A "molten globule" of Torpedo acetylcholinesterase undergoes thiol-disulfide exchange

Torpedo acetylcholinesterase is a disulfide-linked homodimer containing three intramolecular disulfide bonds, as well as a single free thiol on Cys-231. We report that in a "molten globule" state, produced by 1.5 M guanidine hydrochloride, this enzyme undergoes rapid intramolecular thiol-disulfide exchange, in the absence of reducing agents, resulting in the production of novel species. Most strikingly, this results in appearance of enzyme monomers. Chemical modification of the free thiol group prevents these changes. Unfolded acetylcholinesterase, namely in 5 M guanidine hydrochloride, also undergoes intramolecular thiol-disulfide exchange, including production of enzyme monomers, but at a much lower rate. Our data show that the molten globule state, in contrast to the native and unfolded states, is both compact and flexible, thus being especially amenable to thiol-disulfide exchange.