The Structure of Neurospora crassa 3-Carboxy-cis,cis-Muconate Lactonizing Enzyme, a β Propeller Cycloisomerase

Abstract Muconate lactonizing enzymes (MLEs) convert cis,cis -muconates to muconolactones in microbes as part of the β-ketoadipate pathway; some also dehalogenate muconate derivatives of xenobiotic haloaromatics. There are three different MLE classes unrelated by evolution. We present the X-ray structure of a eukaryotic MLE, Neurospora crassa 3-carboxy- cis,cis -muconate lactonizing enzyme ( Nc CMLE) at 2.5 A resolution, with a seven-bladed β propeller fold. It is related neither to bacterial MLEs nor to other β propeller enzymes, but is structurally similar to the G protein β subunit. It reveals a novel metal-independent cycloisomerase motif unlike the bacterial metal cofactor MLEs. Together, the bacterial MLEs and Nc CMLE structures comprise a striking structural example of functional convergence in enzymes for 1,2-addition-elimination of carboxylic acids. Nc CMLE and bacterial MLEs may enhance the reaction rate differently: the former by electrophilic catalysis and the latter by electrostatic stabilization of the enolate.