PMC

Two-State (A) and Three-State (B) Models for the Association of Arylsulfonamides with CA

Three-State Models for the Association of Neutral (A) and Charged (B) Arylsulfonamides with CA

Equilibria for the Association of Arylsulfonamide (ArSO₂NH₂/ArSO₂NH−) with Carbonic Anhydrase (CA–Zn^II–OH₂⁺/CA–Zn^II–OH) (Reproduced with Permission from Ref ; Copyright 2007 Wiley-VCH)

Examples of Zn^II-containing metallo-organic models of the active site of CA: (A) tris(pyrazolyl)borate family of ligand, (B) 1,1,1-tris(aminomethyl)ethane, and (C) 1,5,9-triazacyclododecane.

Linear dependence between K_i (on a logarithmic scale) and p K_a for a series of halogen-substituted unbranched aliphatic sulfonamides (R–SO₂NH₂). Data taken from ref .

Estimated free energies, enthalpies, and entropies (all in kcal mol ⁻¹) for the different structural interactions between fluorinated benzenesulfonamide anions and CA–Zn^II–OH₂⁺. Reproduced with permission from ref . Copyright 2007 Wiley-VCH.

Ribbon rendering of HCA II from two perspectives, with α-helices in red and β-sheets in blue. The N- and C-termini, the C-terminal knot, and the primary residues involved in the initiation of folding and of coordinating the Zn^II cofactor are indicated.

Possible free-energy diagram for the association of an arylsulfonamide with carbonic anhydrase. The values of energy shown for the different states are in kcal mol⁻¹ and have been taken from experimental values for the association of p-nitrobenzene-sulfonamide (3) with HCA II. Data taken from King and Burgen.

Overlay of X-ray structures of HCA I, HCA II, and BCA II, with His residues in the active site highlighted. This image was rendered using POV-Ray 3.5 (www.povray.org). The accession numbers from the protein data bank (PDB) for the rendered structures are 2CAB (HCA I), 2CBA (HCA II), and 1V9E (BCA II).,,

Binding of arylsulfonamide to carbonic anhydrase (CA). (A) Structure of a general arylsulfonamide ligand showing the structural features that can be modified (to a first approximation) independently. (B) The interactions between the different structural components of a general arylsulfonamide ligand and CA.

Variation of pK_a of the arylsulfonamide that will theoretically give the highest affinity ligand (lowest value of ) to carbonic anhydrase II with β (). The curves were generated with a pH of the buffer of 7 (indicated by dotted vertical line).

Model for the binding of compound 84 to HCA II based on the deposited X-ray crystallographic coordinates (PDB/1CNW). Catalytically important residues and residues that contribute to the primary and secondary hydrophobic binding sites for this ligand are shown.

Measuring the binding of CA to self-assembled monolayers using surface plasmon resonance: (A) schematic of the apparatus and the overall molecular structure at the solid–liquid interface, (B) effects of mass transport on measurements of binding, and (C) effects of lateral sterics on binding of CA at densely populated surfaces.

Amino acid sequences of HCA I, HCA II, and BCA II. Sequence homology is denoted by the symbols “*”, “:”, and “.”; “*” represents identical residues, “:” represents charge/polarity conserved residues, and “.” denotes polarity conserved residues. BCA II exists as two variants: an “R” form (shown here), where residue 56 exists as an Arg, and a “Q” form, where it exists as Gln. This residue is underlined above.

Diagram comparing (A) carbon dioxide (putative interactions), (B) an arylsulfonamide, and (C) bicarbonate bound in the active site of HCA II. The arylsulfonamide can be viewed as a transition-state analogue of the hydratase reaction (H₂O + CO₂ ⇆ HCO₃⁻ + H⁺.

Structure of the active site of HCA II bound to compound 109 (shown as a chemical structure). The van der Waals surface of the enzyme is opaque gray, and that of the ligand is translucent purple. Relevant residues are indicated, most notably Leu198 and Pro202 on the hydrophobic wall.

Surface rendering of opposite faces of HCA II (PDB/2CBA) showing (A and B) acidic residues in red and basic residues in blue and (C and D) hydrophobic residues in yellow and polar residues in green. At pH 7–8, the red regions have a negative charge; the blue regions have a positive charge. The arrows indicate the active site.

Mechanism of catalysis of the hydration of CO₂ by HCA II. The putative structures of the species CA–OH and CA–OH₂⁺, discussed in detail in the text, are indicated. We show the formal charge only on the zinc-bound water (and not the histidine residues) to emphasize that this water ligand is acidic (analogous to a hydronium ion being acidic) and adopt this convention throughout the remainder of the review.

Variation of k_on (A) and k_off (B) with the (equilibrium) dissociation constant, , for several classes of arylsulfonamides. Structures for the different series are listed in as follows: m-ester alkyl (114–118), o-ester alkyl (119–123), p-ester alkyl (30–35), p-amide alkyl (37–43), p-alkyl (7–10), p-peptides (63, 66, 223–226), and miscellaneous sulfonamides (1–3, 29, 133, 135, 137, 166, 168), and data listed in .

Regimes of refolding and aggregation of BCA II. Each datum represents rapid dilution of BCA II in 5 M GuHCl to a given final protein and GuHCl concentration. Conditions in the aggregation regime result in the immediate formation of micron-sized particulates. The upper boundary of the aggregation regime is defined by (■). Conditions in the multimer regime (▲) yielded measurable dimeric and trimeric species by CD before aggregation. The lower limit of refolding (●) is the regime where multimers form but do not proceed to form micron-sized particles. Adapted with permission from ref . Copyright 1990 American Chemical Society.