Studies of Enzyme-Ligand Complexes Using Dynamic Fluorescence Anisotropy. I. The Substrate-Binding Site of Malate Dehydrogenase

The coenzyme-binding site in mitochondrial malate dehydrogenase from pig heart was studied using dynamic fluorescence anisotropy decay. The dynamics of the fluorescent ligands NADH and 6-cyano-7-hydroxy-4,8-dimethylcoumarin were used to detect conformational changes at the dihydronicotinamide-and at the adenosine-binding sites, respectively. Addition of the natural substrate L-malate to the complex from enzyme and NADH does not influence the complete immobilization of the dihydronicotinamide group, whereas the stereoisomer D-malate and the substrate-analogue hydroxymalonate form ternary complexes with highly mobile dihydronicotinamide. The dynamics of the fluorescent adenosine-analogue are not influenced by formation of complexes with substrate and substrate-analogues. Thus the conformational changes at the dihydronicotinamide-binding site remain local and are not transmitted to the adenosine-binding site.