Collect. Czech. Chem. Commun. 1983, 48, 668-671

Modification of tyrosine residues of tomato pectin esterase

Oskar Markovič

Institute of Chemistry, Slovak Academy of Sciences, 842 38 Bratislava


Selective acetylation of tyrosine residues in tomato pectin esterase with an 80 molar excess of n-acetylimidazole reduced the activity of the enzyme to 50%. Deacetylation with hydroxylamine restored the original activity of the enzyme. The difference in absorptivities at 278 nm showed that 2 tyrosine residues of the enzyme had been acetylated. Spectrophotometric pH-titration of native pectin esterase revealed that 2 of the 12 tyrosine residues in the enzyme molecule were ionized at pH 9.3-9.5, and the remaining 10 residues at pH > 10.5. Acetylation of the pectin esterase with acetanhydride resulted in an irreversible inhibition of the enzyme. Nitration of the enzyme with tetranitromethane also suggested a role of the tyrosine residues in the catalytic function of the enzyme. However, 10 min after the start of the nitration precipitation set in.