The catalytic mechanism of glucose 6-phosphate dehydrogenases: assignment and 1H NMR spectroscopy pH titration of the catalytic histidine residue in the 109 kDa Leuconostoc mesenteroides enzyme

The chemical shifts of the C(epsilon1) and C(delta2) protons of His-240 from the 109 kDa Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase (G6PD) were assigned by comparing 1H and 13C spectra of the wild-type and mutant G6PDs containing the His-240 to asparagine mutation (H240N). Unambiguous assignment of the His-240 1H(epsilon1) resonance was obtained from comparing 13C-1H heteronuclear multiple quantum coherence NMR spectra of wild-type and H240N G6PDs that were selectively labeled with 13C(epsilon1) histidine. The results from NOESY experiments with wild-type and H240N variants were consistent with these assignments and the three-dimensional structure of G6PD. pH titrations show that His-240 has a pK(a) of 6.4. This value is, within experimental error, identical to the value of 6.3 derived from the pH dependence of kcat [Viola, R. E. (1984) Arch. Biochem. Biophys. 228, 415-424], suggesting that the pK(a) of His-240 is unperturbed in the apoenzyme despite being part of a His-Asp catalytic dyad. The results obtained for this 109 kDa enzyme indicate that 1H NMR spectroscopy in combination with heteronuclear methods can be a useful tool for functional analysis of large proteins.