Structural and dynamic properties of Synechocystis sp. PCC 6803 Hb revealed by reconstitution with Zn-protoporphyrin IX.

In its resting state, the truncated globin of the cyanobacterium Synechocystis sp. PCC 6803 exhibits hexacoordination of the heme iron, with His46 (E10) and His70 (F8, proximal) serving as axial ligands. Diatomic ligands displace the distal His46 (E10) from the ferric and ferrous iron and promote considerable structural changes in the B helix, E helix, and EF regions. Here, Zn(II)-substituted hemoglobin was used to explore the role of distal ligands in stabilizing the heme pocket structure. NMR data showed that the Zn ion was coordinated by the four pyrrole nitrogens and by His70 (F8) only. The proximal side of the Zn-porphyrin adopted a geometry recognizable as that of the wild-type protein. Decoordination of His46 (E10) to form the pentacoordinate Zn resulted in an incomplete transition to the conformation observed in the ferric, cyanide-bound protein. The NMR data also demonstrated that the H helix underwent complex dynamic processes near His117, a residue readily reacting with the wild-type heme 2-vinyl group in a post-translational modification.