Liposome-catalyzed unfolding of acetylcholinesterase from Bungarus fasciatus.

Abstract

The kinetics of thermal inactivation of acetylcholinesterase from the venom of the snake, Bungarus fasciatus, were studied at 45-54 degrees C. An Arrhenius plot reveals an activation energy of 113 kcal/mol. The thermally denatured enzyme displays the spectroscopic characteristics of a partially unfolded 'molten globule' state. The rate of thermal denaturation is greatly enhanced in the presence of unilamellar vesicles of dimyristoylphosphatidylcholine, the energy barrier for the transition being lowered from 113 to 52 kcal/mol. In contrast to our findings for partially unfolded Torpedo californica acetylcholinesterase [Shin et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 2848-2852], the thermally denatured snake enzyme does not remain bound to the liposomes but is released after unfolding and subsequently aggregates. The liposomes thus serve as catalysts for unfolding of the snake enzyme, and its rate of unfolding in the presence of liposomes can be described by the Michaelis-Menten equation (Km = 8 x 10(-7) M). The phospholipid vesicles display a catalytic turnover number of kcat approximately 4 min-1, assuming 15 binding sites per vesicle for the snake acetylcholinesterase.