Cytochrome c550 (Cc550) is a c-type heme protein with a bis-histidinyl axial coordination. It has been described as an extrinsic component of the photosystem II (PSII) membrane complex in cyanobacteria and some eukaryotic algae, although its role within the PSII complex is unknown. Cc550 purified from the diatom Phaeodactylum tricornutum is mostly obtained from the soluble cell extracts, and seems to be truncated in the last two hydrophobic residues of the C-terminal, as deduced from the gene sequence. In addition, Cc550 was absent in purified PSII samples, indicating a low affinity of this protein for the membrane complex. Oxidized Cc550 has been characterized using continuous wave (CW) EPR as well as pulse techniques including ESEEM and HYSCORE. CW measurements reveal a typical low spin heme centre and provide information on the crystal field symmetry related with the axial coordination. ESEEM and HYSCORE experiments show hyperfine coupling of the unpaired spin with nitrogen and hydrogen nuclei in its close vicinity. These interactions are in agreement with the distortion of the heme-axial ligands and loss of the co-planarity of the imidazole ring. The relevant changes in the structure of the single occupied molecular orbital (SOMO) of the heme centre in Cc550 could have implications in their function. Further research is in progress to confirm this point.

Resumen del trabajo presentado al IX Meeting of the Protein Structure and Function Network (Reunión Temática de la Red de Estructura y Función de Proteínas), celebrada en Sevilla (España) del 11 al 13 de noviembre de 2015.

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