LOW-TEMPERATURE THERMOCHROMISM MARKS A CHANGE IN COORDINATION FOR THEMETAL-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-coor dination for the azide complex at 295 K, cleanly convert to spectra re flecting six-coordination at low temperature, with a midpoint for the transition near 200 K. The active site structure is temperature-depend ent, a relatively rigid, distorted octahedral low-temperature Mn compl ex melting with dehydration (or displacement of one of the protein lig ands) to form a five-coordinated complex under physiological condition s. Thermodynamic parameters for the transition estimated from van't Ho ff analysis (Delta H-VH = 5 kcal/mol; Delta S-vH = 22 cal/mol K) are c onsistent with reduced chemical binding and increased fluxionality at room temperature. This thermochromism of MnSD demonstrates the existen ce of distinct isomeric forms of the active site metal complex, whose relative stability depends on the degree of vibrational excitation. Th e marginal destabilization of the six-coordinate anion complex under p hysiological conditions suggests that the enzyme may thermally control the stability of intermediates in a dissociative displacement mechani sm for substrate binding and redox.