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

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Abstract
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.