Methionine ionizing groups

Distinct changes in several properties of lysozyme occur after reaction with ozone. The lytic activity of the ozonized lysozyme shows the same trend at various pHs as the native enz3mie (Fig. 2) this may suggest that the pK values of the ionizable groups involved in catalysis have not been altered by ozonplysis. The amino acid composition of ozonized lysozyme differs from that of the native enz3mie in three residues — methionine, tryptophan and t3H osine. None of the other amino acids is affected by ozone. The extensive loss of enz5miic activity must be ascribed to the oxidative modification of these three amino acid residues in the lysozyme. 

Methylation is rarely of quantitative importance in the metabolism of xenobiotics. The methyl group is transferred from the nucleotide S-adenosyl-L-methionine (SAM) by means of a methyltransferase. The functional groups that undergo methylation include primary, secondary and tertiary amines, pyridines, phenols, catechols, thiophenols. The aza-heterocycle pyridine is metabolized to the A-methylpyridin-ium ion, which is more toxic than pyridine itself (Figure 33.18). The binding properties of the ionized metabolite are disturbed by the loss of its hydrophobic feature, resulting from the polarity inversion. 

II. Amino acids with un-ionized, but polar substituents, such as —OH, —SH, —CO NHj, and a few heterocyclic ones tyrosine, tryptophan, serine, threonine, cysteine, cystine, and methionine. To this group belong also asparagine and glutamine, the acid amides of the acidic amino acids (Group III). The acidic properties of the carboxyl group are lost through amide formation. 

Figure 1 Ionization of amino acids in solutions. pH dependencies of ionic fractions a - amino acids with aliphatic radicals (alanine, glycine, isoleucine, leucine, and valine) b - amino acids with aromatic radical (phenylalanine), sulfur-containing radical (methionine), and nitrogen in the cycle (proline) c -amino di-carboxylic acids (aspartic acid and glutamic acid) d - amino acid with phenol group (tyrosine) e - di-amino acid (lysine) f - dihydroxyphenylalanine (DOPA). One can see that there is no significant difference in ionization of amino acids that have aliphatic radicals while ionization of amino acids with more complicated radicals differs significantly...

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