Mini-myoglobin. The structural significance of haem-ligand interactions

The properties of purified mini-myoglobin, the fragment 32-139 of horse heart myoglobin reconstituted with protohaem, have been investigated from a structural and functional view point. The recovery of secondary structure observed in the carbon monoxide derivative of mini-myoglobin, as shown by circular dichroism, and the overall similarity of the haem pocket to that of myoglobin, as deduced from the fluorescence properties of the complex with 1-anilino-8-naphthalene sulphonate, indicate that, in the presence of the constraints imposed by the haem and its ligands, the miniprotein reacquires a conformation close to that of native myoglobin. These spectroscopic data parallel the conclusions drawn from the results of ligand combination and dissociation kinetics; stopped-flow experiments indicate that carbon monoxide and oxygen bind to mini-myoglobin with rates almost identical with those of myoglobin itself. The significance of mini-myoglobin as a model of an oxygen-carrying protein, with some of the expected functional characteristics of an ancestor haemoprotein, is discussed, with reference to the mosaic structure of the myoglobin gene and the role of different exons in the evolution of proteins.