Fusion of dioxygenase and lignin-binding domains in a novel secreted enzyme from cellulolytic <i>streptomyces sp.</i> SirexAA-E

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Fusion of dioxygenase and lignin-binding domains in a novel secreted enzyme from cellulolytic streptomyces sp.  SirexAA-E

Publication Type:

Journal Article

Source:

Journal of Biological Chemistry, Volume 288, Number 25, p.18574-18587 (2013)

Keywords:

Deconstruction

Abstract:

Streptomyces sp. SirexAA-E is a highly cellulolytic bacterium isolated from an insect/microbe symbiotic community. When grown on lignin-containing biomass, it secretes SACTE_2871, an aromatic ring dioxygenase domain fused to a family 5/12 carbohydrate-binding module (CBM 5/12). Here we present structural and catalytic studies of this novel fusion enzyme, thus providing insight into its function. The dioxygenase domain has the core β-sandwich fold typical of this enzyme family but lacks a dimerization domain observed in other intradiol dioxygenases. Consequently, the x-ray structure shows that the enzyme is monomeric and the Fe(III)-containing active site is exposed to solvent in a shallow depression on a planar surface. Purified SACTE_2871 catalyzes the O2-dependent intradiol cleavage of catechyl compounds from lignin biosynthetic pathways, but not their methylated derivatives. Binding studies show that SACTE_2871 binds synthetic lignin polymers and chitin through the interactions of the CBM 5/12 domain, representing a new binding specificity for this fold-family. Based on its unique structural features and functional properties, we propose that SACTE_2871 contributes to the invasive nature of the insect/microbial community by destroying precursors needed by the plant for de novo lignin biosynthesis as part of its natural wounding response.