Positive activation entropy of Bacillus circulans xylanase catalyzed ONPX₂ hydrolysis: A mechanistic and engineering study

Transition state (TS) stabilization by enzymes greatly accelerates catalytic reactions. For some enzymes, the TS complex has entropy higher than enzyme substrate (ES) complex. But the origin of favorable entropy remains unclear. In this work, we studied the mechanism of Bacillus Circulans xylanase (BCX) 11 catalyzed o-nitrophenyl β-xylobioside (ONPX₂) glycoside hydrolysis. The catalytic reaction exhibits a positive activation entropy, and an increase in ionic strength leads to a decrease in entropy without affecting the activation free energy, indicating that the entropy is predominantly influenced by electrostatic forces. Moreover, NMR measurements of electrostatic attractions within the active site demonstrate a positive entropy, aligning with molecular dynamics (MD) simulations showing that electrostatic interactions contribute to the entropic stabilization of the TS complex. These findings suggest that the positive entropy primarily originates from alterations in electrostatic interactions due to the formation of the oxocarbenium ion at C₁ in the TS. Differences of electrostatic interactions in ES and TS modify hydrogen bonding of surrounding residues in the active site which causes their side chain dynamics and thus conformational entropy changes. Residues critical for the positive activation entropy are identified. A new BCX mutant with an increased activation entropy and catalytic activity is found.