The Complete Amino Acid Sequence of Chromatium High Potential Iron Sulfur Protein

High potential iron-sulfur protein isolated from both autotrophically and heterotrophically grown cells of Chromatium vinosum strain D was found to contain a single 85-amino acid residue polypeptide chain which is covalently linked to the iron-sulfur cluster via 4 residues of cysteine. The molecular weight of the protein, including the 4 atoms of iron and acid labile sulfur, is 9,254. The iron-sulfur complex must be removed from the protein before complete digestion by proteolytic enzymes like trypsin, chymotrypsin, or thermolysin can be attained. Digestion of the holoprotein by these enzymes leads to large peptides which contain the central portion of the sequence and comprise 35 to 45 residues, depending on the enzyme used, as well as residual iron presumably attached to the cysteines in positions 43, 45, 63, and 77. Both ends of the protein are readily accessible to exopeptidases and can be degraded substantially. Cysteines 43 and 45 and the single histidyl residue in position 42 are in a configuration reminiscent of that found at the heme attachment site of c-type cytochromes. The complete amino acid sequence of Chromatium high potential iron-sulfur protein was deduced from analyses of peptides obtained by proteolytic digestion of the apoprotein, the tryptic and chymotryptic core peptides, and of the two large peptides produced by chemical fragmentation of the apoprotein with BrCN. The first 18 residues of the protein were determined by stepwise Edman degradation. This approach was also applied to the core peptides and to both BrCN fragments.