Structure and Function of the Hemoglobins of the Carp, Cyprinus carpio

Amino acid analysis and tryptic peptide mapping indicate that the α chains of the three major hemoglobin components are significantly different, whereas either no or very slight differences were found among the β chains. The amino acid sequence of the first 28 residues of the β chain of Component I indicates that the A helix is probably the same length in both carp and human β chains. The oxygen equilibria of the isolated components, I, II and III, appear to be indistinguishable from one another. ATP enhances the normal alkaline Bohr effect of carp hemoglobin, and CO2 depresses the Bohr effect between pH 7 and 8. Cooperativity as measured by n in Hill's equation is maximal at about pH 6.5 in 0.05 m bis-(2-hydroxyethyl)-imino-tris-(hydroxymethyl)methane, 0.1 m NaCl but shifts to pH 7.5 in the presence of saturating ATP. A simple model suggests that cooperativity in carp hemoglobin may require the presence of a particular set of protonated species, the concentration of which is maximal at about pH 6.5. The model suggests that the alkaline shift in the position of this maximum in the presence of ATP may result from the preferential binding of ATP at low pH.