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Anticarcinogenic Bowman Birk Inhibitor Isolated from Snail Medic Seeds (Medicago scutellata): Solution Structure and Analysis of Self-Association Behavior†
journal contribution
posted on 2003-02-20, 00:00 authored by M. Catalano, L. Ragona, H. Molinari, A. Tava, L. ZettaThe high-resolution three-dimensional structure of a Bowman Birk inhibitor, purified from
snail medic seeds (Medicago scutellata) (MSTI), has been determined in solution by 1H NMR spectroscopy
at pH 5.6 and 27 °C. The structure of MSTI comprises two distinct symmetric domains each composed
of a three-stranded β-sheet containing a VIb type loop, where the active sites are located. A characteristic
geometry of three aromatic residues confers stability to this protein, and we observe that this feature is
conserved in all the Bowman Birk inhibitors of known structure. The two active domains exhibit different
conformational features: the second domain displays higher flexibility and hydrophobicity with respect
to the first one, and these properties have been correlated to a lower trypsin inhibitory specificity, in
agreement with titration studies that have shown a stoichiometric ratio MSTI:trypsin of 1:1.5. NMR analysis
indicated that MSTI undergoes self-association at concentrations higher than 2 mM, and the residues involved in this mechanism are localized at opposite faces of the molecule, having the highest positive and
negative potential, respectively, thus indicating that electrostatic intermolecular interactions are the driving
forces for MSTI association. Most of the residues affected by self-association are highly conserved in
BBIs from different seeds, suggesting a functional relevance for these charged superficial patches, possibly
involved in the interaction with other enzymes or macromolecules, thus triggering anti-carcinogenic activity.