Nuclear magnetic resonance studies on the pKa values and interaction of ionizable groups in bromelain inhibitor VI from pineapple stem.

Bromelain inhibitor VI (BI-VI), a cysteine proteinase inhibitor from pineapple stem, is a unique double-chain molecule composed of two distinct domains A and B. In order to clarify the molecular mechanism of the proteinase-inhibitor interaction, we investigated the electrostatic properties of this inhibitor. The inhibitory activity toward bromelain was revealed to be maximal at pH 3-4 and the gross conformation to be stable over a wide range of pH. Based on these results, pH titration experiments were performed on the proton resonances of BI-VI in the pH range of 1.5-9.9, and pKa values (pKexp) were determined for all carboxyl groups and alpha-amino groups. The pKexp were also compared with theoretical values calculated from the NMR-derived structures of BI-VI. The electrostatic surface potential map constructed using the pKexp values revealed that BI-VI possesses continuous negatively charged and scattered positively charged regions on the molecular surface and both regions appear to serve for docking properly with a basic target enzyme. Furthermore, it was suggested that the ionic interaction of the inhibitor with the target enzyme is primarily important for the inhibition, which seems to involve some carboxyl groups in the inhibitor and a thiol group in the proteinase.