Three-Dimensional Structural Model of Chicken Liver Sulfite Oxidase in its Activated Form

Sulfite oxidase (SO) catalyzes the conversion of sulfite to sulfate in almost all living organisms. In vertebrates, the catalytic process involves a rapid intramolecular electron transfer (IET) step between the molybdenum cofactor in the central domain and the heme in the cytochrome b5 domain. The large distance between redox centers observed in the crystal structure disagrees with the fast IET rates measured experimentally. This conflict was explained by postulating a major rearrangement of the cytochrome b5 domain toward the molybdopterin cofactor. Using steered molecular dynamics and molecular dynamics simulations, we generated a stable 3D structural model for chicken liver SO (CSO) in the activated form characterized by a short electron donor−acceptor distance consistent with the enzymes’ experimentally obtained electron transfer properties. IET rates for the active complex were estimated with the Pathway model. The good agreement between calculated and experimental IET rates supports our structural model for the active CSO.