Abstract
Abstract Liver microsomal apocytochromes b5 from man, monkey (Alouatta fusca), pig, and chicken were subjected to trypsin digestion, and all of the peptides were isolated and characterized. The sum of the residues present in these peptides equaled the total amino acid composition of the corresponding parent cytochrome. For all of these peptides a homologous segment was found in our previously established bovine and rabbit cytochrome sequence. This information provided sufficient evidence to construct a unique amino acid sequence for human, monkey, porcine, and chicken cytochrome b5. Comparison of the six sequences indicated a close similarity between these proteins. The human cytochrome differed from that of chicken in 15 positions but from that of monkey in only 2 positions. With the exception of the avian protein sequence, the amino acid replacements were confined predominantly to the NH2- and COOH-terminal segments of the cytochrome. A segment comprising residues 42 to 72 was invariant in all six cytochromes.
Highlights
The total number of residues derived from the compositions of these peptides, listed in Tables I to IV, are in good agreement with the compositions of the parent proteins
The sequence of two tryptic fragments from the avian cytochrome further strengthens the assumption that the order of tryptic peptides is identical for all the cytochromes 65
The absence of trypsit-sensitive bonds in the avian protein segments comprised of Peptides (T-5)-(T-12) and (T-4)-(T-3) confirms that they are linked in that order
Summary
Methods of Sequence AnalysisThe sequential degradations ofEdman [14] were performed. The coupling with phenylisothiocyanate and its cyclization in anhydrous trifluoroacetic acid were carried out under nitrogen. Terminal residue was determined by amino acid analysis on a fraction of the residual peptide. The number of micromoles of amino acid present in the residual peptide was converted to a molar ratio, by dividing the analytical value of the residue by the mean value of the total composition. The mean value of the composition was obtained by dividing the sum of the analytical values of all amino acids by the assumed number of residues in the peptide. The assumed number of residues, if not evident, is given in parentheses following the molar ratio values. Amino acids present in peptides, before Edman degradations, in yields below 15’% of a residue are not reported or included in the calculations. A 0.0 molar ratio, following an Edman degradation step, represents less than 0.1 residue
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