A FUNCTIONAL-MODEL FOR CARBONIC-ANHYDRASE - THERMODYNAMIC AND KINETIC-STUDY OF A TETRAAZACYCLODODECANE COMPLEX OF ZINC(II)

The hydration of CO2 and dehydration of HCO3- catalyzed by a Zn(II) complex of the macrocyclic tetraamine 1,4,7,10-tetraazacyclododecane ([12]aneN(4), cyclen) (2) were studied in aqueous solution as a functional model for the zinc-containing carbonic anhydrase. The hydration of CO2 by 2 yielding protons and bicarbonate ions is a second-order reaction. The rate constant, k(cat)(h), is (3.3 +/- 0.1) x 10(3) M(-1)s(-1) at 25 degrees C and 0.10 M (NaClO4) ionic strength. A plot of hydration rate vs pH (6.0-9.0) gives a sigmoidal curve with an inflection point at pH 8.1, which is identical to the pK(a) value for the Zn(II)-bound water of [cyclen-Zn-H2O](2+) (2a). Thus, [cyclen-Zn-OH](+) (2b) must play a crucial role in the hydration of CO2. The catalytic activity of 2b is the highest of all studied model complexes and reaches almost one-third of the activity of the carbonic anhydrase variant III. The catalyzed dehydration of HCO3- by 2a is also a second-order reaction. The rate constant, k(cat)(d), is (51 +/- 8) M(-1)s(-1) at 25 degrees C and 0.10 M (NaClO4) ionic strength, which exceeds all data reported for other model complexes. The active species for the dehydration of HCO3- is [cyclen-Zn-H2O](2+) (2a). Monovalent anions inhibit competitively the dehydration of HCO3- by substituting the coordinated H2O (inhibition order is Cl- < Br- < I- < NO3-). A comparison of the structure and activity of 2 with those of (1,5,9-triazacyclododecane)zinc(II) (4) leads to the suggestion that the formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. A unidentate bicarbonate intermediate is most likely to be the active species in the carbonic anhydrase catalyzed reaction.