Nitrate binding to Limulus polyphemus subunit type II hemocyanin and its functional implications

The horseshoe crab, Limulus polyphemus, employs hemocyanin as an oxygen carrier in its hemolymph. This hemocyanin displays cooperative oxygen binding and heterotropic allosteric regulation by protons, chloride ions and divalent cations. Here, we report the crystal structure of Limulus polyphemus subunit type II hemocyanin with a nitrate ion bound in the interface of its first and second domains. Interestingly, the nitrate-binding site coincides with the binding site for the allosteric effector chloride. Oxygen-binding data indeed indicate that nitrate, like chloride, reduces the oxygen affinity of this hemocyanin. The observed binding of two distinct anions to a single site suggests that several other anions may also bind at this site. This opens the intriguing possibility that bicarbonate, which is structurally similar to nitrate and closely linked to respiration, can act as an allosteric effector that lowers the oxygen affinity. Such an effect could be another factor in the repertoire of allosteric regulators of this hemocyanin; however, the physiological implications will be a challenge to decipher, since there exists a complex interplay of effects between bicarbonate, chloride, pH and divalent cations.