Polyamine Binding to Acetylcholinesterase Revealed by Molecular Dynamics and Surface Plasmon Resonance

Acetylcholinesterase (AChE) is a cholinergic enzyme that hydrolyzes acetylcholine to terminate neurotransmission. Inhibition of AChE prevents the breakdown of acetylcholine, leading to its accumulation and thereby providing therapeutic relief for memory deficits in Alzheimer's disease. While the inhibitory effects of synthetic ligands on AChE have been widely studied, the modulation of its activity by endogenous polyamines such as spermine and putrescine remains poorly understood at the molecular level. Previous kinetic studies have shown that polyamines can modulate AChE activity, exhibiting an inhibition effect at substrate concentrations less than ∼200 μM. In this work, we characterized the binding modes of polyamines to AChE using molecular dynamics simulations and binding free energy calculations, and measured the dissociation constants by surface plasmon resonance. Our results show that spermine and putrescine bind to the active-site gorge of AChE by interacting with residues of the peripheral anionic site, the catalytic site, and other important residues within the gorge. As a consequence, they block the pathway of the substrate toward the active site. This theoretical approach helps to understand the mechanism responsible for the inhibitory effects of polyamines on AChE activity observed experimentally.