Reaction of carbon monoxide with hemocyanin: Stereochemical effects of a non-bridging ligand

Abstract

The carbon monoxide binding equilibria and kinetics of a number of molluscan and arthropodal hemocyanins have been investigated employing the visible luminescence of the carbon monoxide-copper complex. Proteins from both phyla, in oligomeric and monomeric form, bind carbon monoxide non-co-operatively; the reaction is largely enthalpy driven is associated with a small unfavourable entropy change. Molluscan hemocyanins display a carbon monoxide affinity ( p 50 = 1 to 10mm Hg) higher than that of arthropodal hemocyanins ( p 50 = 100 to 700mm Hg), and only Panulirus interruptus hemocyanin, among those studied here, exhibits a small Bohr effect. The observed differences in equilibrium constant are kinetically reflected in differences in the carbon monoxide dissociation rate constant, which ranges from 20 to 70 s −1 for molluscan hemocyanins and from 200 to 9000 s −1 for arthropodal hemocyanins; on the other hand the differences in the combination rate constants between the two phyla are considerably smaller. A comparison of the equilibrium and kinetic results shows some discrepancies between the two sets of data, suggesting that carbon monoxide binding may be governed by a complex mechanism. The correlation between the ligand binding properties and the stereochemistry of the active site is discussed in the light of the knowledge that, while oxygen is bound to both copper atoms in a site, carbon monoxide is a “non-bridging” ligand, being bound to only one of the metals.