The Conformational Transition of Sperm Whale Ferrimyoglobin in the Presence of Several Equivalents of Cupric Ion

Abstract

Abstract When sperm whale ferrimyoglobin is mixed, at pH 6.6 to 6.7 and 23°, with 2 to 7 moles of copper(II) ion per mole of protein, at a concentration of the latter of 0.2 mm, the copper(II) ions become bound to the protein within a few seconds, as shown by electron paramagnetic resonance spectroscopy. In a slower process, which can be correlated with a previously described conformational transition, the characteristic high spin iron(III) signal at g = 6 is reduced in intensity without change in shape. The extent of the reduction in the heme iron(III) signal at equilibrium depends linearly on the mole ratio of the copper(II); it is essentially complete at a mole ratio of 7, but not observable at a mole ratio of 1. Throughout most of the course of the process, the reduction in the iron(III) signal parallels the reduction in the Soret peak intensity at 409 mµ. The loss of the signal may be caused by dipolar coupling between paramagnetic ions, but it is significant that it may be produced by zinc ions, which are not paramagnetic but do promote the conformational transition in a similar way. At equilibrium, the copper(II) signal is reduced in intensity and somewhat broadened, but without other striking changes in form, and some degree of dipolar coupling of neighboring paramagnetic ions is indicated. Binding of the 1st copper(II) ion is relatively strong, but the binding of the additional 6 copper(II) ions in the altered complex cannot be distinguished according to a gradation of binding strengths.