Characterization of a Buried Neutral Histidine inBacillus circulansXylanase: Internal Dynamics and Interaction with a Bound Water Molecule

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Abstract
NMR spectroscopy was used to characterize the dynamic behavior of His149 in Bacillus circulans xylanase (BCX) and its interaction with an internal water molecule. Rate constants for the specific acid- and base-catalyzed exchange following bimolecular kinetics (EX2) of the nitrogen-bonded Hε2 of this buried, neutral histidine were determined. At pDmin 7.0 and 30 °C, the lifetime for this proton is 9.9 h, corresponding to a protection factor of ∼107 relative to that predicted for an exposed histidine. The apparent activation energies measured for specific acid and base catalysis (7.0 and 17.4 kcal/mol) indicate that exchange occurs via local structural fluctuations. Consistent with its buried environment, the Nε2−H bond vector of His149 shows restricted mobility, as evidenced by an order parameter S2 = 0.83 determined from 15N relaxation measurements. The crystal structure of BCX reveals that a conserved, buried water hydrogen-bonds to the Hε2 of His149. Strong support for this interaction in solution is provided by the observation of a negative nuclear Overhauser effect (NOE) and positive rotating-frame Overhauser effect (ROE) between His149 Hε2 and a water molecule with the same chemical shift as the bulk solvent. However, the chemical shift of Hε2 (12.2 ppm) and a D/H fractionation factor close to unity (0.89 ± 0.02) indicate that this is not a so-called low-barrier hydrogen bond. Lower and upper bounds on the lifetime of the internal water are estimated to be 10-8 and 10-3 s. Therefore the chemical exchange of solvent protons with those of His149 Hε2 and the diffusion or physical exchange of the internal water to which the histidine is hydrogen-bonded differ in rate by over 7 orders of magnitude.