Anatomy of acetylcholinesterase catalysis: reaction dynamics analogy for human erythrocyte and electric eel enzymes

The anatomy of catalysis (i.e., reaction dynamics, thermodynamics and transition state structures) is compared herein for acetylcholinesterases from human erythrocytes and Electrophorus electricus. The two enzymes have similar relative activities for the substrate o-nitrochloroacetanilide and o-nitrophenyl acetate. In addition, with each substrate K values and solvent deuterium kinetic isotope effects for kES and kE are similar for the two enzymes. Solvent isotope effects in mixed isotopic buffers indicate that the acylation stages of o-nitrochloroacetanilide turnover by the two enzymes are rate-limited by virtual transition states that are weighted averages of contributions from transition states of serial chemical and physical steps. Similar experiments show that the transition states for Vmax of o-nitrophenyl acetate turnover by the two enzymes are stabilized by simple general acid-base (i.e., one-proton) catalysis. These comparisons demonstrate that acetylcholinesterases from diverse sources display functional analogy in that reaction dynamics and transition state structures are closely similar.