Molecular insights of an avian species with low oxygen affinity, the crystal structure of duck methemoglobin

Abstract

Hemoglobin is an iron-containing metalloprotein which plays a major role in the transportation of oxygen from lungs to tissues and carbon dioxide back to lungs. The structural investigations on avian hemoglobin are limited when compared with the enormous work has been carried out on mammalian hemoglobin. Here, the crystal structure of methemoglobin from domestic duck (Anas platyrhynchos), a low oxygen affinity avian species, determined to 2.1 Å resolution is presented. It has been crystallized in orthorhombic space group C2221 with unit cell parameters a = 59.89, b = 109.42 and c = 92.07 Å. The final model is refined to an R-factor and Rfree of 19.5% and 25.2%, respectively. The structural analysis reveals that duck methemoglobin adopts a unique quaternary structure that is distinct from any of the liganded hemoglobin structures. Moreover, it closely resembles the deoxy hemoglobin of bar-headed goose, a high oxygen affinity species. Besides the amino acid αPro 119 located in the α1β1 interface, a unique quaternary structure with a constrained heme environment is attributed for the intrinsic low oxygen affinity of duck hemoglobin.