Modification chelation agents

Certain bifunctional metal chelating agents have been used to investigate protein interactions by virtue of their ability to generate reactive oxygen species that affects protein structure in the immediate vicinity of their modification site. The following sections discuss two applications of such chelate labels, one of which cleaves peptide bonds while the other one causes covalent crosslinks to occur between interacting protein structures. 

In order to increase the solubility of porphyrin and phthalocyanine complexes, several structural modifications have been made, a, jS, y, 6-Tetra-(4-pyridyl)-porphin complexes of copper(II), nickel(II), and zinc(II) have been synthesized (35) and their ultraviolet spectra determined in chloroform and in acid solution. By utilizing sulfonic acid groups to increase solubility, complexes of 4,4, 4",4" -tetrasulfophthalocyanine complexes of many metals were prepared (94j 95). This chelating agent was found to have a ligand field strength comparable to cyanide (94y 95). 

Carboxypeptidase A is one of the most intensely investigated zinc metalloenzymes. The enzyme as isolated contains 1 g-atom of zinc per protein molecular weight of 34,600. Removal of the metal atom either by dialysis at low pH or by treatment with chelating agents gives a totally inactive apoenzyme (46). Activity can be restored by readdition of zinc or one of a number of other di-valent metal ions (47). Through a combined use of chemical modification and transient state kinetic studies, it has been possible to determine the role of zinc in the catalysis of ester and peptide hydrolysis by this enzyme. 

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