4j20
X-ray structure of the cytochrome c-554 from chlorobaculum tepidum
Structural highlights
FunctionC555_CHLTE This basic c-type monoheme cytochrome has been found exclusively in the green photosynthetic bacteria, although its role in bacterial photosynthesis is not established. It has an unusually low redox potential compared with mitochondrial cytochrome c. It is reactive with cytochrome c oxidases but not with reductases. Publication Abstract from PubMedThe cytochrome (Cyt) c-554 in thermophilic green photosynthetic bacterium Chlorobaculum tepidum serves as an intermediate electron carrier, transferring electrons to the membrane-bound Cyt c z from various enzymes involved in the oxidations of sulfide, thiosulfate, and sulfite compounds. Spectroscopically, this protein exhibits an asymmetric alpha-absorption band for the reduced form and particularly large paramagnetic 1H NMR shifts for the heme methyl groups with an unusual shift pattern in the oxidized form. The crystal structure of the Cyt c-554 has been determined at high resolution. The overall fold consists of four alpha-helices and is characterized by a remarkably long and flexible loop between the alpha3 and alpha4 helices. The axial ligand methionine has S-chirality at the sulfur atom with its CepsilonH3 group pointing toward the heme pyrrole ring I. This configuration corresponds to an orientation of the lone-pair orbital of the sulfur atom directed at the pyrrole ring II and explains the lowest-field 1H NMR shift arising from the 181 heme methyl protons. Differing from most other class I Cyts c, no hydrogen bond was formed between the methionine sulfur atom and polypeptide chain. Lack of this hydrogen bond may account for the observed large paramagnetic 1H NMR shifts of the heme methyl protons. The surface-exposed heme pyrrole ring II edge is in a relatively hydrophobic environment surrounded by several electronically neutral residues. This portion is considered as an electron transfer gateway. The structure of the Cyt c-554 is compared with those of other Cyts c, and possible interactions of this protein with its electron transport partners are discussed. Structure analysis and characterization of the cytochrome c-554 from thermophilic green sulfur photosynthetic bacterium Chlorobaculum tepidum.,Yu LJ, Unno M, Kimura Y, Yanagimoto K, Oh-Oka H, Wang-Otomo ZY Photosynth Res. 2013 Sep 20. PMID:24052268[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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