l‐galactonate dehydratase is part of the fungal path for d‐galacturonic acid catabolism

Summary An l‐galactonate dehydratase and the corresponding gene were identified from the mould Hypocrea jecorina (Trichoderma reesei). This novel enzyme converts l‐galactonate to l‐threo‐3‐deoxy‐hexulosonate (2‐keto‐3‐deoxy‐l‐galactonate). The enzyme is part of the fungal pathway for d‐galacturonic acid catabolism, a pathway which is only partly known. It is the second enzyme of this pathway after the d‐galacturonic acid reductase. l‐galactonate dehydratase activity is present in H. jecorina cells grown on d‐galacturonic acid but absent when other carbon sources are used for growth. A deletion of the l‐galactonate dehydratase gene in H. jecorina results in a strain with no growth on d‐galacturonic acid. The active enzyme was produced in the heterologous host Saccharomyces cerevisiae and characterized. It exhibited activity with l‐galactonate and d‐arabonate where the hydroxyl group of the C2 is in l‐ and the hydroxyl group of the C3 is in d‐configuration in the Fischer projection. However, it did not exhibit activity with d‐galactonate, d‐gluconate, l‐gulonate or d‐xylonate where the hydroxyl groups of the C2 and C3 are in different configuration.