Site-binding ability, anion

As discussed previously, zwitterionic polymers such as poly (sulfobetaine)s have thermoresponsive UCST properties. The water solubility of poly(sulfobetaine) depends on salt concentration. The site-binding ability of the cation and the anion allows polymer chains to preferentially complex the... 

A topical relation between the esteratic and anionic sites is implicit in any consideration of enzyme action in which the two sites participate. It seems reasonable to assume that the two sites are spaced to accommodate choline esters. The proximity of the sites is indicated by the ability of prostigmine to inhibit the reaction with thioacetic acid, which does not involve interaction with the anionic site. The necessity for binding acetylcholine at both sites for efficient catalysis has been used to explain inhibition by excess substrate. When high concentrations of acetylcholine are present, it is possible for one molecule to interact with the anionic site of the enzyme, while a second molecule associates with the... 

The classic example of permeation as an anion is the ability of oxyanions of toxic metals to traverse cell membranes on either the phosphate or sulfate carriers (reviewed by Wetterhahn-Jennette 1981 Clarkson 1993). Vanadate and arsenate are structurally similar to phosphate and compete with phosphate for transport, as well as intracellular binding sites. Indeed, their toxicity is thought to be directly related to this competition (Clarkson 1993). Similarly, chromate, selenate, and molybdate are structurally similar to sulfate, and are substrates for sulfate transporters. 

Bivalent inhibitors of thrombin have been synthesized to bind the anion-binding exosite and active (catalytic) site of thrombin simultaneously. By coupling the carboxy terminal fragment of hirudin to a tripeptide (D-Phe-Pro-Arg) by including a spacer molecule, both the anion exosite and the catalytic site are blocked. An example of such a molecule is Hirulog, which has 20 amino acids and has a Kj of 2 nM (61). Its ability to block the active site has been questioned, since thrombin has been shown to cleave the Arg-Pro bond of Hirulog slowly in vivo (58). In addition to hirudin and hirudin-like compounds, three other classes of site-directed thrombin inhibitors deserve mention. 

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