Re-evaluation of the kinetic mechanism of the choline acetyltransferase reaction

Abstract

Initial velocity patterns for human placental choline acetyltransferase show a series of converging lines for both the forward and reverse reaction; with Kacetyl coenzyme A = 11.9 micron, Kcholine = 0.41 mM, KCOA = 8.8 micron, and Kacetylcholine = 1.3 mM. The relative rates of acetylcholine synthesis (Vf) to acetylcholine breakdown (Vr) is 4.5. Product inhibition by acetylcholine is competitive with respect to choline and noncompetitive with respect to acetyl-CoA, while product inhibition by choline is competitive with respect to acetylcholine and noncompetitive with respect to coenzyme A. Chlorocholine, diethylaminoethanol, and acetylaminocholine were used as dead-end inhibitors and shown to inhibit competitively with respect to acetylcholine, and noncompetitively with respect to choline, acetyl-CoA, and CoA. At high choline concentrations, uncompetitive substrate inhibition is observed, and inhibition by acetylaminocholine changes from noncompetitive to competitive. Comparing the reactivity of dimethylaminoethanol to choline, and acetyldimethylaminoethanol to acetylcholine, the maximal velocities obtained with these analogues was approximately 25% of the natural substrates. These data are not consistent with the previously proposed ordered Theorell-Chance reaction mechanism, and have been interpreted in terms of a random binding mechanism.