LIPOSOME-CATALYZED UNFOLDING OF ACETYLCHOLINESTERASE FROM BUNGARUS-FASCIATUS

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 d enaturation is greatly enhanced in the presence of unilamellar vesicle s of dimyristoylphosphatidylcholine, the energy barrier for the transi tion being lowered from 113 to 52 kcal/mol. In contrast to our finding s for partially unfolded Torpedo californica acetylcholinesterase [Shi n et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 2848-2852], the ther mally denatured snake enzyme does not remain bound to the liposomes bu t is released after unfolding and subsequently aggregates. The liposom es 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 (K-m = 8 x 10(-7) M). The phospholipid vesi cles display a catalytic turnover number of k(cat) similar to 4 min(-1 ), assuming 15 binding sites per vesicle for the snake acetylcholinest erase.