Structural, functional and conformational properties of rat hemoglobins

The oxygen equilibrium properties of rat total hemoglobin show pH dependence. Thus oxygen affinity and cooperativity, which are significantly reduced at pH 6.0, show increase with increasing pH. Conformational studies using nitrosyl derivatives indicate that in rat nitrosyl hemoglobin the R to T equilibrium is shifted towards the T state in going from pH 7.0 to pH 6.0. Under similar conditions human hemoglobin A shows no significant changes in cooperativity or conformation. These results indicate that a destabilized oxy structure (R) exist in rat hemoglobins at pH 6.0. Alterations in the heme pockets and alpha 1 beta 2 contact points could lead to these structural characteristics. Crystallization of rat hemoglobins is dependent on an oxy-like structure since all liganded ferrous derivatives and methemoglobin form crystals in the pH range of 7.0-8.0. Deoxygenation of oxyhemoglobin crystals or addition of inositol hexakisphosphate to nitrosyl or methemoglobin derivatives result in solubilization of the crystals. Thus the quaternary and/or tertiary structural changes involved in the transition of a R to T state disrupt the crystal structure. The precise positioning of the complementary sites involved in the interaction of amino acids between rat hemoglobin tetramers seems to occur only when the molecules are in the quaternary or tertiary R conformation. This is in contrast to the polymerization of human deoxyhemoglobin S, where gel formation occurs only in the T state.