IDENTIFICATION OF AXIAL LIGANDS OF CYTOCHROME C(552) FROM NITROSOMONAS-EUROPAEA

Cytochrome c(552) from Nitrosomonas europaea was analyzed by visible, EPR and MCD spectroscopies. The visible and MCD data show that histidi ne and methionine are the axial ligands to the heme iron of the ferric protein. The EPR spectrum of the cytochrome shows an atypical highly axial low spin (HALS) type signal with g-values that make it difficult to identify the axial ligands. These results reinforce the value of n ear-infrared MCD spectroscopy for assigning ligands in ferric heme sys tems and point out the difficulties in using only EPR spectroscopy for the same purpose. The description of another c-cytochrome exhibiting a HALS-type EPR signal will eventually be helpful in explaining the ph ysical basis for this unusual signal.