Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Plant Physiology 48:130-136 (1971)
© 1971 American Society of Plant Biologists

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Articles

L-Phenylalanine Ammonia-Lyase (Maize)

Evidence for a Common Catalytic Site for L-Phenylalanine and L-Tyrosine 1

Evelyn A. Havira

Philip D. Reidb,2 and Herbert V. Marsh, Jr.b

a Department of Biochemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504, Department of Botany and Department of Plant and Soil Sciences, University of Massachusettes, Amherst, Massachusetts 01002

L-Phenylalanine ammonia-lyase (E.C. 4.3.1.5) from maize is active with L-tyrosine and L-phenylalanine and exhibits atypical Michaelis-Menten kinetics with both substrates. With phenylalanine as a substrate, the pH optimum is 8.7 and with tyrosine, 7.7. The estimated Km at high substrate concentrations is 0.27 mM for phenylalanine and 0.029 mM for tyrosine. However, the Vmax with phenylalanine is eight times higher than the Vmax with tyrosine when both are measured at pH 8.7, and 7 times higher when both are measured at their pH optima. The following evidence leads us to the conclusion that there is a common catalytic site for both substrates: (a) It is impossible to appreciably alter the ratio of the two activities during purification and isoelectric focusing. (b) The ratio of the products formed in mixed substrate experiments is in good agreement with the ratio predicted from the estimated Km values. (c) NaBH4 reduces both activities to the same degree and L-phenylalanine, L-tyrosine, cinnamate, and p-coumarate protect both activities against NaBH4 reduction to the same degree. In contrast, the enzyme isolated from potato, which does not act on L-tyrosine, is not protected against reduction by either L-tyrosine or p-coumarate. However, both enzymes appear to have a dehydroalanine-containing prosthetic group.

2 Present address: Department of Plant Sciences, University of California, Riverside, Calif. 92502.

1 This work was supported in part by Grant GB-12428 from the National Science Foundation.






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