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4-Nitro-L-histidine as a substrate for histidine ammonia-lyase: The role of β-hydrogen acidity in the rate-limiting step

1979; Elsevier BV; Volume: 87; Issue: 1 Linguagem: Inglês

10.1016/0006-291x(79)91685-1

1090-2104

Claude B. Klee, Kenneth L. Kirk, Louis A. Cohen,

Plant nutrient uptake and metabolism

The Km for the interaction of 4-nitro-L-histidine with histidine ammonia-lyase (reduced enzyme, pH 8.0) is comparable to that for L-histidine, while Vmax is 18 that for the natural substrate. With the analog, addition of Cd+2 effects a small decrease in Km but fails to alter Vmax; the normal deuterium isotope effect for removal of the β-hydrogen (1.5–2.0) is eliminated; and enzyme-catalyzed incorporation of solvent tritium into substrate occurs to a much greater extent than into histidine. Thus, the nitro group increases the acidity of the β-hydrogen and the stability of the conjugate carbanion to such a degree that CH bond cleavage now precedes rate-limiting CN bond cleavage.