Molecular Insights of an Avian Species with Low Oxygen Affinity, the Crystal Structure of Duck T-State Methemoglobin.

Hemoglobin (Hb) is the key metalloprotein within red blood cells involved in oxygen transportation from lungs to body cells. The heme-iron atom inherent within Hb effectuates the mechanism of oxygen transportation and carbon dioxide removal. Structural investigations on avian Hb are limited when compared with the enormous work has been carried out on mammalian Hb. Here, the crystal structure of T-state methemoglobin (T-metHb) from domestic duck (Anas platyrhynchos), a low oxygen affinity avian species, determined to 2.1Å resolution is presented. Duck T-metHb crystallized in the orthorhombic space group C222₁ with unit cell parameters a = 59.89, b = 109.42 and c = 92.07Å. The final refined model with R-factor: 19.5% and R_free: 25.2% was obtained. The structural analysis reveals that duck T-metHb adopts a unique quaternary structure that is distinct from any of the avian liganded Hb structures. Moreover, it closely resembles the deoxy Hb of bar-headed goose, a high oxygen-affinity species. Besides the amino acid αPro119 located in the α1β1 interface, a unique quaternary structure with a constrained heme environment is attributed for the intrinsic low oxygen-affinity of duck Hb. This study reports the first protein crystal structure of low oxygen-affinity avian T-metHb from Anas platyrhynchos.