The heme and Fe4S4 cluster in the crystallographic structure of Escherichia coli sulfite reductase.

Isolated hemoprotein subunits of Escherichia coli NADPH:sulfite reductase catalyze the 6-electron reduction of SO2-3 to S2-. The prosthetic groups of the hemoprotein, a siroheme and a Fe4S4 cluster, have been shown by spectroscopy to be tightly coupled. We have crystallized the isolated hemoprotein subunits and produced a 3-A electron density map by x-ray crystallography. A single heavy atom derivative and the native anomalous scattering (from the protein's 5 Fe and several S) were used to calculate the phases. In the electron density map, the cluster has a geometry similar to other Fe4S4 clusters. Both the cluster and the siroheme are found near the surface of the protein. The siroheme and the Fe4S4 cluster pack next to each other in the structure, apparently with a common ligand, consistent with a cysteine S gamma, shared by the siroheme Fe and one of the cluster Fe. The distance from the siroheme Fe to the center of the cluster is 5.5 A and the distance from the siroheme Fe to the nearest cluster Fe is 4.4 A. The edge of the siroheme macrocycle appears to be in Van der Waals contact with a cubane S atom of the cluster. The sixth coordination position of the siroheme Fe appears unoccupied and is quite exposed to the solvent. Some possible implications of the proposed structure on the role of the bridged siroheme-Fe4S4 cluster in catalysis are discussed.