Novel Histidine−Heme Covalent Linkage in a Hemoglobin

When treated with dithionite at neutral pH, the recombinant hemoglobin from Synechocystis sp. PCC 6803 reconstituted with ferric heme undergoes a rapid chemical reaction resulting in the attachment of the heme group to the polypeptide chain. The nature of the cross-linked species was studied by NMR and mass spectral methods. 1H NMR data indicated that the 2-vinyl group was the reacting moiety of the heme. Mass spectrometry of pepsin digests located the site of attachment within a 12-mer at the C-terminal end of the protein. Homonuclear and 1H−15N NMR data identified the modified residue as His117, which underwent addition to the vinyl Cα through the imidazole Nε. Dithionite treatment of the globin reconstituted with Zn protoporphyrin IX sample did not lead to 2-vinyl group modification, suggesting that the chemical reduction of the heme iron facilitated the attachment.