Physical studies and synthetic modeling of the molybdenum-containing enzyme sulfite oxidase.

Abstract

This research has been directed at the study of both the enzyme sulfite oxidase and molybdenum model chemistry. A modification of a previously published procedure has been used to purify sulfite oxidase in high yield which is well-suited for experiments requiring prosthetically intact enzyme and which is not contaminated with extraneous heme or with other redox active proteins. Laser flash photolysis was used to study the reaction of photoproduced 5-deazariboflavin, lumiflavin, and riboflavin semiquinone radicals with the redox centers of purified sulfite oxidase. Two distinctly different intramolecular electron transfer processes were observed between the molybdenum and heme sites of the enzyme, and these assignments were supported by flash photolysis studies of the cyanide-inactivated enzyme and the sulfite oxidase heme peptide. Microcoulometric experiments on sulfite oxidase have shown that the enzyme requires the addition of three electrons for complete subunit reduction. Midpoint potentials for the Mo(VI)/Mo(V), Mo(V)/Mo(IV), and Fe(III)/Fe(II) couples have been obtained under varied buffer conditions. The midpoint potentials obtained under High-pH and Low-pH conditions provided a means for reductively titrating the enzyme to the Mo(V) oxidation state for EXAFS studies. EXAFS of sulfite oxidase under High-pH and Low-pH conditions have provided the first example of a structural study of the three accessible oxidation states (Mo(VI), Mo(V), and Mo(IV)). A biologically relevant synthetic model for the formation of the Mo(V) Low-pH form of sulfite oxidase has been developed. The Mo(V) model compound closely resembles the minimum coordination environment for the Mo(V) Low-pH form of sulfite oxidase as determined by EXAFS. Using synchrotron radiation, molybdenum L-edge x-ray absorption spectra have been obtained for a variety of oxomolybdenum(V) compounds which serve as models for sulfite oxidase. An attempt has been made to correlate unique features of the molecules to the observed 2P → 4d electronic transitions.