Properties and structure of a low-potential, penta-heme cytochrome c 552 from a thermophilic purple sulfur photosynthetic bacterium Thermochromatium tepidum
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The thermophilic purple sulfur bacterium Thermochromatium tepidum possesses four main water-soluble redox proteins involved in the electron transfer behavior. Crystal structures have been reported for three of them: a high potential iron–sulfur protein, cytochrome c′, and one of two low-potential cytochrome c 552 (which is a flavocytochrome c) have been determined. In this study, we purified another low-potential cytochrome c 552 (LPC), determined its N-terminal amino acid sequence and the whole gene sequence, characterized it with absorption and electron paramagnetic spectroscopy, and solved its high-resolution crystal structure. This novel cytochrome was found to contain five c-type hemes. The overall fold of LPC consists of two distinct domains, one is the five heme-containing domain and the other one is an Ig-like domain. This provides a representative example for the structures of multiheme cytochromes containing an odd number of hemes, although the structures of multiheme cytochromes with an even number of hemes are frequently seen in the PDB database. Comparison of the sequence and structure of LPC with other proteins in the databases revealed several characteristic features which may be important for its functioning. Based on the results obtained, we discuss the possible intracellular function of this LPC in Tch. tepidum.
KeywordsPurple sulfur bacteria Electron transfer Cytochrome c Multiheme Crystal structure Thermochromatium tepidum
Electron paramagnetic resonance
High potential iron–sulfur protein
Multiheme cytochrome c
Single wavelength anomalous diffraction
The authors thank Drs. M. Suga and Y. Umena for their help in the structural analysis of LPC, Dr. Y. Kashino for his help in identifying the LPC in the early stage of this work, Dr. M. T. Madigan for providing the Tch. tepidum strain, the staff members at beamlines BL41XU and BL26B1 of SPring-8 for their help in data collection, and the staff members of the Faculty of Science, Okayama University for their help in N-terminal sequencing and DNA sequencing experiments. This work was supported by the National Key R&D Program of China (2017YFA0503700), a Strategic Priority Research Program of CAS (XDB17000000), a CAS Key Research Project for Frontier Science (QYZDY-SSW-SMC003), and a JSPS KAKENHI Grant Number JP17H06433. JHC acknowledges the financial support provided by China Scholarship Council (CSC). AB was supported in part by the French Infrastructure for Integrated Structural Biology (FRISBI) ANR-10-INBS-05. The sequences were deposited in GenBank under the Accession Number MH000336, and the coordinate and the structure factors were deposited in the Protein Data Bank under Accession Number 5ZE8.
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