Low-temperature thermochromism marks a change in coordination for the metal ion in manganese superoxide dismutase

We have observed thermochromism (temperature-dependent absorption) for anion complexes of manganese superoxide dismutase indicating a change in coordination number for the metal complex at low temperatures. The ligand field spectra for the Mn(III) ion, characteristic of five-coordination for the azide complex at 295 K, cleanly convert to spectra reflecting six- coordination at low temperature, with a midpoint for the transition near 200 K. The active site structure is temperature-dependent, a relatively rigid, distorted octahedral low-temperature Mn complex melting with dehydration (or displacement of one of the protein ligands) to form a five-coordinated complex under physiological conditions. Thermodynamic parameters for the transition estimated from van't Hoff analysis (ΔH(vH) = 5 kcal/mol; ΔS(vH) = 22 cal/mol K) are consistent with reduced chemical binding and increased fluxionality at room temperature. This thermochromism of MnSD demonstrates the existence of distinct isomeric forms of the active site metal complex, whose relative stability depends on the degree of vibrational excitation. The marginal destabilization of the six-coordinate anion complex under physiological conditions suggests that the enzyme may thermally control the stability of intermediates in a dissociative displacement mechanism for substrate binding and redox.