Crystal structure and redox properties of a novel cyanobacterial heme-protein with a His/Cys heme axial ligation and a per-arnt-sim (PAS)-like domain [Molecular Biophysics]

Photosystem II (PSII) catalyzes the light-induced water oxidation leading to the generation of dioxygen indispensable for sustaining aerobic life on Earth. The PSII reaction center is composed of D1 and D2 proteins encoded by the psbA and psbD genes, respectively. In cyanobacteria, different psbA genes are present in the genome. The thermophilic cyanobacterium Thermosynechococcus elongatus contains 3 psbA genes, psbA1, psbA2 and psbA3 and a new c-type heme protein, Tll0287, was found to be expressed in a strain expressing the psbA2 gene only, but the structure and function of Tll0287 are unknown. Here we solved the crystal structure of Tll0287 at a 2.0 Å resolution. The overall structure of Tll0287 was found to be similar to some kinases and sensor proteins with a per-arnt-sim (PAS)-like domain, rather than to other c-type cytochromes. The 5th and 6th axial ligands for the heme were Cys and His, instead of the His/Met or His/His ligand pairs observed for most of the c-type hemes. The redox potential, E1/2, of Tll0287 was -255 ± 20 mV versus normal hydrogen electrode at pH values above 7.5. Below this pH value, the E1/2 increased by ≈57 mV/pH unit at 15°C, suggesting the involvement of a protonatable group with a pKred = 7.2 ± 0.3. Possible functions of Tll0287 as a redox sensor under micro-aerobic conditions or a cytochrome subunit of an H2S-oxidising system, are discussed in view of the environmental conditions in which psbA2 is expressed as well as phylogenetic analysis, structural and sequence homologies.

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