Structure and Ligand Binding Properties of Myoglobins Reconstituted with Monodepropionated Heme:Functional Role of Each Heme Propionate Side Chain

摘要

TWO heme propionate side chains,which are attached at the 6 and 7 positions of the heme framework,are linked with Arg45 and Ser92,respectively,in sperm whale myoglobin.To evaluate the role of each propionate,two kinds of one-legged hernins,6-depropionated and 7-depropionated protohemins,were prepared and inserted into the apomyoglobin to yield two reconstituted proteins.Structural data of the reconstituted myoglobins were obtained via an X-ray crystallographic analysis at a resolution of 1.1- 1.4 A and resonance Raman spectroscopy.It was found that the lack of the 6-propionate reduces the number of hydrogen bonds in the distal site and clearly changes the position of the Arg45 residue with the disrupting Arg45-Asp60 interaction.In contrast,the removal of the 7-propionate does not cause a significant structural change in the residues of the distal and proximal sites.However,the resonance Raman studies suggested that the coordination bond strength of the His93-Fe bond for the protein with the 7-depropionated protoheme slightly increases compared to that for the protein with the native heme.The O_2 and CO ligand binding studies for the reconstituted proteins with the one-legged hemes provide an important insight into the functional role of each propionate.The lack of the 6-propionate accelerates the O_2 dissociation by ca.3-fold compared to those of the other reconstituted and native proteins.The lack of the 7-propionate enhances the CO affinity by 2-fold compared to that of the protein with the native heme.These results indicate that the 6-propionate clearly contributes to the stabilization of the bound O_2,whereas the 7-propionate plays an important role in the regulation of the Fe-His bond.