Tryptophanase and tyrosine phenol-lyase

The closely related bacterial enzyme tyrosine phenol-lyase [137] has an even wider substrate and reaction specificity than tryptophanase, including the remarkable ability to cleave both D- and L-tyrosine and to interconvert D- and L-alanine. As already discussed and summarized in Tables 1 and 2, the stereochemistry at C-/J in all the a,/3-elimination and -replacement reactions of this enzyme studied so far is always retention [108,109,129]. This includes the a, -elimination of L- as well as of D-tyrosine. The fate of the a-hydrogen of L-tyrosine in this reaction has been probed in preliminary experiments (H. Kumagai, E. Schleicher and H.G. Floss, unpublished results), and the results tentatively suggest transfer of deuterium from the a-position to C-4 of the resulting phenol. Attempts to demonstrate intramolecularity of this transfer have so far been inconclusive. The base abstracting H-a in this enzyme may be histidine [138]. 

A system has recently been developed to examine the question whether any internal return of the H-a can be demonstrated in the interconversion of L- and D-alanine catalyzed by this enzyme. L-[2-2H]Alanine in HzO or unlabeled L-alanine in 2H20 are converted into D-alanine with tyrosine phenol-lyase in the presence of excess D-amino acidiacetyl-coenzyme A acetyl transferase [139] and acetyl-coenzyme A to ensure single turnover conditions. The resulting TV-acetyl-D-alanine is analyzed for its deuterium content by mass spectrometry and NMR. Initial data indicate 12% transfer of a- H from L-alanine to the D-isomer in 2HzO (S.-j. Shen, H. Kumagai and H.G. Floss, unpublished results), but these results need to be verified. If they are confirmed, racemization by a single base mechanism would be indicated. 

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Studies of the recycling of the cofactor in an enzyme reactor comparison of reactions using free or immobilized nicotinamide adenine dinucleotide Investigations of the role of carbonic anhydrases in vivo Purification of tryptophanase and tyrosine phenol-lyase... 

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