Hagfish hemoglobins - Structure, function, and oxygen-linked association

Cyclostomes, hagfishes and lampreys, contain hemoglobins that are monomeric when oxygenated and polymerize to dimers or tetramers when deoxygenated. T he three major hemoglobin components (HbI, HbII, and HbIII) from the hagfis h Myxine glutinosa have been characterized and compared with lamprey Petrom yzon marinus HbV, whose x-ray crystal structure has been solved in the deox ygenated, dimeric state (Heaslet, H. A., and Royer, W. E., Jr. (1999) Struc ture 7, 517-526). Of these three, HbII bears the highest sequence similarit y to P. marinus HbV. In HbI and HbIII the distal histidine is substituted b y a glutamine residue and additional substitutions occur in residues locate d at the deoxy dimer interface of P. marinus HbV. Infrared spectroscopy of the CO derivatives, used to probe the distal pocket fine structure, brings out a correlation between the CO stretching frequencies and the rates of CO combination. Ultracentrifugation studies show that HbI and HbIII are monom eric in both the oxygenated and deoxygenated states under all conditions st udied, whereas deoxy HbII forms dimers at acidic pH values, like P. marinus HbV. Accordingly, the oxygen affinities of HbI and HbIII are independent o f pH, whereas HbII displays a Bohr effect below pH 7.2. HbII also forms het erodimers with HbIII and heterotetramers with HbI. The functional counterpa rts of heteropolymer formation are cooperativity in oxygen binding and the oxygen-linked binding of protons and bicarbonate. The observed effects are explained on the basis of the x-ray structure of P. marinus HbV and the ass ociation behavior of site-specific mutants.