Histidine ammonia-lyase. The use of 4-fluorohistidine in identification of the rate-determining step.
The alpha,beta eliminations of NH3 from L-histidine and 4-fluoro-L-histidine by histidine ammonia-lyase appear to occur by similar mechanisms, although a large difference in Vmax for the two reactions was observed. Both reactions were shown to be reversible with an equilibrium constant of 4 to 5. The presteady state kinetics of the deamination of 4-fluoro-L-histidine indicates that the rate-determining step precedes the dissociation of ammonia from the enzyme. The isotope effect of 1.4 to 2.0 observed with 4-fluoro-DL-[beta-2-H2]histidine or DL-[beta-2-H2]histidine or DL-[beta-2-H2]histidine indicates that the C-H bond breakage is at least partially rate-determining for the deamination of both substrates.
D000642:Ammonia-Lyases / Q000378:metabolism*; D002250:Carbon Radioisotopes; D002855:Chromatography, Thin Layer; D003903:Deuterium; D006639:Histidine / Q000031:analogs & derivatives; D006638:Histidine Ammonia-Lyase / Q000378:metabolism*; D007700:Kinetics; D011549:Pseudomonas / Q000201:enzymology; D014316:Tritium; D014560:Urocanic Acid / Q000378:metabolism
D002250:Carbon Radioisotopes; D014316:Tritium; D006639:Histidine; D003903:Deuterium; D000642:Ammonia-Lyases; D006638:Histidine Ammonia-Lyase; D014560:Urocanic Acid