Direct detection of acetylcholinesterase inhibitor binding with an enzyme-based surface plasmon resonance sensor

Acetylcholinesterase (AChE) inhibitors are potentially lethal but also have applications as therapeutic drugs for neurodegenerative diseases such as Alzheimer’s. Enzyme inhibitor binding are difficult to be detected directly by surface plasmon resonance (SPR) due to their small molecular weight. In this article, we describe the detection of AChE inhibitor binding by SPR without the use of competitive binding or antibodies. AChE was immobilized on the gold surface of an SPR sensor through covalent attachment to a self-assembled monolayer (SAM) of a COOH-terminated alkanethiol. The activity of the immobilized protein and the surface density were determined by using a standard photometric assay. Binding of two reversible inhibitors, which are used as therapeutic drugs, was detectable by SPR without the need to further modify the surface or the use of other reagents. The binding affinities (K_A) obtained from the fits were 3.8 × 10³ M⁻¹ for neostigmine and 1.7 × 10³ M⁻¹ for eserine, showing a higher affinity of the sensor for neostigmine. We believe that the SPR sensor’s ability to detect these inhibitors is due to conformational changes of the enzyme structure on inhibitor binding.