Identification and characterization of a recombinant cognate hemoglobin reductase from Synechocystis sp. PCC 6803

One popular and relevant proposed function for cyanobacterial hemoglobin (Synechocystis Hb) is anaerobic nitrite reductase in vivo. During such reduction reactions, the hexacoordinated heme iron atom of SynHb is oxidized from the ferrous (Fe⁺²) to ferric (Fe⁺³) state and prevent damage by limiting the concentration of toxic metabolites such as nitrite. In order to perform these functions in vivo, there must be a mechanism that converts inactive Fe⁺³-SynHb back to the active Fe⁺²-SynHb to accomplish the nitrite reductase function. Here, we report a cognate reductase protein for Synechocystis hemoglobin which can reduce the Fe⁺³-SynHb to Fe⁺²-SynHb, thus lending a support to the proposed nitrite reductase function. This reductase is also able to reduce pentacoordinate Hbs such as myoglobin but with lower affinity compared to hexacoordinate SynHb. Insilico model of reductase protein-cyanobacterial hemoglobin complex revealed that the heme active site of Hb faces the catalytic center of the reductase protein and several amino acids in the interface interacts non-covalently thus favoring their interaction. Overall, our in vitro study provides the basic foundation for the understanding of the specific molecular mechanism of action and interaction of the SynHb reductase protein, which need to be investigated in further detail.