Chloride binding

Selected entries from Methods in Enzymology [vol, page(s)] Description, 231, 97 232, 130 mechanism, 232, 97-99 role of hemoglobin A histidyl residues, 232, 130-133, 138 role of hemoglobin chloride-binding site, 231, 225-226 H NMR analysis, 232,... 

Figure 12-6 Drawing showing the overall polypeptide chain fold and relative positioning of the three structural domains of human pancreatic a-amylase. Also drawn are the locations of the calcium and chloride binding sites. Overlaid is the placement of a modified form of the inhibitor acarbose (p. 607) that binds in the active site cleft. MolScript drawing courtesy of G. Sidhu and G. Brayer.

The justification for (A) is manifold. NMR as interpreted by Jardetsky and his colleagues indicates that His 119 and 12 would be almost fully protonated at pH 4.5 in the protein and the complex. The pK of 3 -CMP is 5.7. Hummel and Witzel (456) showed that one proton is released when the complex is formed. Loeb and Saroff (475) detected chloride binding at low pH. Irie showed KC1 to be a competitive inhibitor of C > p hydrolysis, and Riiterjans and Witzel showed that the pfC values... 

A functional role for Cr as a gate keeper has been suggested [153], namely, to prevent HO binding at the Mn center, a process that would favor a 2e oxidation process leading to H202 formation, instead of the normal 4e oxidation required to produce 02. The preceding hypothesis implies that the site of 02 evolution is the same as that of chloride binding i.e., Cr binds to the Mn center. 

When small ligands like water or chloride bind to a metal, the coordination number is determined by how many ligands can pack round the central metal ion, a number relating to repulsion between the donor atoms directly in contact with the metal, a so-called first-order ... 

Also shown in Fig. 5 are the 3 extrinsic polypeptides which bind to the inner surface of the PS II reaction centre and which are involved in the chloride binding associated with the water-splitting reaction. These polypeptides may make up a pocket surrounding the manganese atoms of the 02-evolving enzyme. The four manganese atoms are probably bound to the PS II reaction centre and may be arranged as a distorted cube (see Chapter 6). 

Kavanaugh JS, Rogers PH, Case DA, Arnone A. High resolution x-ray study of deoxyhemoglobin Rothschild 37b Trp to Arg a mutation that creates an intersubunit chloride-binding site. Biochemistry 1992 31 4111-4121. 

Although the cellular mechanism of action has not been fully elucidated, furosemide (frusemide) appears to bind competitively to the chloride-binding site of the Na, K" ,2Cl -cotransporter on the luminal surface of the epithelial cells in the thick ascending limb of the loop of Henie, thereby inhibiting the reabsorption of sodium, potassium and chloride ions (Hinchcliff Muir 1991,... 

Receptor 16 was reported to form a 1 1 complex with chloride ions in chloroform (K = 500 M ), exhibiting fast exchange on the NMR time scale (73). Such hosts, containing four tin binding sites, were shown to be considerably more effective than mononuclear organotin compounds for chloride binding. In 1991, Newcomb and co-worker (74) published modeling studies for these tin based hosts as well as a crystal structure of 17. 

The trinuclear receptor 18 and its propyl linked analog 19 have been reported by Jurkschat et al. (75, 76). Receptor 18 was shown to transport chloride and bromide ions from water through dichloromethane, but the process was slow. Crystallography of 19a indicated chloride binding between a pair of tin atoms, while NMR studies indicated that 19b involved all three tin atoms in the binding process. No quantitative evaluation of binding affinities was attempted. 

The electrophoretic data of Alberty and Marvin (1951) on bovine serum albumin, unlike those discussed above, were obtained in unbuffered solutions. They show that chloride ion binding increases as the solution becomes more acid the indicated net charge on the protein is therefore less than that predicted from the titration curve. The extent of chloride binding found by these authors agrees well ivith that determined by Scatchard, Scheinberg, and Armstrong (1950a) for human serum albumin from dialysis equilibrium and electromotive force data. 

Figure 3.32 Encapsulating ligands for anions Schmidtchen s chloride-binding ligand (top left), Anslyn s nitrate-binding cryptand (bottom left), Steed s chloride-detecting ferrocenyl podand (right)...

---