Carbon-13 Nuclear Magnetic Resonance as a Probe of Side Chain Orientation and Mobility in Carboxymethylated Human Carbonic Anhydrase B

13C NMR spectra of [1-13C]- and [2-13C]carboxymethyl His-200 human carbonic anhydrase B have been obtained as a function of pH and in the presence and absence of the active site Zn(II) or Cd(II) ion. Chemical shifts of the 1-13C show that the carboxyl is sensitive to two ionization processes, with apparent pKas of 7.2 and 9.9, respectively. These are assigned to the deprotonation of the Nπ of His-200 and the breaking of the coordination bond between the carboxyl oxygen and the Zn(II) ion, respectively. Assignment of the lower pK, to that of the Nπ is supported by the observation of this same ionization in the chemical shift of the 2-13C resonance showing the signal from the methylene carbon to undergo the same upfield shift as is observed on the ionization of the Nπ in Nτ-carboxymethylated histidine. The high pH ionization process is not reflected in the resonance of the methylene carbon. No changes in the chemical shifts vs. pH are observed for both the [1-13C]- and [2-13C]carboxymethyl apocarbonic anhydrase, suggesting that the pK, of the Nπ has shifted at least 1 pH unit to acid pH, and must reflect significant conformational changes in the active center. Cd(II) carboxymethyl carbonic anhydrase shows 13C chemical shifts identical with those of the apoenzyme. Since the Cd(II) at the active site is known to bond external donor groups very weakly, the data suggest that the changes in conformation are related to Zn(II)-carboxylate coordination. Changes in the mobility of the carboxymethyl group have been assessed by a relaxation analysis which relates the relative line widths of the central and outer lines of the [13C]methylene triplet to the internal rotational motion of the group relative to the protein. At neutral pH this group shows no internal motion, compatible with its coordination to the active site metal ion. At pH 10.6 significant internal motion is present, compatible with breaking of the coordination bond in competition with -OH binding.