Structure-function relationships in iron and manganese superoxide dismutases.

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Superoxide dismutases (SODs) are critical enzymes that catalyze the dismutation of superoxide radicals (O2-) into oxygen and hydrogen peroxide.
• Understanding the structural and mechanistic details of MnSOD and FeSOD is essential for comprehending cellular defense against oxidative stress.

Purpose of the Study:

• To refine structural models of Thermus thermophilus MnSOD and E. coli FeSOD in their oxidized states.
• To elucidate the coordination geometry, substrate binding, and mechanistic aspects of these metalloenzymes.

Main Methods:

• X-ray crystallography to obtain high-resolution structural data for MnSOD and FeSOD.
• Structural refinement using crystallographic data to R-factors of 0.186 (MnSOD) and 0.22 (FeSOD).
• Analysis of enzyme complexes (e.g., azide complex) and reduced forms to investigate reaction mechanisms.

Main Results:

• Refined structures support a solvent molecule as the fifth ligand to Mn(III) and Fe(III), with a trigonal bipyramidal coordination geometry.
• The substrate-entry channel involves residues from both subunits, with conserved basic residues potentially facilitating superoxide approach.
• Analysis of the FeSOD azide complex suggests O2- binds to a sixth site during turnover without displacing the solvent ligand.

Conclusions:

• The structural and mechanistic data provide insights into the catalytic cycle of MnSOD and FeSOD.
• The findings suggest that conserved residues play roles in substrate binding and enzyme stability.
• The study proposes a mechanistic scheme accommodating various experimental data, including pH-dependent titration behaviors.