CIS,CIS-MUCONATE LACTONIZING ENZYME FROM TRICHOSPORON-CUTANEUM - EVIDENCE FOR A NOVEL CLASS OF CYCLOISOMERASES IN EUKARYOTES

The absolute stereochemical courses of cis,cis-muconate lactonizing enzyme (MLE; EC 5.5.1.1) from Trichosporon cutaneum (TcMLE) and chloromuconate cycloisomerase (MLE II; EC 5.5.1.7) from Pseudomonas sp B13 have been determined from H-1 NMR measurements. Both cycloisomerases convert cis,cis-muconate to (4S)-muconolactone by a syn lactonization, the absolute stereochemical outcome of which is identical to that observed with MLE from Pseudomonas putida. The regiochemical courses of cyclization of 3-halo-cis, cis-muconates by TcMLE and MLE II have been characterized and shown to differ in a halogen substituent dependent manner, suggesting at least a different active site architecture of the two MLEs. Moreover, the regiochemical preferences of MLE II and TcMLE parallel results previously observed for the nonenzymatic lactonization of the 3-halomuconates at pH 1-6 and in concentrated HCl, respectively, in which alternate mechanisms of cyclization were proposed [Pieken, W. A., and Kozarich, J. W. (1990) J. Org. Chem. 55, 3029-3035]. Complementary DNA clones encoding TcMLE have been isolated from phenol induced T. cutaneum cDNA using the polymerase chain reaction. The deduced amino acid sequence does not exhibit any similarity to that of MLE from P. putida. It does however, exhibit moderate sequence similarity (21% residue identity, 14 gaps) with 3-carboxy-cis,cis-muconate lactonizing enzyme (CMLE; EC 5.5.1.5) from Neurospora crassa, which catalyzes a regiochemically analogous and stereochemically identical lactonization reaction with 3-carboxymuconate. The limited data available suggest that the fungal CMLE and yeast MLE are representative of a unique class of eucaryotic cycloisomerases which have evolved convergently with the bacterial MLEs.