Bicarbonate Inhibition of Carbonic Anhydrase Mimics Hinders Catalytic Efficiency: Elucidating the Mechanism and Gaining Insight toward Improving Speed and Efficiency

Abstract

Carbonic anhydrase (CA) mimics are often studied with a focus on the hydration of CO 2 for atmospheric carbon capture. Consequently, the reverse reaction (dehydration of HCO 3 − ) has received minimal attention, so much so that the rate-limiting step of the dehydration reaction in CA mimics is currently unknown. The rate-limiting step of the hydration reaction is reported to be the bicarbonate-bound intermediate step, and thus is susceptible to product inhibition. It is not, however, clear if this inhibition is a consequence of an increase in the rate of the competing dehydration reaction or resulting from the strong affinity of bicarbonate to the mimic. To address this, insight into the dehydration reaction kinetics is needed. We therefore report the most comprehensive study of a CA mimic to date. The dehydration profile of the fastest small-molecule CA mimic, ZnL1S, was characterized, and consequently evidence for the rate-limiting step for the dehydration reaction was seen to be the bicarbonate-bound intermediate step, much like the hydration reaction. This experimental validation of the rate-limiting step was achieved through a variety of methods including NMR experiments and the effect of inhibitors, substrate concentration, and metal center on activity. With this understanding, an improvement in the favorability of the rate-limiting step was achieved, resulting in decreased bicarbonate inhibition. Thus, an increase in the mimic's k cat for both reactions was observed, resulting in the largest rate constants of any small-molecule CA mimic reported to date (28 093 and 579 M −1 s −1 for hydration and dehydration, respectively). Enzyme-like k cat /k m values were obtained for ZnL1S (5.9 × 10 5 M −1 s −1 for CO 2 hydration), and notably there is only a difference of 2.5 orders of magnitude from the enzyme, the closest of any CA mimic reported in the literature. The results from this work can be applied to the development and improvement of future and existing mimics toward attaining increased activities.

Kinetic study of catalytic CO 2 hydration by metal-substituted biomimetic carbonic anhydrase model complexes

“…Recently, a computational study of the DFT wave function has shown that the N4 ligand has monodentate ground state HCO3− binding modes with Zn, while the N3 ligand allows bidentate ground state HCO3− binding modes [16]. This shows that the N4 ligand has better catalytic efficiency than that of the N3 ligand because of the lower dissociation energy of HCO3− (HCO3− release step) [32]. Moreover, compared with the N3 ligand, the N4 ligand is less likely to dimerize during the formation of the metal complex.…”

Kinetic Study of CO2 Hydration by Small-Molecule Catalysts with A Second Coordination Sphere that Mimic the Effect of the Thr-199 Residue of Carbonic Anhydrase

“…The final step entails the regeneration of the catalyst in its active form, that is, the deprotonation of the Zn-bound water to form the Zn-bound hydroxide (pKa = 7; Figure 4). Among these steps, deprotonation is known to be the rate-determining step (RDS) [30,[34][35][36]. The structure of natural CA includes a tridentate N3 ligand coordinated to and surrounding the Zn ion.…”

“…This result indicates that (2) is more efficient than (1) in the hydration reaction through the nucleophilic attack of Zn-bound hydroxide (active form) on CO 2 . Many researchers have attempted to lower the pK a of Zn-bound water by increasing the Lewis acidity of Zn 2+ , using ligands that attract more electrons [17,28,30,36]. However, increasing the acidity of Zn 2+ increases the binding force for the Zn-bound bicarbonate, which is not effective for the release of bicarbonate.…”

Biomimetic Catalysis of CO2 Hydration: A Materials Perspective

“…16 The mineral rocks used for the majority of aqueous CO 2 carbonation are silicate rocks of calcium and magnesium. 13,17,18 However, the use of industrial waste materials for CO 2 mineralization is gaining interest. 17−20 Leaching of the alkali earth metals is a slow process that can be accelerated by the use of enhancers in acidic or basic media.…”

“…Then this acid is reacted with an alkali metal ion (from natural rocks) to be precipitated as a mineral carbonate. − The process mimics natural weathering of rocks and is kinetically limited . The mineral rocks used for the majority of aqueous CO 2 carbonation are silicate rocks of calcium and magnesium. ,, However, the use of industrial waste materials for CO 2 mineralization is gaining interest. − Leaching of the alkali earth metals is a slow process that can be accelerated by the use of enhancers in acidic or basic media . However, the hydration of CO 2 is another rate limiting step with pH dependence and has slow kinetics in mildly basic solutions .…”