THE ACTIVE-SITE METAL COORDINATION GEOMETRY OF CADMIUM-SUBSTITUTED ALCOHOL-DEHYDROGENASE - A THEORETICAL INTERPRETATION OF PERTURBED-ANGULAR-CORRELATION OF GAMMA-RAY MEASUREMENTS

The structure of eleven complexes of cadmium-substituted alcohol dehyd rogenase with or without coenzyme and with different non-protein cadmi um ligands has been estimated by combined quantum chemical and molecul ar mechanical geometry optimisations. The geometry of the optimised co mplexes is similar to the crystal structure of cadmium-substituted alc ohol dehydrogenase, indicating that the method behaves well. The optim ised structures do not differ significantly (except for the metal bond lengths) from those of the corresponding zinc complexes, which shows that cadmium is a good probe of zinc coordination geometries. The elec tric field gradients at the cadmium nucleus have been calculated quant um chemically at the MP2 level with a large cadmium basis set, and the y have been used to interpret experimental data obtained by perturbed angular correlation of gamma-rays. The experimental and calculated fie ld gradients (all three eigenvalues) differ by less than 0.35 a.u. (3. 4.10(21) Vm(-2)), the average error is 0.11 a.u., and the average rela tive error in the two largest eigenvalues of the field gradients is 9% . Calculated field gradients of four-coordinate structures agree bette r with the experimental results than do those of any five-coordinate m odel. Thus, the results indicate that the catalytic metal ion remains four-coordinate in all examined complexes. Two measurements are best e xplained by a four-coordinate cadmium ion with Glu-68 as the fourth li gand, indicating that Glu-68 probably coordinates intermittently to th e catalytic metal ion in horse liver alcohol dehydrogenase under physi ological conditions.