Primary Structure of Chromatium tepidum High-Potential Iron-Sulfur Protein in Relation to Thermal Denaturation

A high-potential ferredoxin (HiPIP) has been purified from the thermophilic purple sulfur bacterium Chromatium tepidum. Most of the properties of this protein, including absorption and electron paramagnetic resonance spectra as well as redox potential, are identical to those of the similar protein isolated from the mesophilic organism Chromatium vinosum. The similarity extends to the amino acid sequences, which share 74 of the 83 residues composing the primary structure of C. tepidum HiPIP. The latter has been determined by sequencing overlapping peptides and precisely measuring the molecular mass of the holoprotein (9136 Da) by electrospray ionization mass spectrometry. The most significant difference between these sequences involves a stretch of 8 amino acids, which is shortened by two residues and notably changed in C. tepidum HiPIP. This region had been identified in the three-dimensional structure of C. vinosum HiPIP as both a link between two strands of a twisted β sheet coordinating the [4Fe-4S] cluster and an area of strong interaction of the molecule with the solvent. These data have been used to discuss the molecular basis for the slightly improved thermal stability of C. tepidum HiPIP, as compared to C. vinosum HiPIP. Based on the physiological differences distinguishing C. tepidum from other small-sized Chromatiaceae, the presence of an abundant HiPIP in C. tepidum indicates that involvement as electron acceptor for the previously proposed thiosulfate oxidizing activity in C. vinosum may not be the sole function in all purple sulfur bacteria.