Enzymatic and nonenzymatic dehydration reactions of L-arogenate

L-Arogenate, an immediate precursor of either L-tyrosine, L-phenylalanine, or both in many microorganisms and plants, may undergo two types of dehydration reactions that yield products of increased stability. Under acidic conditions, a facile aromatization attended by loss of the C-4 hydroxyl and the C-1 carboxyl moieties results in quantitative conversion to L-phenylalanine. When aromatization was largely prevented by maintaining pH in the range of 7.5-12, a second dehydration reaction occurred in which the alanyl side chain and the carboxyl group at C-1 formed a lactam ring to yield spiro-arogenate. The latter reaction occurs at 100 degrees C, roughly 50% conversion being obtained in 2 h. The product formed from L-arogenate was authentic spiro-arogenate, as demonstrated by high-performance liquid chromatography and thin-layer chromatography identification procedures. Further confirmation was obtained by 1H nuclear magnetic resonance, ultraviolet spectroscopy, and mass spectrometry. Thus far, the conversion of L-arogenate to spiro-arogenate is not known to be enzyme catalyzed. The other dehydratase reaction, however, is catalyzed in nature by an enzyme denoted arogenate dehydratase. An improved assay is described for this in which [3H]dansyl derivatives of L-arogenate (substrate) and L-phenylalanine (product) are separated by using bidimensional thin-layer chromatography. The radioactive reaction product is then quantitated. This assay was used to study partially purified arogenate dehydratase from Pseudomonas diminuta, an organism that depends upon the arogenate pathway for L-phenylalanine biosynthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and structure determination of a novel spiro-gamma-lactam, spiro-arogenate

The eucaryotic microorganism, Neurospora crassa, is able under specified conditions (Zamir, L.O., Jung, E., and Jensen, R.A. (1982) J. Biol. Chem. 258, 6492-6496) to synthesize a cyclohexadienyl derivative of prephenic acid having the novel structure of a spiro-gamma-lactam. This L-gamma-(spiro-4-hydroxy-2,5-cyclohexadienyl)-pyroglutamate is herein given the trivial name, spiro-arogenate, to indicate its close relationship to the amino acid, L-arogenate. Spiro-arogenate is quantitatively converted to phenylalanine at mildly acidic pH and can be converted to arogenate by boiling at basic pH. The structure of spiro-arogenate was established through the application of spectroscopic techniques (ultraviolet, 1H-NMR, 13C-NMR, and mass spectrometry). The 1H-NMR and 13C-NMR spectra of spiro-arogenate isolated as the natural product conformed to the spectrum of spiro-arogenate prepared by chemical synthesis by S. Danishefsky and co-workers (Danishefsky, S., Morris, J., and Clizbe, L.A. (1981) J. Am. Chem. Soc. 103, 1602-1604). Circular dichroism established the S configuration of the asymmetric carbon at C-8 of spiro-arogenate.