Abstact

The Antarctic fish Pagothenia bernacchii has one major haemoglobin, Hb1 (over 95% of the total blood content). Hb1 has a strong alkaline Bohr effect and at low pH exhibits the reduced ligand affinity and co-operativity that comprise the Root effect. We have determined the complete amino acid sequence of P. bernacchii Hb1 and also the structure of its carbonmonoxy derivative by X-ray crystallography, to a resolution of 2.5 A. The crystallographic R-factor of the refined structure is 18%. The three-dimensional structure of this fish haemoglobin is similar to that of human haemoglobin A, with a root-mean-square difference in main-chain atom positions of 1.4 A after superimposition of the two structures, despite only 48% homology of their amino acid sequences (including insertion of a single residue in the CD region of the fish alpha-chain). Large structural differences occur only at the N and C termini of both the alpha- and beta-chains. Neither these nor other smaller structural differences provide any obvious explanation of the Root effect of this or other fish haemoglobins.