A MODEL FOR LOCAL MELTING OF METALLOPROTEIN STRUCTURE

Metalloprotein complexes that are able to explore a range of structure s will isomerize as the temperature is raised when the isomeric state is coupled to a soft mode in the vibrational spectrum. A model based o n a local mode description of the metal center provides insight into t he driving forces for the isomerization reaction, which is the local a nalog of global melting transitions well-known for cooperative systems (proteins, nucleic acids, and crystals). Thermal isomerization of the active site in manganese superoxide dismutase appears to be an exampl e of this type of transition, the azide complex converting between six - and five-coordinate forms at a temperature T-m = 220 K. In superoxid e dismutase, mode melting defines the pathway for ligand binding at am bient temperatures, stabilizing the functional form of the active site . Thermal isomerization may be a very general characteristic of comple x molecules in solution or in the gas phase, reflecting a transition i n molecular dynamics.