The binding, synergistic and structural characteristics of BsEXLX1 for loosening the main components of lignocellulose: Lignin, xylan, and cellulose

• BsEXLX1 was found to preferentially bind to xylan when loosening lignocellulose.
• The binding between heterogenous expressed BsEXLX1 and substrates did not saturate.
• Residue-125, 126, and 171 of BsEXLX1 were less important for the binding to xylan.
• Residue-125 and 171 play more important role in structural maintenance of BsEXLX1.
• The synergistic activity of BsEXLX1 is not correlated with its specific binding.

The bacterial expansin, BsEXLX1, has been studied as a model to understand the synergistic effect of expansins on lignocellulose degradation and the structure-function relationships of expansins. However, the specific mechanism is still poorly understood. In this study, the binding, synergistic and structural characteristics of BsEXLX1 for loosening the main components of lignocellulose: lignin, xylan, and cellulose, were further characterized. Results showed that BsEXLX1 preferentially binds to xylan over lignin or cellulose under various conditions. The binding of BsEXLX1 to all substrates increased linearly with the initial concentration of BsEXLX1. But the changing rate of binding (i.e., slope of the line; k value) varied with the incubation temperature. Interestingly, the binding of BsEXLX1 to substrates did not saturate. Mutating residue-125, 126 or 171 indicated their importance for binding, but they were less important for binding to xylan. Through binding assays and homologous modeling, it was concluded that residue-125 and 171 play more important roles in the structural maintenance of BsEXLX1. By comparing the synergistic activity of BsEXLX1 or its mutants, it was found that synergistic activity is not correlated with specific binding. All these results can lead deeper understand about the mechanism of wall loosening by expansins, and further promote the application of expansins in lignocellulose degradation.

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