Studies on the Mechanism and Stereospecificity of the Urocanase Reaction

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Abstract
Nuclear‐magnetic‐resonance spectroscopy was used to follow the course of the urocanase reaction. It was shown that 3‐(4′‐hydroxyimidazol‐5′‐yl)‐propionate is the true enzymic product and that it spontaneously tautomerises to recemic 3‐(imidazol‐4′‐one‐5′‐yl)‐propionate. Conversion of urocanate in deuterium oxide by a crude‐enzyme preparation from Pseudomonas putida led to a product containing two adjacent CH2H‐groups. Oxidative degradation of this product afforded optically pur (RR)‐[2H2]succinic acid. This finding defines the sterochmistry of proton addition to the double bond of urocate.