title: Clustered Genes Encoding 2-Keto-l-Gulonate Reductase and l-Idonate 5-Dehydrogenase in the Novel Fungal d-Glucuronic Acid Pathway.
abstract: D-Glucuronic acid is a biomass component that occurs in plant cell wall polysaccharides and is catabolized by saprotrophic microorganisms including fungi. A pathway for D-glucuronic acid catabolism in fungal microorganisms is only partly known. In the filamentous fungusAspergillus niger, the enzymes that are known to be part of the pathway are the NADPH requiring D-glucuronic acid reductase forming L-gulonate and the NADH requiring 2-keto-L-gulonate reductase that forms L-idonate. With the aid of RNA sequencing we identified two more enzymes of the pathway. The first is a NADPH requiring 2-keto-L-gulonate reductase that forms L-idonate, GluD. The second is a NAD+requiring L-idonate 5-dehydrogenase forming 5-keto-gluconate, GluE. The genes coding for these two enzymes are clustered and share the same bidirectional promoter. The GluD is an enzyme with a strict requirement for NADP+/NADPH as cofactors. The kcatfor 2-keto-L-gulonate and L-idonate is 21.4 and 1.1 s-1, and the Km25.3 and 12.6 mM, respectively, when using the purified protein. In contrast, the GluE has a strict requirement for NAD+/NADH. The kcatfor L-idonate and 5-keto-D-gluconate is 5.5 and 7.2 s-1, and the Km30.9 and 8.4 mM, respectively. These values also refer to the purified protein. ThegluDdeletion resulted in accumulation of 2-keto-L-gulonate in the liquid cultivation while thegluEdeletion resulted in reduced growth and cessation of the D-glucuronic acid catabolism.
authors.affiliation: VTT Technical Research Centre of Finland Ltd Espoo, Finland.
authors.forename: Joosu
authors.initials: J
authors.lastname: Kuivanen
authors.affiliation: VTT Technical Research Centre of Finland Ltd Espoo, Finland.
authors.forename: Mikko
authors.initials: M
authors.lastname: Arvas
authors.affiliation: VTT Technical Research Centre of Finland Ltd Espoo, Finland.
authors.forename: Peter
authors.initials: P
authors.lastname: Richard
keywords: 2-keto-L-gulonate; Aspergillus; D-glucuronate; D-glucuronic acid; L-idonate; fungi; metabolism