Improving the efficiency and sustainability of chitin bioconversion through a combination of Streptomyces chitin-active-secretomes and mechanical-milling†

Abstract

Chitin, particularly α-chitin, is the most abundant and highly recalcitrant form, fortified by an intricate network of hydrogen bonds. Efficient valorization of α-chitin requires mild pre-treatment and enzymatic hydrolysis. Streptomyces spp. secrete chitin-active CAZymes that can efficiently tackle the recalcitrant problem of chitin biomass. To better understand the potential of Streptomyces spp., a comparative analysis was performed between the novel isolate, Streptomyces sp. UH6 and the well-known chitin degraders, S. coelicolor and S. griseus. Growth studies and FE-SEM analysis revealed that all three Streptomyces spp. could utilize and degrade both α- and β-chitin. Zymogram analysis showed expression of 5–7 chitinases in the secretomes of Streptomyces strains. The chitin-active-secretomes produced by Streptomyces sp. UH6 and S. griseus were optimally active at acidic pH (pH 4.0 and 5.0) and 50 °C. Time-course degradation of α- and β-chitin with the secretomes generated N-acetyl-D-glucosamine (GlcNAc) and N,N-diacetylchitobiose [(GlcNAc)₂] as the predominant products. Further, the highly crystalline α-chitin was subjected to pre-treatment by ball-milling, which reduced the crystallinity from 88% to 56.6% and increased the BET surface area by 3-folds. Of note, the activity of all three Streptomyces secretomes was improved by a mild pre-treatment, while Streptomyces sp. UH6 secretome displayed improved GlcNAc and (GlcNAc)₂ yields by 14.4 and 9.6-folds, respectively. Overall, our results suggest that the Streptomyces chitin-active-secretomes, particularly Streptomyces sp. UH6, can be deployed for efficient valorization of chitin biomass and to establish an economically feasible and eco-friendly process for valorizing highly recalcitrant α-chitin.