CYTOCHROME-C(6) FROM MONORAPHIDIUM-BRAUNII - A CYTOCHROME WITH AN UNUSUAL HEME AXIAL COORDINATION

A soluble monoheme c-type cytochrome (cytochrome c6) has been isolated from the green alga Monoraphidium braunii. It has a molecular mass of 9.3 kDa, an isoelectric point of 3.6 and a reduction potential of 358 mV at pH 7. The determined amino acid sequence allows its classification as a class-I c-type cytochrome. The ferric and ferrous cytochrome forms and their pH equilibria have been studied using H-1-NMR, ultraviolet/visible, EPR and Mossbauer spectroscopies. The pH equilibria are complex, several pK(a) values and pH-dependent forms being observed. The amino acid sequence, the reduction-potential value and the visible and NMR spectroscopies data in the pH range 4-9 indicate that the heme iron has a methionine-histidine axial coordination. However, the EPR and Mossbauer data obtained for the ferricytochrome show that in this pH range two distinct forms are present: form I, g(z) = 3.27, g(y) = 2.05 and g(x) = 1.05; form II, g(z) = 2.95, g(y) = 2.29 and g(x) = 1.43. While form I has crystal-field parameters typical of a methionine-histidine coordination, those associated with form II would suggest a histidine-histidine axial ligation. This possibility was extensively analyzed by spectroscopic methods and by chemical modification of a histidine residue. It was concluded that form II actually corresponds to an unusual type of methionine-histidine axial coordination. Straightforward examples of this type of coordination have recently been found in other c-type hemeproteins [Teixeira, M., Campos, A. P, Aguiar, A. P., Costa, H. S., Santos, H., Turner, D. L. & Xavier, A. V. (1993) FEBS Lett. 317, 233 - 236], corroborating our proposal. Since both forms, with very distinct crystal-field parameters, are shown to have the same reduction potential, it may be concluded that the axial and rhombic distortions of the heme-iron ligand field cannot be directly correlated with the heme-reduction potential. The pH-dependence studies have also shown that the form I and form II are interconvertible, with pK(a) almost-equal-to 5. To establish a possible physiological significance for this process, in particular for the interaction of the cytochrome with the membrane-bound electron-transfer complexes b6f and photosystem 1, the effect of surfactants on the spectroscopic characteristics of cytochrome c6 has been studied.