pH effects on the binding of oxygen to non-vertebrate monomeric hemoglobins. A linked function model

Monomeric invertabrate hemoglobins with high oxygen affinity usually contain a tyrosine in the distal region of the heme. This feature has stimulated investigations revealing that one of the properties resulting from the presence of the distal tyrosines is a decreased off rate on the binding of oxygen, thus developing the high affinity. Despite that fact that the pK value of the tyrosine group differs significantly from the groups it replaces little attention has been paid to the pH dependence of the binding of oxygen to the high affinity hemoglobins. Such a pH dependence has been reported on two of the monomeric hemoglobins with relatively low oxygen affinity and one monomeric hemoglobin of intermediate affinity. The pH data of these hemoglobins has been analysed with a linked function model involving the hydrogen ion. pK values required for the low-affinity hemoglobins vary from 4.5 to 7.5. When applied to the high-affinity hemoglobins, the linked function model provides reasonable values for the binding parameters. These pK values vary from 3.0 to 9.0.