Web Release Date: December 5,
The Mechanism of MIO-Based Aminomutases in 
-Amino Acid Biosynthesis
and
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, Department of Chemistry, Division of Pharmaceutical Sciences, and University of Wisconsin National Cooperative Drug Discovery Group, University of Wisconsin-Madison, Madison, Wisconsin 53705
Received August 20, 2007
Abstract:
-Amino acids are widely used building blocks in both natural and synthetic compounds. Aromatic -amino acids can be biosynthesized directly from proteinogenic 
-amino acids by the action of MIO (4-methylideneimidazole-5-one)-based aminomutase enzymes. The uncommon cofactor MIO plays a role in both ammonia lyases and 2,3-aminomutases; however, the precise mechanism of the cofactor has not been resolved. Here we provide evidence that the electrophilic cofactor uses covalent catalysis through the substrate amine to direct the elimination and subsequent readdition of ammonia. A mechanism-based inhibitor was synthesized and the X-ray cocomplex structure was determined to 2.0 Å resolution. The inhibitor halts the chemistry of the reverse reaction, providing a stable complex that establishes the mode of substrate binding and the importance of tyrosine 63 in the chemistry. The proposed mechanism is consistent with the biochemistry of aminomutases and ammonia lyases and provides strong support for an amine-adduct mechanism of catalysis for this enzyme class.
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