An Iron-Containing Superoxide Dismutase from Anacystis nidulans

Abstract

Superoxide dismutase (SOD) was isolated and purified from Anacystis nidulans to near electrophoretic homogeneity. The enzyme has a molecular weight of 37, 500, as determined by gel filtration and SDS-gel electrophoresis. The enzyme molecule consists of two subunits of identical molecular weight. Proton-induced X-ray elemental analysis (PIXE) showed that the SOD of A. nidulans is an iron-containing enzyme; the Fe:enzyme mol ratio was found to be 1. The EPR spectra indicated that the active center contains high-spin ferric ion. Based on quantitative EPR data, we conclude that essentially all iron ions were detected in the EPR experiments and were present in the Fe³⁺ high-spin form. Since the Fe: enzyme mol ratio was 1, we suggest that each enzyme contains a 1-Fe³⁺ active center.
Effective g'-values were calculated from computer-simulated spectra and analysis of the g'-value anisotropy of the ±3/2 Kramers doublet made the calculation of crystal field parameters possible. The symmetry of the Fe³⁺ ion in the SOD molecule was found to be close to rhombic (E/D=0.240).