The Oxygen Transport Efficiency of Arthropod Hemocyanins

The efficiency in O2 transport of an O2 carrier is best expressed by the slope of O2 equilibrium curve (dS/dP), where S and P denote the fractional O2 saturation of the carrier and the partial O2 pressure, respectively. The aim of this study is to characterize the O2 binding to arthropod hemocyanins (Hcs) in term of the O2 transport efficiency, and for that purpose we have reexamined the previously published O2 equilibrium data and their mathematical interpretations. Examination of the data for 6- and 12-meric Hcs from lobsters (Panulirus japonicus, Procambarus clarkii) revealed the relationship Pdmax < P50 ≈ Pnmax, where Pdmax, P50 and Pnmax denote the P at which dS/dP is maximized, P at half-saturation and P at which the Hill coefficient is maximized, respectively. On the other hand, 48-meric Hcs from horseshoe crabs (Tachypleus gigas, and Limulus polyphemus) gave the relationship Pdmax ≤ P50 < Pnmax, when the O2 binding is cooperative. The results indicate that Hc functions most effectively at P values lower than that expected from the maximal degree of cooperativity, similarly to the previous results obtained for human hemoglobin (Hb) [6]. The profile of dS/dP vs. P curves for lobster Hcs was similar to that of Hb, showing a single maximum and a relatively symmetric shape. On the other hand, the curves for horseshoe crab Hcs showed multiple maxima tailing in the low P range, indicating that the O2 transport efficiency is high even under hypoxic circumstances. As far as the O2 transport efficiency is concerned, horseshoe crab Hcs are regarded as unique, and the features seem to reflect their uniqueness in the molecular architecture and the mode of allosteric transition.