Catalytic Mechanism of α-Class Carbonic Anhydrases: CO2 Hydration and Proton Transfer

Abstract

The carbonic anhydrases (CAs; EC 4.2.1.1) are a family of metalloenzymes that catalyze the reversible hydration of carbon dioxide (CO2) and dehydration of bicarbonate (HCO3 (-)) in a two-step ping-pong mechanism: [Formula: see text] CAs are ubiquitous enzymes and are categorized into five distinct classes (α, β, γ, δ and ζ). The α-class is found primarily in vertebrates (and the only class of CA in mammals), β is observed in higher plants and some prokaryotes, γ is present only in archaebacteria whereas the δ and ζ classes have only been observed in diatoms.The focus of this chapter is on α-CAs as the structure-function relationship is best understood for this class, in particular for humans. The reader is referred to other reviews for an overview of the structure and catalytic mechanism of the other CA classes. The overall catalytic site structure and geometry of α-CAs are described in the first section of this chapter followed by the kinetic studies, binding of CO2, and the proton shuttle network.

Structural elucidation of the hormonal inhibition mechanism of the bile acid cholate on human carbonic anhydrase II

“…Cholate is seen to bind to the zinc ion in a bivalent interaction centralized between the two O atoms of the carboxyl group which displaces the zinc-bound solvent molecule and water 1 (W1) of the proton-transfer water network (Boone et al, 2014). The carboxyl O atoms are also at potential hydrogen-bonding distance from two well ordered water molecules observed in the unbound active site termed the deep water (D W ), which acts as a place holder for CO 2 binding, and W2, which is involved in the transfer of a proton from the zinc-bound solvent to His64 (Boone et al, 2014). The D W molecule has been displaced 0.9 Å further into the hydrophobic pocket of the CO 2 -binding site towards Trp209 upon binding cholate, whereas the zinc-bound solvent molecule W2 is in an comparable position to that in the unbound structure.…”

“…The carbonic anhydrases (CAs; EC 4.2.1.1) are mostly zinccontaining metalloenzymes that are important in acid-base homeostasis as they catalyze the reversible hydration of CO 2 into bicarbonate and a proton (Boone et al, 2014). Several isoforms of human CAs (HCAs) along the gastrointestinal tract, including HCA II, are inhibited by the bile acids (BAs; Milov et al, 1992), the primary products of cholesterol catabolism (Staels & Fonseca, 2009).…”

Crystal structure of the most catalytically effective carbonic anhydrase enzyme known, SazCA from the thermophilic bacterium Sulfurihydrogenibium azorense

“…Conversely, the second step of the reaction consists of the regeneration of the zincbound hydroxide by means of the transfer of a proton from the zinc-bound water molecule to the external buffer. [3][4][5] This step is rate limiting and is generally assisted by a residue, which acts as proton shuttle. This residue is located in the enzyme active site and is connected to the zinc-bound water molecule through a well ordered hydrogen bonded water network.…”

“…1,2 This catalytic reaction consists of two separate steps, described by equations (1) and (2). [3][4][5] EZn 2+ -OH -+ CO 2 …”

“…[6][7][8][9] In the majority of human α-CA isoforms, this residue is a histidine (e.g., His64 in hCA II, IV, VII, IX, etc). 2,5 For a long time α-CAs were studied mainly for their medicinal chemistry applications as drug targets; only recently, in parallel to these uses, these enzymes have seen an increasing interest in their employment as biocatalysts for carbon dioxide sequestration processes and biofuel production. [10][11][12] However, their efficient utilization in these processes is generally strongly limited by the poor stability and activity that these enzymes present in the harsh conditions typical of the CO 2 capture processes, particularly the very high temperatures.…”