Design of Fluorescent Substrates and Potent Inhibitors of CYP73As, P450s That Catalyze 4-Hydroxylation of Cinnamic Acid in Higher Plants

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Abstract
CYP73As are the major functional cytochromes P450 in higher plants. Several of them have been shown to catalyze the 4-hydroxylation of cinnamic acid, the first oxidative step in the synthesis of lignin, flavonoids, coumarins, and other phenylpropanoids. The coding sequence for CYP73A1, the enzyme from Helianthus tuberosus, has been isolated and expressed in yeast. Previous studies indicate that the yeast-expressed enzyme is capable of metabolizing cinnamic acid and several small, planar molecules but with low efficiency. Using this we further examined how CYP73A1 could bind and metabolize a set of possible alternate substrates. We show here that naphthalenes, quinolines, and indoles substituted with an aldehyde, a carboxylic, or a sulfonic acid group make good ligands and substrates for CYP73A1. The best ligands are hydroxynaphthoic acids, which show higher affinity than cinnamate. Naphthalene, 2-naphthol, and molecules with two-carbon side chains, such as natural and synthetic auxins, are not substrates of this enzyme. Methyl-2-naphthoate and 2-hydroxy-1-naphthoic acid are strong ligands of CYP73A1 but are not metabolized. Uncoupling and low spin conversion induced by these compounds suggest that their positioning in the heme pocket is inadequate for catalysis. These compounds can act as potent inhibitors of the second step of the phenylpropanoid pathway, the first described so far. The molecule which most closely mimics cinnamic acid, 2-naphthoic acid, is metabolized with a catalytic turnover and efficiency similar to those measured with the physiological substrate. Using this compound we designed a fluorometric assay to measure the catalytic activity of CYP73As. This assay was then used to monitor the CYP73As activity in microsomes from transgenic yeast and several plant species.