Functional Properties of Hb Pasadena, α₂β₂ 75(E 19) LeuArg

Abstract

Functional properties of Hb Pasadena (β75[E19]Leu Arg) as well as correlations of its structure-function relationships with those of normal hemoglobin are presented. the homeotropic and heterotopic properties of the isolated variant were investigated by automatic recording oxygen equilibrium analysis. These studies indicate an increase in 0₂ affinity but normal cooperativity. the decrease of 0₂ affinity in response to organic phosphate (DPG and IHP) was found to be normal. However, the cooperativity of Hb Pasadena is less affected by IHP than is that of Hb A. the alkaline Bohr effect was found to be somewhat decreased, although the temperature effect is nearly normal. the autoxidation rate is increased by a factor of nearly 2 compared to normal. These observations are consistent with the conclusion that the β(E19)75 leucyl hydrophobic residue of Hb A associates with β(A8)11 Val, β(A12)15 Trp and β(H11)133 Val to form a hydrophobic cluster between the A, E and H helics which thereby stabilizes the heme of the β chain in the deoxy ferrous state. the experimental results also suggest that the amino acid substitution in Hb Pasadena mainly disrupts the interior conformation of the s subunit without affecting subunit interfaces. Thus it is predicted that Hb Pasadena should have a normal crystal structure in either the liganded (R) or unliganded (T) state. Its increased 0₂ affinity can be explained partially by a decrease of proton binding and partially by a shift of the R:T equilibrium towards the R conformation.