PMID: 22674800
Paz A, Roth E, Ashani Y, Xu Y, Shnyrov VL, Sussman JL, Silman I, Weiner L
Structural and functional characterization of the interaction of the photosensitizing probe methylene blue with Torpedo californica acetylcholinesterase.
Protein Sci. 2012 Jun 1. doi: 10.1002/pro.2101.
The photosensitizer, methylene blue (MB), generates singlet oxygen that irreversibly inhibits Torpedo californica acetylcholinesterase (TcAChE). In the dark it inhibits reversibly. Binding is accompanied by a bathochromic absorption shift, used to demonstrate displacement by other AChE inhibitors interacting with the catalytic 'anionic' subsite (CAS), the peripheral 'anionic' subsite (PAS), or bridging them. MB is a noncompetitive inhibitor of TcAChE, competing with reversible inhibitors directed at both 'anionic' subsites, but a single site is involved in inhibition. MB also quenches TcAChE's intrinsic fluorescence. It binds to TcAChE covalently inhibited by a small organophosphate, but not an organophosphate containing a bulky pyrene. Differential scanning calorimetry shows a 8 degrees increase in the denaturation temperature of the MB/TcAChE complex relative to native TcAChE, and a >2-fold increase in cooperativity of the transition. The crystal structure reveals a single MB stacked against Trp279 in the PAS, oriented down the gorge towards the CAS; it is plausible that irreversible inhibition is associated with photo-oxidation of this residue and others within the active-site gorge. The kinetic and spectroscopic data showing that inhibitors binding at the CAS can impede binding of MB are reconciled by docking studies showing that the conformation adopted by Phe330, midway down the gorge, in the MB/TcAChE crystal structure, precludes simultaneous binding of a second MB at the CAS. Conversely, binding of ligands at the CAS dislodges MB from its preferred locus at the PAS. The data presented demonstrate that TcAChE is a valuable model for understanding the molecular basis of local photo-oxidative damage.