Modification of tyrosine residues of tomato pectin esterase

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Abstract
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.

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