Anionic inhibitors

Some examples of organic anionic inhibitors are sodium phosphates, thioureas and sodium MBT phosphionates (mercaptobenzothiazole). Some examples of organic cationic inhibitors are amines, amides, quaternary ammonium salts, and imidazoline. 

Molybdate is always used in conjunction with other anion inhibitors, not only to reduce the cost of the inhibitor program, but also because, through synergism, much-improved barrier films are produced when coupled with nitrite or silicate. 

Loss of copper. Solutions and crystals of ceruloplasmin that have been allowed to stand for several weeks partially lose their blue color and this may indicate changes in copper oxidation state and/or loss of copper. A series of experiments are required to determine whether any copper loss is site preferential and how this is affected by anionic inhibitors. 

III.b.1.1. Anion inhibitors. Perchlorate, periodate, pertechnetate and thiocyanate (a naturally occurring goitrogen) are classified as iodide pump inhibitors, antagonizing iodide transport through competitive inhibition. This effect can be overcome by large dose of iodides. Perchlorate is used to block reuptake of iodide in cases of amiodarone induced hyperthyroidism and for the perchlorate discharge test . 

ESR spectra of type 2 Cu suggest the presence of three to four nitrogen ligands, while bound water has also been implicated by proton relaxation rate measurements. A number of anionic inhibitors bind to type 2 Cu. These results suggest that substrates may bind to the type 2 copper centres in oxidases. 

As suggested in Figure 13.7, binuclear or multinuclear surface complexes tend to block surface sites. A much higher activation energy is involved in detaching simultaneously two metal centers from the surface hence dissolution is retarded by binuclear surface species. This retardation is especially pronounced when the effect of surface coordination with an inhibitor upon surface protonation is not unfavorable. For example, in the reaction of an anionic inhibitor, L, ... 

The ability of zinc in carbonic anhydrase to become five-coordinate is also confirmed by the structural studies on enzyme-inhibitor complexes discussed in Section 62.1.4.2.1. There is much evidence for the coordination of anionic inhibitors to the metal, while the competitive inhibitor imidazole gives a five-coordinate centre. Sulfonamides are powerful inhibitors which bind directly to the zinc and also interact with the protein. The sulfonamide acetazolamide has significant affinity for the apoenzyme. It is probable " that the first interaction between the enzyme and aromatic sulfonamides is a hydrophobic interaction between the aromatic ring and residues in the active site cavity, followed by ionization of the SO2NH2 group prior to complex formation. Sulfonamides only bind to the zinc and cobalt enzymes, i.e. the two metals that give an active enzyme. 

Sultones were the earliest anionic stabilisers used in cyanoacrylates but fell from favour because of their potential carcinogenicity. Chelates of boric acid derivatives with polyhydroxy compounds also were considered as anionic inhibitors. Anionic inhibitors are normally added at concentrations between 0.001 and 0.01% depending on the application. 

Insights into how substrate interacts with the active site of the PAPs may be gleaned from an investigation of the tetraoxo anion inhibitor complexes. These studies have focused on the effects of the substrate analogue and product phosphate, but comparative data on arsenate and molybdate are also informative. The various observations are summarized in Table... 

In 10 mM acetate buffer at pH 5.5 and with 4 /tM EDTA present to reduce the catal5dic effects of iron salts, the order of inhibition was determined to be CN- > Ng- > F- > I- > NO3- > Cl- > Br- > OCN" > SCN- > SeCN- > (CIO4-, tetraborate, boric acid, phosphate, sulfate and cacodylate). Clearly the strongest inhibitors are those with metal binding capabilities although this features alone does not readily correlate the series. Indeed, there are complexities which are apparent imder detailed scrutiny and which suggest a very compUcated pattern of inhibitory action by these anions. Curzon and his co-workers and other investigators have carried out detailed studies of the inhibition by azide, cyanide, the halides, and mixtures of various anionic inhibitors. 

Suspecting that the preference of ODCase for anionic inhibitors might indicate the presence of a cationic active site amino acid residue, perhaps acting as a general acid in catalysis, Smiley and Jones sought to identify critical amino acid residues by site-directed mutagenesis, with particular attention to lysine residues [19]. By the late 1980s, quite a few ODCase sequences had... 

E7.5. Corrosion of passive metals occurs by pitting in chloride solutions. This pitting can be prevented by the addition of anionic inhibitors. At a critical ratio of adsorbed chloride ions to adsorbed anion, the passive film is displaced by chlorides, allowing a pit to initiate. 

Next, the monomer is redistilled from an acid and a radical inhibitor, in scrupulously dry equipment, under high vacuum. The monomer is stabilized by adding small amounts of radical and anion inhibitors. After redistillation the appropriate modifiers are added, the inhibitor levels are adjusted, and the adhesive is packaged. 

The above discussion bears heavily on the analysis of the patterns of anionic inhibition of BCA catalysis. Anionic inhibitors, depending on the choice of pH and substrate, are able to inhibit solely through the apparent Km of HCOo dehydration activity largely through only the apparent ) of CO2 hydration activity... 

We propose that HCO3 can occupy the anionic binding sites common to all anionic inhibitors and that HCO3 can also inhibit CO2... 

We propose that the catalysis of Zinc(II) carbonic anhydrase proceeds through a variety of four- and five-coordinate Zinc(II) species in rapid equilibrium. The extensive and stimulating series of spectral studies recently undertaken by Bertini and colleagues (30-32) has strongly suggested to us that the expansion of the coordination sphere of metal-substituted carbonic anhydrase is an important mode of accommodation of anions in the enzyme active site. X-ray evidence(23) and Cl NMR data(34) as well as other spectral studies, have localized the binding of simple anionic inhibitors to the inner coordination sphere of the zinc atom in the acidic form of carbonic anhydrase. 

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