Spectral, Conformational and Chemical Properties of Opossum Methemoglobin

Abstract

Opossum methemoglobin differs from methemoglobin A in spectral, spin state, conformational and chemical properties. The primary structural alterations in opossum hemoglobin, including the critical substitution at a58 (E7) His ---f Gln result in the following properties. (a) Major contribution of the spectral transitions due to inositol hexakisphosphate binding arises from the x chains. (b) The aquomet to hydroxymet (high-spin to low-spin) transition as a function of pH is slightly retarded resulting in considerable high spin at alkaline pH. (c) The tertiary conformation (t) around the / 3 hemes, upon transition to a T quaternary state, differs from the known hemoglobin t tertiary structure. (d) Both a and ,9 hemes are susceptible to rapid reduction by ascorbic acid (the reduction rate being tenfold faster than that of methemoglobin A). These properties suggest that the heme environments in both the a and subunits of opossum hemoglobin are different from those of human hemoglobin A.The complete amino acid sequence of the a and /j chains of opossum (Didelphius marsupialis) hemoglobin reveals 83 residue differences when compared to human hemoglobin A TI]. The a-chain heme environment of opossum hemoglobin is modified due to the presence of glutamine in place of the distal histidine at position a58 (E7) and also by the presence of a methionine residue at a25 (B6), which by virtue of its larger size compared with the glycine residue in hemoglobin A can alter the stability and rigidity of B and