The Reactivity of the Tyrosyl Residues of Cytochrome b,*

The reactivities of the tyrosyl residues of calf and rabbit cytochrome bs have been examined by acylation, iodination, and titration of the proteins. These experiments distinguish residues 6 and 7 as the most exposed and reactive residues, which ionize normally and can be acetylated or iodinated to form the monoiodo or diiodo derivatives. Tyrosine 30, which is common to both cytochrome preparations and “buried” in a nonpolar environment according to x-ray data, is iodinated more slowly without affecting the heme binding. However, this latter iodination does produce a structural change reflected in a less stable protein conformation. The calf liver heme protein contains a fourth tyrosyl residue at position 27 in a nonpolar peptide sequence of nine amino acids which can be isolated as a tryptic peptide. This residue, either in the intact protein or the isolated peptide fragment, is extremely unreactive and can be iodinated or ionized only in concentrated urea solutions.