Metallocarbonic anhydrases

Most work has involved the cobalt(II) carbonic anhydrase, and some illustrations will be given. Work with metallocarbonic anhydrases has been reviewed.482... 

Zinc can be removed from carbonic anhydrase on dialysis against a chelating agent at pH about 5 (37, 38). The apoenzyme is inactive but the gross conformation of the protein is maintained (37, 39, 40). The metal-chelating site can accomodate any of the divalent transitional metal ions from Mn2+ to Zn2+ as well as Cd2+ and Hg2+ (38,41). Most of these metallocarbonic anhydrases have low activities or are inactive, however. Only Zn2+ and Co2+ are efficient activators. As shown in Table 3, this narrow metal-ion specificity is observed for the CO 2 hydration as well as for the esterase reactions. 

Reaction of the apoenzyme with various metal ions allows the preparation of other metallocarbonic anhydrases and the Cu(II), Co(II) and Co(III) enzymes have been prepared. The cobalt(II) enzyme shows some catalytic activity. The d-d spectrum of the cobalt(II) derivative, which is high spin, is markedly pH dependent (Fig. 5-19). A plot of the molar absorbance at 640 nm versus pH does not follow a simple pattern, as expected for a single ionising group, but is consistent with two ionising groups with pK values of < 6 and > 7. 

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