Recombinant rice xylanase-inhibiting protein inhibits GH11 endo-xylanases through competitive inhibition

The ricexip gene, which encodes the rice xylanase-inhibiting protein (riceXIP), was recombinantly expressed in Pichia pastoris GS115 under the control of AOX1 promoter. Recombinant riceXIP (rePriceXIP) was secreted into the supernatant and purified to homogeneity with the use of Ni-affinity resin. The molecular mass of rePriceXIP was approximately 44.0 kDa. RePriceXIP significantly inhibited the activity of GH11 xylanases in a concentration-dependent manner. The optimal inhibitory activity of rePriceXIP on GH11 xylanases (TfxA_CD214, reBaxA454, and TfxA_CD526) occurred at 40 °C for 30 min. The IC₅₀ values of rePriceXIP inhibiting TfxA_CD214, reBaxA454, and TfxA_CD526 (0.5 U) were 45, 40, and 40 nM, respectively. The K_i of rePriceXIP on TfxA_CD214 xylanase was 12.2 nM. Increasing the concentration of rePriceXIP did not change V_max, but increased K_m, thereby suggesting that the inhibition was competitive. Analysis of the fluorescence intensities revealed that the K_q values for TfxA_CD214, reBaxA454, and TfxA_CD526 exceeded 2.0 × 10¹⁰ L mol^-1•s^-1, implying that static quenching occurred. Circular dichroism spectroscopy demonstrated that rePriceXIP bound to xylanase, thereby changing the secondary structure and reducing the catalytic activity of xylanase. Lower levels of hydrolytes are released from beechwood xylan by xylanases in the presence of rePriceXIP.