Despite the enzyme’s near-diffusion control processivity, which might suggest an exposed and easily-accessible catalytic domain, its active site lies buried in a deep and narrow gorge. The surface of this cavity includes fourteen strictly conserved residues.  Among those, a single tryptophan residue engages the ACh ammonium moiety in a cation-pi interaction; this is believed by Sussman et al. to be significantly responsible for pre-enzymatic ACh capture.