Studies on cytochrome c oxidase, VII. Isolation and chemical characterization of polypeptide VII

Polypeptide VII of cytochrome c oxidase was isolated and purified by gel filtration on Bio-Gel P-10 in 10% acetic acid. Automatic Edman degradation of this peptide chain was not successful, because it is blocked at the N-terminus. The amino acid analysis shows a relatively high content of hydrophilic residues (54%). On the basis of this analysis and the apparent molecular weight by sodium dodecyl sulfate gel electrophoresis and gel filtration, a chain length of about 80 residues was calculated. Among the tryptic peptides one blocked heptapeptide was found. Cleavage of this peptide with thermolysin gave two peptide fragments, one of which was not retained on a cation exchange resin. Mass spectrometric sequence determination of this peptide revealed the structure Ac-Ala-Glu-Asp for the N-terminus of polypeptide VII. Treatment with carboxypeptidase A at two different pH values showed that the C-terminal amino acid is isoleucine and the penultimate amino acid is lysine.