Characterization of a covalent polysulfane bridge in copper-zinc superoxide dismutase

Abstract

In the course of studies on human copper-zinc superoxide dismutase (SOD1), we observed a modified form of the protein whose mass was increased by 158 mass units. The covalent modification was characterized, and we established that it is a novel heptasulfane bridge connecting the two Cys111 residues in the SOD1 homodimer. The heptasulfane bridge was visualized directly in the crystal structure of a recombinant human mutant SOD1, H46R/H48Q, produced in yeast. The modification is reversible, with the bridge being cleaved by thiols, by cyanide, and by unfolding of the protein to expose the polysulfane. The polysulfane bridge can be introduced in vitro by incubation of purified SOD1 with elemental sulfur, even under anaerobic conditions and in the presence of a metal chelator. Because polysulfanes and polysulfides can catalyze the generation of reactive oxygen and sulfur species, the modification may endow SOD1 with a toxic gain of function.

Figure 1

A. The mass spectrum of wild-type SOD1; B. the mass spectrum of the double mutant P62A/P66A.

Figure 2

Mass spectra of SOD1 mutants obtained by the direct infusion method. A) G93A; B). P62A/P66A.

Figure 3

Mass spectra of P62A/P66A after exposure to 6M guanidine. A) The full spectrum B) Expanded view of the dimer region showing different forms of covalent dimeric P62A/P66A with varying length of polysulfanes; C) Expanded view of the monomer region showing different forms of monomer P62A/P66A with varying length of polysulfanes.

Figure 3

Mass spectra of P62A/P66A after exposure to 6M guanidine. A) The full spectrum B) Expanded view of the dimer region showing different forms of covalent dimeric P62A/P66A with varying length of polysulfanes; C) Expanded view of the monomer region showing different forms of monomer P62A/P66A with varying length of polysulfanes.

Figure 4

P62A/P66A after treatment with 10 mM KCN for 90 min.

Figure 5

Composite annealed omit electron density calculated at 1.75 Å resolution with coefficients 2Fo-Fc of the dimer interface region of apo-H46R/H48Q (contoured at 1σ). The polysulfane chain is shown in yellow. Water molecules have been removed from the image for clarity.

Figure 6

Human SOD1 purified from red blood cells through the DEAE elution step.