Covalent structure of the membranous segment of horse cytochrome b5. Chemical cleavage of the native hemoprotein.

The complete covalent structure of the membranous segment of horse liver cytochrome b5 has been determined. This peptide spans residues 91 to 133 of the cytochrome molecule, and contains the segment responsible for the association of the hemoprotein with microsomal or synthetic vesicles. Two peptides, residues 91 to 127 and 128 to 133, comprising the entire membranous moiety were isolated from a tryptic digest of urea-denatured apoprotein. The membranous segment (residues 91 to 127) could be separated from all other tryptic peptides by a single gel filtration step. Trypsin digestion of succinylated cytochrome produced similar peptides, residues 89 to 127 and 128 to 133. The covalent structures for residues 89 to 127 and 128 to 133 were derived from automated sequenator analysis of tryptic peptides. Chemical cleavage at tryptophanyl, or methionyl residues, or both, by the method of Ozols and Gerard ((1977) J. Biol. Chem. 252, 5986-5989) provided the overlapping peptides from which the following unique sequence was deduced: (formula: see text).