ORIGINAL RESEARCH: Assaying sensitivity of fungal xylanases to proteinaceous inhibitors from a rye extract: Two GH10 family xylanases resistant to XIP-like inhibitors

Xylanase Inhibitors: Defense Players in Plant Immunity with Implications in Agro-Industrial Processing

Xylanase inhibitors (XIs) are plant cell wall proteins largely distributed in monocots that inhibit the hemicellulose degrading activity of microbial xylanases. XIs have been classified into three classes with different structures and inhibition specificities, namely Triticum aestivum xylanase inhibitors (TAXI), xylanase inhibitor proteins (XIP), and thaumatin-like xylanase inhibitors (TLXI). Their involvement in plant defense has been established by several reports. Additionally, these inhibitors have considerable economic relevance because they interfere with the activity of xylanases applied in several agro-industrial processes. Previous reviews highlighted the structural and biochemical properties of XIs and hypothesized their role in plant defense. Here, we aimed to update the information on the genomic organization of XI encoding genes, the inhibition properties of XIs against microbial xylanases, and the structural properties of xylanase-XI interaction. We also deepened the knowledge of XI regulation mechanisms in planta and their involvement in plant defense. Finally, we reported the recently studied strategies to reduce the negative impact of XIs in agro-industrial processes and mentioned their allergenicity potential.

Experimental and in silico studies of competitive inhibition of family GH10 Aspergillus fumigatus xylanase A by Oryza sativa xylanase inhibitor protein

The family GH10 Aspergillus fumigatus xylanase A (AfXylA10) gene, afxyla10 was cloned and recombinantly expressed in Pichia pastoris X33. The optimum temperature and pH of reAfXylA10 was 53 °C and 7.0, and Mn²⁺ remarkably activated the catalytic activity. The recombinant Oryza sativa xylanase inhibitor protein, rePOsXIP significantly inhibited reAfXylA10 with inhibition constant (K_i) of 177.94 nM via competitive inhibition and decreased the concentration of hydrolysate from beechwood xylan. Optimal inhibition of rePOsXIP on reAfXylA10 occurred at 45 °C for 40 min. The fluorescence of reAfXylA10 was statically quenched by rePOsXIP, indicating the formation of reAfXylA10-rePOsXIP complex during their interaction. Furthermore, molecular dynamics (MD) simulations were performed to obtain the detailed information on enzyme-inhibitor interaction. The binding free energy (ΔG) of AfXylA10-OsXIP complex was -30 ± 9 kcal/mol by MM-PBSA calculation, and the α-7 helix of OsXIP anchored in the catalytic cleft of AfXylA10 by competition with the xylan substrate. K239_OsXIP stably interacted with the catalytic site E140_AfXylA10 through hydrogen bond and vdW interaction. Intermolecular hydrogen bonds T104_AfXylA10/V99_AfXylA10-Q5_OsXIP, R256_AfXylA10-E235_OsXIP, D155_AfXylA10-Y243_OsXIP and D145_AfXylA10-R194_OsXIP on the upper of the TIM barrel were essential for strengthening the stability of complex. Therefore, these non-covalent interactions (NCI) played key role in the interaction between AfXylA10 and OsXIP.