Crystallization and preliminary crystallographic analysis of the high-potential iron-sulfur protein from Thermochromatium tepidum

The high-potential iron-sulfur protein (HiPIP) is an electron carrier between the photosynthetic reaction centre and the cytochrome bc₁ complex in the electron-transfer chain of photosynthesis. The purified HiPIP from Thermochromatium tepidum (formerly Chromatium tepidum) was crystallized in a solution of 1.4 M ammonium sulfate and 0.1 M sodium citrate pH 3.5. The crystals diffract X-rays beyond 1.4 Å resolution and belong to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 47.12 (6), b = 59.59 (10), c = 23.62 (3) Å. The structure was preliminarily solved by the molecular-replacement method. The crystal structure of HiPIP from T. tepidum showed that the proteins exist as monomers, although HiPIPs from several other species can form dimers.