| Abstract: | The fluorescent probe 8-anilinonaphtalene-1-sulfonate (ANS) binds at the active site of the Naja melanoleuca snake venom phospholipase A2, thus protecting the enzyme against active-site-directed chemical modification. Both hydrophobic and electrostatic interactions are involved in the binding. At pH 7.5, a binding constant of 100 microM was determined, which improved twofold upon addition of the enzymatic cofactor Ca2+. The pH dependence of the ANS binding in the absence and presence of Ca2+ ions showed a perturbation of a group with a pKa value of 5.2, which could be assigned to the carboxylate group of the Ca2+-binding ligand Asp49 at the active site of the protein. Monomeric concentrations of the substrate analog n-decylphosphocholine displace ANS from the protein, indicating again that both ligands bind at the active site. Binding studies with several modified N. melanoleuca enzymes showed that a loss of enzymatic activity on aggregated substrates was correlated with a loss of affinity for the active site bound ANS molecule. It is suggested therefore, that the fluorescent ANS probe can detect structural rearrangements at the active site, which are important for enzymatic activity. |
