Interaction of lanthanide ions with Panulirus interruptus hemocyanin: Evidence for vicinity of some of the cation binding sites

The interaction of a number of lanthanide ions (namely terbium, praseodymium, erbium, lanthanum, gadolinium and europium) with Panulirus interruptus hemocyanin has been studied.Results from O₂-binding experiments indicate that all these ions may substitute for calcium as allosteric effectors of hemocyanin. Addition of the lanthanides to deoxygenated Panulirus hemocyanin saturated with Tb³⁺ results in a quenching of the terbium luminescence. The highly efficient quenching observed in the case of Eu³⁺ may indicate energy-transfer between Tb³⁺ and Eu³⁺. Since energy-transfer between lanthanides is only effective over very short distances, the data suggest that some of the cation binding sites of Panulirus hemocyanin are clustered.