High-affinity tamoxifen analogues retain extensive positional disorder when bound to calmodulin

Area of Science:

• Biochemistry
• Structural Biology
• Pharmacology

Background:

• Calmodulin (CaM) is a crucial calcium-binding protein involved in cellular signaling.
• Idoxifene, a tamoxifen derivative, is a potent inhibitor investigated for breast cancer treatment.
• Understanding drug-protein interactions is vital for therapeutic development.

Purpose of the Study:

• To elucidate the structural and dynamic aspects of the complex formed between calcium-loaded calmodulin (CaM) and idoxifene.
• To investigate the binding mechanism and affinity of idoxifene to CaM.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy to determine structural constraints.
• Fluorescence measurements to assess binding dynamics and affinity.
• Analysis of intermolecular Nuclear Overhauser Effect (NOE) restraints.

Main Results:

• High-affinity binding of idoxifene to CaM (nanomolar range) was observed.
• Idoxifene molecules are located near CaM's hydrophobic patches but do not fill the pocket.
• Multiple idoxifene orientations and rapid positional switching were required to satisfy NOE restraints.
• CaM adopts a conformation where N- and C-terminal domains are in close proximity.

Conclusions:

• The CaM-idoxifene complex exhibits high-affinity binding without losing significant positional dynamics.
• Idoxifene's binding mode differs from lower-affinity antagonists, highlighting unique interactions.
• This study provides insights into how potent inhibitors achieve high affinity through dynamic interactions.