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Regulation of phenylalanine ammonia-lyase enzyme in Annona fruit: Kinetic characteristics and inhibitory effect of ammonia

In this work, we analyzed the kinetic properties of phenylalanine ammonia-lyase (PAL) extracted from "cherimoya" (Annona cherimola Mill.) fruits ripened at ambient temperature (20C) and stored under several environmental conditions, including high CO 2 levels (20%) and low temperature (6C). The effect of different ammonia-related compounds on cherimoya PAL activity was also evaluated. PAL exhibited two different K m values for L-phenylalanine (L-Phe) and negative substrate cooperativity, with Hill coefficient (n app ) values reaching 0.64 and 0.71 for low temperature and high CO 2 levels, respectively. The kinetic analysis revealed that ammonia produced mixed inhibition of PAL enzyme, with inhibition constants (K i and K i ') values of 0.57 ± 0.2 mM and 2.54 ± 0.2 mM. We propose that the regulation of PAL by ammonia inhibition and the negative cooperativity may be essential in adjusting the active phenylpropanoid metabolism in Annonas to the requirement of L-Phe and in consequence, to the carbon skeleton demand for other anabolic pathways.