Abstract
The inducible enzyme catalysing the conversion of cis,cis-muconate to (+)-muconolactone was purified 300-fold from the yeast Trichosporon cutaneum, grown on phenol. The enzyme has a sharp pH optimum at pH 6.6. It reacts also with several monohalogen derivatives and with one monomethyl derivative of cis,cis-muconate, but not with cis,trans- or trans,trans-muconate or 3-carboxy-cis,cis-muconate. In contrast with the corresponding enzymes in bacteria, the yeast enzyme does not require added divalent metal ions for activity and is not inhibited by EDTA. The purified enzyme can be resolved into two peaks by isoelectric focusing. The two forms have pI 4.58 (cis,cis-muconate cyclase I) and pI 4.74 (cis, cis-muconate cyclase II), respectively. Each of these is homogenous on polyacrylamide-gel electrophoresis in the absence or presence of sodium dodecyl sulphate. The two enzyme forms have the same molecular weight (50000) as determined by gel filtration and by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. They have the same Km value (25 microM) for cis,cis-muconate. They differ with respect to their content of free thiol groups. cis, cis-Muconate cyclase I contains one thiol group, essential for activity, but relatively stable upon storage. cis, cis-Muconate cyclase II contains two thiol groups that are readily oxidized during storage with concomitant loss of activity.