rationalize solution data in some detail.