Spectroscopic and kinetic aspects of Elephas maximus hemoglobin

In comparison with myoglobin and human and Glycera dibranchiata hemoglobins, the heme distal side amino acid exchanges within the heme environment of elephant tetrameric hemoglobin (Hbe) only slightly affect the electronic and ESR spectra of Hbe(III) and Hbe(II) derivatives, several of which were prepared and characterized by optical and ESR spectroscopy. Addition of 2,3-bisphosphoglycerate [Gri(2,3)P2] or inositol hexakisphosphate to Hbe(II)NO causes tension in the Fe-N(proximal His) bond, although the behaviour differs in detail from that of HbA(II)NO. There are two equilibrium states of Hbe having significantly different kinetics for the Hbe(III)----Hbe(II) reaction of Hbe(III)NO. This autoreduction occurs in the form of two parallel processes, which collapse into one intermediate rate in the presence of Gri(2,3)P2. The temperature dependences of the rates enable deduction of delta H0 and delta S0 for the linked equilibrium, and yield linear Eyring plots for Hbe(III)NO, from which activation parameters were estimated on the basis of a previously described mechanism.