Minimal structural requirements for root effect: Crystal structure of the cathodic hemoglobin isolated from the antarctic fish Trematomus newnesi.

The cathodic Hb component of the Antarctic fish Trematomus newnesi (HbCTn) is a Root-effect protein. The interpretation of its functional properties in relation to its sequence is puzzling. Indeed, HbCTn sequence is characterized by an extremely low histidyl content, and in particular by the lack of His146b and His69b, which are believed to be important in Bohr and Root effects, resp. Furthermore, previous analyses suggested that the local environment of Asp95a, Asp99b, and Asp101b should not be appropriate for the formation of Asp-Asp interactions, which are important for the Root effect. Here, we report the high-resoln. crystal structure of the deoxy form of HbCTn. Our data provide a structural interpretation for the very low oxygen affinity of the protein and insights into the structural determinants of the Root effect protein. The structure demonstrates that the presence of Ile41a and Ser97a at the a1b2 interface does not prevent the formation of the inter-Asp interactions in HbCTn, as previous studies had suggested. The present data indicate that the hydrogen bond formed between Asp95a and Asp101b, which is stabilized by Asp99b, is per se sufficient to generate the Root effect, and it is the minimal structural requirement needed for the design of Root-effect Hbs