Surface plasmon resonance, fluorescence, and circular dichroism studies for the characterization of the binding of BACE-1 inhibitors

The mechanism of action underlying β-secretase 1 (BACE-1) inhibition was characterized by a surface plasmon resonance (SPR) method using primary amino groups to immobilize OM99-2, a well-known highly potent peptidic BACE-1 inhibitor, on the carboxyl groups of the dextran layer of a sensor chip. The diluted BACE-1 was mixed with buffer or the test compound and the mixture was flushed through the chip. BACE-1 binding to the immobilized peptide inhibitor was quantified. This SPR method was used to identify BACE-1 inhibitor binding sites and the mechanism of action (competitive/noncompetitive) and to validate findings of fluorescence resonance energy transfer (FRET) inhibition studies. To support this, a multimethodological approach (circular dichroism and fluorescence spectroscopy) was applied in parallel to FRET inhibition studies to characterize the binding modes of peptidic and nonpeptidic BACE-1 inhibitors. Circular dichroism spectroscopy served to correlate the conformation of BACE-1 with enzymatic activity and to monitor secondary structure changes upon ligand binding. In a complementary approach, direct fluorescence spectroscopy was used to characterize different BACE-1 inhibitor binding sites. The influence of pH and inhibitors on BACE-1 secondary structure was also elucidated. This multimethodological approach was applied to identify binding modes of bis(7)-tacrine and myricetin in comparison with well-known peptidic inhibitors.