ELECTRONIC STATE OF HEME IN CYTOCHROME OXIDASE-III - MAGNETIC-SUSCEPTIBILITY OF BEEF-HEART CYTOCHROME-OXIDASE AND SOME OF ITS DERIVATIVES FROM 7-200 K - DIRECT EVIDENCE FOR AN ANTI-FERROMAGNETICALLY COUPLED FE(III)-CU(II) PAIR

Abstract

The temperature dependence of the paramagnetic susceptibility of cytochrome oxidase and some of its derivatives was measured from 7-200.degree. K. The results obtained for the fully oxidized (resting) enzyme correspond exactly to the requirements of the model recently proposed by Palmer et al. in which the enzyme possesses 2 magnetically isolated spin S = 1/2 centers and a spin-coupled S = 2 center. The S = 2 center paramagnetism was interpreted as arising from a [cytochrome a33+(S = 5/2)--- Cuu2+(S = 1/2)] antiferromagnetically coupled Fe .cntdot. Cu binuclear complex of total spin S = 2 with -J .gtoreq. 200 cm-1. In addition, the wide temperature range used in the present studies permitted an analysis of present and other available data (T < 4K measurements) which readily accommodates results from this and other laboratories so that a fully consistent picture of the magnetic centers in cytochrome oxidase now appears to be available. Anomalous magnetic behavior for the oxidized enzyme .cntdot. cyanide complex has been interpreted in terms of an antiferromagnetic exchange interaction operating in the binuclear complex [cytochrome a33+ .cntdot. CN-(S = 1/2)---Cuu2+(S = 1/2)] with -J .simeq. 40 cm-1. A structural model for the [cytochrome a33+-bridge-Cuu2+] center is advanced in which an imidazolate ion serves as the bridging ligand in a manner similar to that found in superoxide dismutase.