Fortifying the Rasamsonia emersonii secretome with recombinant cellobiohydrolase (GH7) for efficient biomass saccharification.

Abstract

GH7 cellobiohydrolases (CBH1s) are essential for depolymerizing crystalline cellulose, yet the hypercellulolytic thermophile Rasamsonia emersonii secretes them only in low amounts, leaving a gap in its native enzyme cocktail. To see whether a cognate CBH1 could fill this gap and how it stacks up against the industrial workhorse strain Trichoderma reesei Cel7A, we codon optimized the R. emersonii gene (Rem_GH7CBHI), expressed it in Pichia pastoris and purified the recombinant enzyme for structural and functional analysis. The 57 kDa protein retains the canonical GH7 βsandwich tunnel, but an AlaforTyr substitution leaves the channel more open than that of T. reesei, potentially easing substrate entry. Consistent with this architecture, Rem_GH7CBHI binds cellotriose tightly and exhibits a low Kₘ of 0.25 mM. Biochemical characterization revealed the optimal activity at pH 5.0, 60 °C and retaining about 60% activity after 1 h at 70 °C. Adding Rem_GH7CBHI together with an endogenous AA9 LPMO to the native R. emersonii secretome (M36) boosted saccharification of steamacidpretreated rice straw to levels comparable with commercial Cellic CTec3 at the same total protein loading. These results position Rem_GH7CBHI as a thermostable, highaffinity alternative to T. reesei Cel7A and a costeffective addition to tailored enzyme cocktails for highsolids lignocellulose biorefineries.