Organophosphate spin-label studies of inhibited esterases, alpha-chymotrypsin and cholinesterase

ESR study of free and immobilized elastase

Porcine pancreatic elastase (EC 3.4.21.11) has been immobilized on polyacrylamide beads using glutaraldehyde ad bridging reagent without important loss of catalytic activity. A nitroxide spin label, 1-oxyl-2,2,5,5-tetramethyl-4-piperidinyl-ethylphosphonofluoridate, reacting covalently with the serine-195 residue of the active centre of free elastase was used as a conformational and dynamical electron spin resonance probe. This signal is quenched by (Cu2+) which bind specifically at the active site at a distance of 7 A from the nitroxide group. This distance is not significantly affected by the fixation on the solid support. The electron spin resonance lineshape analysis indicates some mobility of the spin label with respect to the native protein. This restricted motion, which is pH dependent, is not noticeably modified by the immobilization of the enzyme. This immobilization has therefore induced no large conformational change of the protein in the vicinity of the active centre. Thermal denaturation of elastase in homogeneous solution is irreversible. Immobilization on the polyacrylamide beads results in 70% reversibility, but the temperature of denaturation is not modified.

An ESR study of the influence of some physico-chemical factors on the conformation of a postsynaptic acetylcholinesterase

1. In a previous ESR study of a membrane acetylcholinesterase (EC 3.1.1.7) we found, contrary to observations by other authors, spectra indicating that the active serine might be located in a pocket of the enzyme surface. In order to inquire into this possibility, ESR spectra were studied under the influence of different physico-chemical factors known to cause an unfolding of proteins. 2. The active serine of the postsynaptic membrane acetylcholinesterase of Torpedo marmorata electric organ was spin labeled using 1-oxyl-2, 2, 6, 6-tetramethyl-4-piperidinyletoxyphosphonofluoridate. 3. The effect of the chosen physico-chemical factors was an increase in the rotational freedom of spin labels; this result corroborates the suggestion that the active center of our acetylcholinesterase preparation is located in a pocket.