Studies of enzyme-mediated reactions. Part II. Stereochemistry of the elimination of ammonia from L-tyrosine catalysed by the enzyme from maize

Abstract

A stereoselective synthesis has been used to yield two samples of (2RS)-tyrosine ( DL-tyrosine) in which the (2S)-form of one carries a (3R)-[3-³H₁] label and that of the other, a (3S)-[3-³H₁] label. The configurations of these products at C-3 have been rigorously determined by degradation via aspartic acid and malic acid to fumaric acid, the final step being enzymic. It has been proved, by use of the labelled samples of tyrosine, that the enzyme from maize eliminates the 3-pro-S hydrogen atom of (2S)-tyrosine together with ammonia to give trans-p-hydroxy-cinnamic acid. Experiments with stereospecifically labelled (2S)-phenylalanine have shown that the degree of 3-pro-S specificity of the maize enzyme is indistinguishable from that of the potato enzyme. Studies of partial conversions with the maize enzyme have confirmed that the same active site acts on both (2S)-tyrosine and (2S)-phenylalanine.