The human carbonic anhydrase II catalyzed hydration of CO2 and dehydration of HCO3- at low and high buffer concentrations and at low and high substrate concentrations were studied using a pH-indicator method at pressures up to 130 MPa. Activation volumes, ΔV⧧, for khcat, khcat/Khm, kdcat, and kdcat/Kdm are 0, −9 ± 1, +9 ± 1 (+10 ± 3 in D2O), and +14.0 ± 1.2 cm3 mol-1 in H2O solution, respectively; they were derived from rate constants obtained in the buffer concentration independent region. The value of ΔV⧧ for both intra- and intermolecular proton-transfer steps is zero. The solvent kinetic isotope effect on kdcat is 3.6 ± 0.4 (H2O/D2O) at pressures up to 100 MPa. Analysis of these results indicates that nucleophilic attack of the Zn2+ bound hydroxyl moiety on CO2 is associative, and that the CO2 release and the substitution of H2O by HCO3- on the zinc center are dissociative in character. The CO2 release and a proton transfer are concerted steps during the dehydration reaction. A reaction volume profile...

Carbonic Anhydrase Catalysis: A Volume Profile Analysis

Publisher Summary The terms ‘labile’ and ‘inert’ represent the two extremes of kinetic reactivity. The solvent molecules in the vicinity of the ion differ in properties from those in the bulk solution. A series of progressively ordered and electro-stricted hydration sheaths surround the ion. This chapter focuses on traditional and simple inert metal-ion complexes, mainly octahedral, in which the rate of displacement of at least one ligand permits it to be defined as labile relative to the normal reactivity of complexes of that metal ion. This reactivity may be an inherent property of the ligand as a consequence of poor nucleophilicity. Alternatively, lability may be induced as a result of effects, such as the influence of other nonleaving donors, reactions of the ligand with reagents external to the complex ion, or solvation phenomena. It is convenient to ‘scale’ lability in terms of the rate of substitution of a ligand. A ligand-based order of lability derived for a particular d3 or d6 metal ion can be largely transposed to another, at least as regards the more reactive ligands. Studies of water-exchange reactions of aqua transition metal ions have implied that the structure of the ion in the ground state is an important factor in defining lability. Ion charge plays a role, because more highly charged ions are less likely to exchange rapidly. A great deal of information exists about pentaamine complexes with one leaving group, particularly for cobalt(III) but also for chromium(III) and other metal ions.

Leaving Groups on Inert Metal Complexes with Inherent or Induced Lability

The influence of dissolved carbon dioxide on trace metal speciation in seawater

Publisher Summary The importance of the biochemistry of hydration of CO2 and dehydration of HCO3- in an aqueous environment has led to extensive and invigorating research on the enzyme carbonic anhydrase (CA) pertaining to its structural details, metal ion cofactor, its coordination environment, and kinetic activity. Emphasis is placed on the integration of the kinetic and mechanistic concepts derived from the studies on model systems with the available kinetic, chemical, and structural information on the enzyme carbonic anhydrase. In recent years, significant progress has been made in the synthesis, structure, and magnetic properties of polynuclear carbonato complexes of transition metal ions. The use of heteropoly anions to trap CO2, and convert it to metal carbonates and related products is of recent origin. It is beyond doubt that some model carbonato complexes in the past have been very useful to understand the mechanism of action of CA. Substitution of the metal ion at the active site of the native CAs reduces catalytic activity significantly; but, "what distinguishes active from inactive metals" is still unanswered.

Carbonato complexes: models for carbonic anhydrase

A new reaction route for the synthesis of different cobalt(III)nitrilotriacetato complexes. The crystal structure of Cs2[Co(nta)(CO3)]·H2O

Mechanistic information from the effect of pressure on the formation and acid-catalyzed aquation reactions of (carbonato)pentaamminecobalt(III), -rhodium(III), and -iridium(III) ions in aqueous solution

Mechanistic information from the effect of pressure on the formation and acid-catalyzed aquation reactions of (carbonato)pentaamminecobalt(iii), -rhodium(iii), and -iridium(iii) ions in aqueous solution

Kinetics and mechanisms of the formation, substitution, and aquation reactions of sulfur-bonded (sulfito)amminecobalt(III) complexes in aqueous solution

Mechanistic information on fast reactions of transition metal complexes using rapid scan spectrophotometry. III. The acid-catalyzed aquation of a series of Co(III) carbonato complexes in aqueous solution [1]

U. Spitzer

Papers

Mechanistic information from the effect of pressure on the formation and acid-catalyzed aquation reactions of (carbonato)pentaamminecobalt(III), -rhodium(III), and -iridium(III) ions in aqueous solution

Mechanistic information from the effect of pressure on the formation and acid-catalyzed aquation reactions of (carbonato)pentaamminecobalt(iii), -rhodium(iii), and -iridium(iii) ions in aqueous solution

Kinetics and mechanisms of the formation, substitution, and aquation reactions of sulfur-bonded (sulfito)amminecobalt(III) complexes in aqueous solution

Mechanistic information on fast reactions of transition metal complexes using rapid scan spectrophotometry. III. The acid-catalyzed aquation of a series of Co(III) carbonato complexes in aqueous solution [1]