ELECTROOPTICAL MEASUREMENTS DEMONSTRATE A LARGE PERMANENT DIPOLE-MOMENT ASSOCIATED WITH ACETYLCHOLINESTERASE

Acetylcholinesterase (AChE) from krait (Bungarus fasciatus) venom is a soluble, nonamphiphilic monomer of 72 kDa, This snake venom AChE has been analyzed by measurements of the stationary and the transient elec tric dichroism at different field strengths, The stationary Values of the dichroism are consistent with the orientation function for permane nt dipoles and are not consistent with the orientation function for in duced dipoles, The permanent dipole moment obtained by least-squares f its for a buffer containing 5 mM MES is 1000 D, after correction for t he internal directing field, assuming a spherical shape of the protein . The dipole moment decreases with increasing buffer concentration to 880 D at IO mM MES and 770 D at 20 mM MES. The dichroism decay time co nstant is 90 ns (+/-10%), which is clearly larger than the value expec ted from the size/shape of the protein and indicates contributions fro m sugar residues attached to the protein, The dichroism rise times obs erved at low field strengths are larger than the decay times and, thus , support the assignment of a permanent dipole moment, although it has not been possible to approach the limit where the energy of the dipol e in the electric field is sufficiently low compared to kT. The experi mental Value for the permanent dipole moment is similar to that calcul ated for a model structure of Bungarus fasciatus AChE, which has been constructed from its amino acid sequence, in analogy to the crystal st ructure of AChE from Torpedo californica.