The role of acidic dissociation of substrate's phenol group in the mechanism of tyrosine phenol-lyase.
pH dependencies of the main kinetic parameters for the reaction of tyrosine phenol-lyase (TPL) with L-tyrosine were studied earlier at pH from 6.0 to 9. 5. At this range, L-tyrosine, whose pK(a) for the phenol hydroxyl is 10.5, exists as the zwitterion. It was concluded that zwitterion is the only "active" form for any tyrosine-like substrate. In the present work, we examined pH dependencies for 2-fluorotyrosine, 3-fluorotyrosine, 3,5-difluorotyrosine, 2,5-difluorotyrosine, 2,6-difluorotyrosine, and 3-chlorotyrosine which are more acidic than tyrosine. Respective pK(a)'s are 9.2, 8.7, 7.3, 7.9, 8,35, and 8. 3. At higher pH, most of these substrates exist predominantly as anions, having two negative charges at the carboxylic and phenol groups, and one positive charge at the amino group. No decrease of k(cat)/K(m) values attributable to acidic dissociation of the phenol group was found. From comparison of theoretical curves with the experimental data, we conclude that most likely, both zwitterion and anion forms of 3-fluorotyrosine, 3,5-difluorotyrosine, 2,5-difluorotyrosine, 2,6-difluorotyrosine, and 3-chlorotyrosine may be bound and subsequently catalytically transformed by TPL. The reactivities of the two forms are quite comparable. The roles of catalytic groups in the active site, especially Arg381 and Thr-124, are discussed.[1]References
- The role of acidic dissociation of substrate's phenol group in the mechanism of tyrosine phenol-lyase. Faleev, N.G., Axenova, O.V., Demidkina, T.V., Phillips, R.S. Biochim. Biophys. Acta (2003) [Pubmed]
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