SATURATION MAGNETIZATION OF UREASES FROM KLEBSIELLA-AEROGENES AND JACK BEAN - NO EVIDENCE FOR EXCHANGE COUPLING BETWEEN THE 2 ACTIVE-SITE NICKEL IONS IN THE NATIVE ENZYMES

Saturation magnetization data for native jack bean and Klebsiella aerogenes ureases in buffers ranging from pD 6.4 to pD 9.7 have been collected at four fixed fields over the temperature range from 2 to 200 K. There is no evidence in these data sets for exchange coupling between the two nickel(II) ions at the active site of either urease. This conclusion contrasts with that of a previous report that the active site nickel ions of native jack bean urease are weakly exchange coupled (Clark, P. A.; Wilcox, D. E. Inorg. Chem. 1989, 28, 1326-1333). Each of our eight saturation magnetization data sets can be fitted by assuming that all nickel ions are magnetically isolated and comprised of a population of high-spin (S = 1) and low-spin (S = 0) species, with the diamagnetic contribution increasing with increasing pD.