Fluorescence, circular dichroism, NMR, and docking studies of the interaction of the alkaloid malbrancheamide with calmodulin

A new malbrancheamide analogue, isomalbrancheamide B (3), along with three known compounds, malbrancheamide (1), isomalbrancheamide (2), and premalbrancheamide (4), were isolated in higher yields from the alkaloid fraction of the fungus Malbranchea aurantiaca. The interaction of the alkaloids 1--4 with calmodulin (CaM) was analyzed using different enzymatic, fluorescence, spectroscopic, nuclear magnetic resonance (NMR), and molecular modelling techniques. On the basis of the enzymatic and fluorescence experiments, malbrancheamides 1--3 are classical CaM inhibitors. Compound 4, however, did not quench the extrinsic fluorescence of the CaM biosensor indicating that it could be a functional inhibitor. Circular dichroism, NMR, and molecular modelling studies revealed that 1 binds to CaM in the same hydrophobic pocket than the chlorpromazine and trifluoperazine, two classical CaM inhibitors. Thus, malbrancheamide and related monochlorinated analogues are compounds with a high potential for the development of new therapeutic agents, involving CaM as their molecular target.