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Stereochemical studies on tyrosine phenol-lyase from Escherichia intermedia have shown that the α,β-elimination reactions of ʟ-serine and ᴅ- and ʟ-tyrosine proceed with retention of config­uration at C-β. Stereospecifically (β-tritiated ʟ-serine is slowly racemized at C-β Deuterium from the α-position of ʟ-tyrosine is partially transferred to C-4 of the phenol formed when the α,β- elimination reaction is carried out in H 2 O, although no transfer of α- 1 H in  2 H 2 O was seen. The result favors tautomerization of the p-hydroxyphenyl to a cyclohexadienonyl moiety prior to carbon-carbon bond cleavage. In the conversion of ʟ- to ᴅ-alanine catalyzed by tyrosine phenol- lyase, some a-hydrogen recycling is observed, pointing to a single-base racemization mechanism. Attempts to demonstrate cofactor motion during racemization by NaBH 4  reduction of [ 3 H]PLP- enzyme: ᴅ- and ʟ-alanine complexes failed, but showed that, as in other PLP enzymes, the holoenzyme is reduced preferentially from the Re face with respect to C-4′ of PLP and enzyme- substrate complexes preferentially from the Si face.

Stereochemistry and Mechanism of Reactions Catalyzed by Tyrosine Phenol-Lyase from Escherichia intermedia