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Structural aspects of tyrosine phenol-lyase catalytic activity

A homotetrameric, pyridoxal-5'-phosphate (PLP)-dependent enzyme tyrosine phenol-lyase (TPL EC 4.1.99.2) catalyses the reversible hydrolytic cleavage of L-tyrosine to phenol and ammonium pyruvate (the b-elimination of L-tyrosine) [R. S. Phillips, et al. Biochim. Biophys. Acta 1647 (2003) 167– 172]. TPL is used in biotechnology, because in vitro it also catalyses the b-elimination reactions of some other amino acids and their derivatives, b-replacement reactions, racemization of alanine and the synthesis of some L-tyrosine derivatives (e.g. 3, 4-dihydroxy-L-phenylalanine (L-DOPA) – the precursor in the synthesis of dopamine [S.-G. Lee, et al. Enzyme Microb. Technol. 25 (1999) 298– 302]). The b-elimination is mechanistically interesting reaction which proceeds via several intermediate steps, including the Cb-Cg bond cleavage. In order to understand structural events during the catalysis, we determined the X-ray structures of several different forms of TPL from Citrobacter freundii, including the structures of complexes which resemble the key reaction intermediates. The previously known crystal structures of the apoenzyme [A. A. Antson, et al. Biochemistry 32 (1993) 4195-4206] and the holoenzyme complexed with a substrate analogue [B. Sundararaju, et al. Biochemistry 36 (1997) 6502– 6510] were solved at a lower resolution and using the data obtained from the crystals grown at pH 6.0. In contrast, we studied the crystals grown at pH 8.0, which is closer to the physiological pH. There are two subunits in the asymmetric unit constituting the "catalytic dimer". Active sites are located at the interface between the two noncrystallographically related subunits. In most of the solved structures one active site is in the closed, while the other one is in the open conformation. Certain favourable interactions between the substrate molecule and the active site residues are only present in the closed conformation, so it is assumed that the closing of the active site during the enzymatic reaction is important for the enzyme catalytic efficiency.

tyrosine phenol-lyase; reaction mechanism; enzymatic activity

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