Oxidative pathway for <scp>L</scp>-rhamnose degradation in <i>Pullularia pullulans</i>

Abstract

Growth of the fungus Pullularia pullulans on L-rhamnose induces the synthesis of L-rhamnofuranose dehydrogenase and other enzymes active in dissimilation of L-rhamnose (6-deoxy-L-mannose). The present findings indicate that at pH 7.0 a lactonase is active in the hydrolysis of L-rhamnono-γ-lactone. The resulting L-rhamnonate is then dehydrated into 2-keto-3-deoxy-L-rhamnonate by an L-rhamnonate dehydratase. It is also shown that, in the extracts, 2-keto-3-deoxy-L-rhamnonate aldolase participates in the catalysis of the cleavage of 2-keto-3-deoxy-L-rhamnonate to form pyruvate and L-lactaldehyde. The presence of D-glucose (0.2%) together with the inducer in the induction medium gives rise to approximately 50% repression of dehydrogenase synthesis, 80% repression of dehydratase synthesis, and 87% repression of aldolase synthesis. Induction of the enzymes of this nonphosphorylative pathway is completely inhibited in the presence of high levels of D-glucose (2%). It is suggested that these enzymes are active in the metabolism of L-rhamnose by the following pathway: L-rhamnose → L-rhamnono-γ-lactone → L-rhamnonate → 2-keto-3-deoxy-L-rhamnonate → pyruvate plus L-lactaldehyde, and that this is believed to be the route for L-rhamnose dissimilation in this microorganism.