High-resolution nitrogen-15 nuclear magnetic resonance studies of alpha-lytic protease in solid state. Direct comparison of enzyme structure in solution and in the solid state.

Histidine enriched in 15N in the imidazole nitrogens was incorporated into the catalytic triad of alpha-lytic protease, and high-resolution solid-state 15N NMR spectra of lyophilized enzyme powders were recorded. The lyophilized powders were prepared from aqueous solutions with pH values ranging from 4.9 to 9.3. The behavior of the 15N resonances as a function of "pH" in these solid samples closely parallels that observed previously in the corresponding solution-state study, with the exception that in the powders proton exchange at His-57 is slow on the NMR time scale whereas in solutions it is fast. Thus, the 15N isotropic shifts demonstrate that the N pi-H tautomer of His-57 predominates in powders prepared at high pH and that N pi(H) participates in a strong hydrogen bond, as the hydrogen-bond donor, in powders prepared at both high pH and low pH. The simplest interpretation of these results is that the active site catalytic triad structure of Asp-His-Ser is maintained in these lyophilized powders. Because Asp-102 and His-57 are sequentially separated, their interaction in these lyophilized powders suggests that the tertiary structures of alpha-lytic protease in the powder and in solution are very similar. The 15N isotropic shifts further indicate that His-57 located within the intact triad in lyophilized enzyme powders has what can be taken as a normal "pKa" for a histidyl residue, undergoing a transition from the protonated to the neutral state with a midpoint between pH 6.0 and 7.0.