Background
The biodegradation of chitin is an important part of the carbon and nitrogen cycles in nature. Speeding up the biotransformation of chitin substrates can not only reduce pollution, but also produce high value-added products. However, this process is strictly regulated by the catalytic efficiency of the chitinolytic machinery. Therefore, it is necessary to study the mode of action and compound mechanisms of different chitin-degrading enzymes in depth to improve the catalytic efficiency of the chitinolytic machinery.
Results
The thermophilic bacterium
Streptomyces
sp. F-3 showed comparatively high chitin degradation activities. To elucidate the mechanism underlying chitin hydrolysis, six chitin degradation-related enzymes were identified in the extracellular proteome of
Streptomyces
sp. F-3, including three chitinases (
Ss
Chi18A,
Ss
Chi18B, and
Ss
Chi18C) from the GH18 family, one GH19 chitinase (
Ss
Chi19A), one GH20 β-
N
-acetylhexosaminidase (
Ss
GH20A), and one lytic polysaccharide monooxygenase (
Ss
LPMO10A) from the AA10 family. All were upregulated by chitin. The heterologously expressed hydrolases could withstand temperatures up to 70 °C and were stable at pH values of 4 to 11. Biochemical analyses displayed that these chitin degradation-related enzymes had different functions and thus showed synergistic effects during chitin degradation. Furthermore, based on structural bioinformatics data, we speculated that the different action modes among the three GH18 chitinases may be caused by loop differences in their active site architectures. Among them,
Ss
Chi18A is probably processive and mainly acts on polysaccharides, while
Ss
Chi18B and
Ss
Chi18C are likely endo-non-processive and displayed higher activity on the degradation of chitin oligosaccharides. In addition, proteomic data and synergy experiments also indicated the importance of
Ss
LPMO10A, which could promote the activities of the hydrolases and increase the monosaccharide content in the reaction system, respectively.
Conclusions
In this article, the chitinolytic machinery of a thermophilic
Streptomyces
species was studied to explore the structural basis for the synergistic actions of chitinases from different GH18 subfamilies. The elucidation of the degradation mechanisms of these thermophilic chitinases will lay a theoretical foundation for the efficient industrialized transformation of natural chitin.
Electronic supplementary material
The online version of this article (10.1186/s13068-019-1472-1) contains supplementary material, which is available to authorized ...