Crystal structure of Trematomus newnesi haemoglobin re-open the Root effect question

As new structural data have become available, somewhat contrasting explanations of the Root effect in fish haemoglobins (Hb) have been provided. Hb 1 of the Antarctic fish Trematomus newnesi has a nearly pH-independent oxygen affinity, in spite of 95 % sequence identity with Hb 1 of Trematomus (previously named Pagothenia) bernacchii that has a strong Root effect. Here, the 2.2 Angstrom R-state structure of Trematomus newnesi Ho 1 is presented. The structure is similar to that of Root effect fish Kbs from Spot and T. bernacchii, suggesting that the differences in the pH dependence cannot be related to the modulation of the R-state. In comparison to T, bernacchii Hb 1, the role of the three mutations Thr41 (C6)alpha --> Ile, Ala97 (G3)alpha --> Ser and His41 (C7)beta --> Tyr at the alpha(1)beta(2)-interface is discussed.