Enhancement of the arabinoxylan degradation ability of Clostridium saccharobutylicum

In the future, chemical precursors will need to be produced from renewable rather than finite fossil resources. One novel precursor could be butanol. It can be produced biologically by ABE fermentation using solventogenic Clostridia. This process is economically viable, if low-value and abundant biomass is used as substrate for fermentation [Thieme et al. 2020]. As by-products from the milling process wheat and rye brans fulfil these criteria. The fermentable carbohydrates were mainly arabinoxylan (40%, 57%) and starch (37%, 17%) [Knudsen KEB 1997].

We found that the good solvent producer Clostridium saccharobutylicum DSM 13864 could only degrade 50% of pure wheat arabinoxylan. To enhance degradation, we fermented pure arabinoxylan in buffered medium and added heterologously expressed arabinofuranosidase (Axh43A), xylanase (Xyn11A, Xyn10B) and β-xylosidase (Bxl3B) from the hemicellolytic thermophile Thermoclostridium stearcorarium DSM8532 to support degradation [Bröcker Jannis 2019].

It was shown that the tested arabinofuranosidase (Axh43A) alone efficiently supports the native xylanolytic enzymes to increase degradation from 50% to 82%. This effect appears to be due to the side-chain cleavage of double substituted xyloses, which synergistically supports endogenous xylanolytic enzymes. A plasmid-based heterologous expression of this enzyme with a signal peptide in C. saccharobutylicum resulted in improved substrate degradation.

Due to the complex nature of the carbohydrates in lignocellulose, a large repertoire of glycoside hydrolases is required to effectively degrade the substrate. Extracellular saccharolytic enzymes can be exploited to improve the degradation properties of a solventogenic Clostridium. Using our genetic system for C. sacchaobutylicum, a stable heterologous expression of a tailored mix of enzymes produced by the solventogenic strain itself could eliminate the need for supplementation with external enzymes.