Circular dichroism and conformational transitions of leucoagglutinin.

Conformational transitions of leucoagglutinin from kidney bean (Phaseolus vulgaris) were studied by the circular dichroism (CD) probe. The CD spectrum of leucoagglutinin in the far ultraviolet zone was similar to the spectrum of concanavalin A, indicating the presence of high amount of the beta pleated sheet structure. The CD spectrum of the leucoagglutinin in the near ultraviolet zone was similar to the CD spectra of peanut and soybean agglutinins. The conformation of leucoagglutinin was not significantly affected by alkali at pH 9.4 or 10.4; however, at pH 11.2 the conformation was disrupted. Appearance of a new CD band at 255 nm was related to ionized OH groups of tyrosine side chains. Acidification of the strongly alkaline solutions leads to a loss of all near ultraviolet bands and formation of large amounts of pleated sheet structure in this denatured lectin. Native leucoagglutinin was resistant to acid, as only a slight weakening of the CD bands was observed at pH values as low as 2.1 to 2.4. These changes (at pH 2.5--2.8) were facilitated by EDTA, and the demetallized lectin slowly precipitated from these solutions. Sodium dodecyl sulfate (SDS) disorganized the native structure of leucoagglutinin, and the secondary structure was reorganized into new conformations of moderate alpha-helix contents. These transitions were strongly promoted by acid, enhancing the helix content to about 30%.