Binding Kinetics versus Affinities in BRD4 Inhibition.

Area of Science:

• Biochemistry
• Molecular Biology
• Drug Discovery

Background:

• Bromodomains (BRDs) are protein modules that bind to acetylated lysine on histones.
• Human BRD4 is a key drug target for various diseases, with several inhibitors developed.
• The precise inhibition mechanism, particularly binding kinetics, remains unclear.

Purpose of the Study:

• To elucidate the inhibition mechanism of BRD4 inhibitors, focusing on binding kinetics.
• To investigate the role of dynamic loop motions and binding pocket size in inhibitor efficacy.
• To explain the differential activity between (+)-JQ1 and (-)-JQ1 enantiomers.

Main Methods:

• Classical molecular dynamics (MD) simulations.
• Quantum mechanics/molecular mechanics (QM/MM) MD simulations.
• Analysis of dynamic characteristics of the ZA-loop in BRD4.

Main Results:

• The ZA-loop's dynamic characteristics and its correlation with binding pocket size were revealed.
• (-)-JQ1 showed thermodynamic feasibility but kinetic infeasibility for BRD4 binding.
• (+)-JQ1 demonstrated both thermodynamic and kinetic feasibility, explaining its high inhibitory effect.
• Specific residues (L92/L94/Y97 in ZA-loop, Asn140 in BC-loop) were identified as critical for (+)-JQ1 binding/release kinetics.

Conclusions:

• Ligand binding kinetics and flexible loop motions are crucial for BRD inhibitor design, beyond static binding affinity.
• The findings provide insights for developing selective inhibitors targeting the BRD family.
• Understanding dynamic interactions can optimize drug design for BRD-related diseases.