UCLA

Manganese-bound superoxide dismutase (MnSOD) is a very important antioxidant enzyme. The mechanism by which MnSOD removes O₂^— involves product inhibition, that is, reduction of O₂^— occurs through either a "prompt protonation" pathway, or an "inner-sphere" pathway, with the latter leading to formation of an observable Mn-peroxo complex. Human MnSOD is more gated toward the "inner-sphere" pathway than bacterial enzymes.

To study whether product inhibition is a common feature to eukaryotic MnSODs, we studied a mitochondrial MnSOD from the eukaryote model organism Saccharomyces cerevisiae (ScMnSOD). To our surprise, ScMnSOD was found to display the highest catalytic efficiency at high levels of O₂^— among MnSODs that had been characterized. To understand further the mechanism of product inhibition, we compared ScMnSOD with another yeast MnSOD, the cytosolic MnSOD from Candida albicans (CaMnSODc). CaMnSODc, like ScMnSOD, is less inhibited than human and bacterial MnSODs. Although the active site of yeast MnSODs closely resembles that of MnSODs from other organisms, spectroscopic studies suggest the presence of a six-coordinate Mn³⁺ species in oxidized yeast MnSODs.

To explore further the origin of the fast catalysis by yeast MnSODs, the Y34F (a strictly conserved second-sphere residue) form of ScMnSOD was created. Y34F ScMnSOD has a novel catalytic mechanism, in which protonation of the Mn-peroxo complex occurs through a fast pathway at neutral pH, leading to a putative six-coordinate Mn³⁺ species, which actively oxidizes O₂^— in the catalytic cycle. Because wild-type and the mutant yeast MnSOD both rest in the 2+ state and become six-coordinate when oxidized up from Mn²⁺, six-coordinate Mn³⁺ species could also actively function in the mechanism of wild-type yeast MnSODs.

ScMnSOD is a tetramer, while CaMnSODc is a dimer or loose tetramer, even though they are similar in many ways. Investigations of their crystal structures suggest that when CaMnSODc is in the dimeric form, its N-terminal regions are highly disordered, hindering it from forming a tetramer in solution. To further investigate the physiological significance of the tetramer structure, we mutated two residues (Lys182/Ala183 in ScMnSOD, Lys184/Leu185 in CaMnSODc) at the dimer interface in the two yeast MnSODs. We find that the dimer interface, which is critical for MnSOD activity, is reinforced by tetramer formation.