Thermodynamics of effector binding to hemocyanin: influence of temperature

Hemocyanins are allosterically regulated oxygen carriers freely dissolved in the blood of many crustaceans. The natural modulator urate accumulates under hypoxic conditions in the hemolymph of Homarus vulgaris, and increases the oxygen affinity of the respiratory pigment. Other non-physiological effectors such as caffeine, dimethylxanthines and methylxanthines are also known to modulate the oxygen-binding properties of hemocyanin. In order to gain insight into the thermodynamic driving forces of these interactions we analyzed the binding of several urate analogs to dodecameric hemocyanin at different temperatures by employing isothermal titration calorimetry (ITC). All investigated non-physiological effectors including caffeine, dimethylxanthines and methylxanthines bind exothermically to hemocyanin and the binding processes are enthalpy driven. Furthermore, we demonstrated a strong temperature dependent binding of caffeine and dimethylxanthines to the hemocyanin of the European lobster and could show that the binding properties of the effectors to the urate-binding site depend on the hydrophobicity of a given molecule.