Catalysis by Acetylcholinesterase in Two-Hydronic-Reactive States and Isotope Effects.
Abstract
2 Low (2) H20 effects (1.0-1.5) for the parameter k(cat)/K(m) in the hydrolysis of various substrates by acetylcholinesterase is due to normal (2) H20 effects (1.8-2.8) for the parameter k(cat) and (2) H20 effects of 1.0-2.5 for the parameter K(m). The analysis and interpretations of 2 H20 effects in the literature utilizing the parameter k(cat)/Ka, which lead to the proposal of isotope insensitivity of the catalytic steps and the hypothesis of a rate-limiting substrate induced-fit conformational change, are incorrect. Since k(cat), is the only parameter that can represent the hydron-transfer step solely, the (2) H20 effect can most appropriately be evaluated by using this parameter. The improved binding -0.84 to -2.09 kj/mol in H20 obscures the normal (2) H20 effect on k(cat) when the ratio k(cat)/Km is utilized. Consistent with this, reversible inhibition constant, K(I(com), for phenyltrimethylammonium lead to K, = 24.5 +/- 3.5 PM and KI= 39 +/- 3 uM, a 2H20 effect of 1.59 +/- 0.26. pH-dependence of and k(cat)/K(m)in 2H20 are subject to variability of the pK(app) values, as evaluated in terms of the two-hydronic-reactive states (EH and EH2) of AcChE. Such effects are result of uneven decrease in 2H2O of the kinetic parameters k(cat), k(cat)/K(m) for the EH2 state relative to k(cat), k(cat)/km for the EH state, leading to variable shifts in pK(app) between 0.5 and 1.2 pH units.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 13, 1993
- Accession Number
- ADP008830
Entities
People
- Erdjan Salih
Organizations
- Brandeis University