Expression and characterization of cyanobacterium heme oxygenase, a key enzyme in the phycobilin synthesis

An efficient bacterial expression system of cyanobacterium Synechocystis sp. PCC 6803 heme oxygenase gene, ho‐1 , has been constructed, using a synthetic gene. A soluble protein was expressed at high levels and was highly purified, for the first time. The protein binds equimolar free hemin to catabolize the bound hemin to ferric‐biliverdin IX α in the presence of oxygen and reducing equivalents, showing the heme oxygenase activity. During the reaction, verdoheme intermediate is formed with the evolution of carbon monoxide. Though both ascorbate and NADPH‐cytochrome P450 reductase serve as an electron donor, the heme catabolism assisted by ascorbate is considerably slow and the reaction with NADPH‐cytochrome P450 reductase is greatly retarded after the oxy‐heme complex formation. The optical absorption spectra of the heme‐enzyme complexes are similar to those of the known heme oxygenase complexes but have some distinct features, exhibiting the Soret band slightly blue‐shifted and relatively strong CT bands of the high‐spin component in the ferric form spectrum. The heme‐enzyme complex shows the acid‐base transition, where two alkaline species are generated. EPR of the nitrosyl heme complex has established the nitrogenous proximal ligand, presumably histidine 17 and the obtained EPR parameters are discriminated from those of the rat heme oxygenase‐1 complex. The spectroscopic characters as well as the catabolic activities strongly suggest that, in spite of very high conservation of the primary structure, the heme pocket structure of Synechocystis heme oxygenase isoform‐1 is different from that of rat heme oxygenase isoform‐1, rather resembling that of bacterial heme oxygenase, Hmu O.