Porphyrins as Detectors of Internal Electric Field in Heme Proteins

Abstract

T , respectively. No changes in highresolution static crystallographic T(deoxy)-and R(oxy)-quaternary and tertiary structures of Hb and their heme environment, as well as the axial coordination structures of the deoxy-heme (n Fe-His = 215 /cm ) and the oxy-heme (n Fe-O2 = 567 /cm ) in solution, are observed, despite K T and K R values are changed as much as 100-and 2,000-folds, respectively. Thus, the assumption that the low-affinity state is caused by the inter-dimeric salt-bridge-linked constraints, the out-of-plane shift of the heme Fe, and the allosteric core constraint in the T(deoxy)-Hb is no longer valid. Although these constraints are completely absent in R(oxy)-Hb, its O 2 -affinity is modulated as much as 2,000-folds by its interaction with heterotropic effectors. The effector-linked modulation of thermal fluctuation of the protein may be responsible.