Carbonic anhydrase metal chelate enzyme

Carboxypeptidase A was the first metalloenzyme where the functional requirement of zinc was clearly demonstrated (9, 92). In similarity to carbonic anhydrase, the chelating site can combine with a variety of metal ions (93), but the activation specificity is broader. Some metal ions, Pb2+, Cd2+ and Hg2+, yield only esterase activity but fail to restore the peptidase activity. Of a variety of cations tested, only Cu2+ gives a completely inactive enzyme. In the standard peptidase assay, cobalt carboxypeptidase is the most active metal derivative, while it has about the same esterase activity as the native enzyme ((93, 94), Table 6). Kinetically, the Co(II) enzyme shows the same qualitative features as the native enzyme (95), and the quantitative differences are not restricted to a single kinetic parameter. 

Because of the ease with which dimercaptopropanol can be broken down in the body there is a danger that chelation, followed by breakdown, will simply result in the translocation of the metal ions to other tissues such as brain or liver. High doses of dimercaptopropanol can adversely affect a number of essential metal-activated enzymes, such as catalase, carbonic anhydrase and peroxidase, and also produce dangerous systemic effects. Dimercaptopropanol cannot be used to remove cadmium because its cadmium complex is toxic to kidney tissue54). 

Chelation therapy. Treatment of carbonic anhydrase with high concentrations of the metal chelator EDTA (ethylenediaminetetraacetic acid) results in the loss of enzyme activity. Propose an explanation. 

Catalysis Hydration and dehydration reactions are catalyzed by various bases, by certain metal chelates, and by the enzyme carbonic anhydrase. Natural waters may contain some natural catalysts. Reaction 81 is essentially a catalysis of reaction 79. The conversion of CO2 into H2CO3 or HCO by both mechanisms is... 

It could be shown, moreover, (Vallee and Neurath, 1955) that five times recrystallized carboxypeptidase was completely inhibited by metal chelating agents, such as 8-OHQ-5SA and 1-10 phenthroline at concentrations of 10 W,Q ,Q Dat concentrations of 10" Af, and some 30% byEDTA at 10 M. These are all known to form complexes with zinc in simple systems. In these experiments, the buffered enzyme solutions were incubated with the chelating agent at pH 7.5, 4°C., for 1 hour prior to the addition of the substrate. Inhibition did not occur when these chelating agents were first incubated with an equimolar amount of zinc, cupric, or ferrous ions. Sodium diethyldithiocarbamate, zincon, sulfanilamide, and diamox, the latter two employed because of their effect on carbonic anhydrase, had little, if any, effect on carboxypeptidase activity. DPN, nicotinamide, and A-methylnicotinamide, examined because of their effect on the ADH sys-... 

The zinc atom in carbonic anhydrases is as strongly attached to the enzyme molecule as that of enolases. It cannot be removed by electrodialysis or exchanged with the radioactive zinc of the medium. But if the enzyme is incubated at pH 5 in the presence of a strong chelating agent, the zinc is detached from the protein and the enzyme is inactivated. The enzyme is reactivated by incubating the metal-free protein with Zn or Co. However, the activity of the cobalt enzyme is only half that of the zinc enzyme. ... 

Polymerization is found assisting the immobilization of carbonic anhydrase on the surface of vesicles. In these examples, polymerization is stabilizing the vesicular structures as well as eliminating the motional properties of lipids i.e., lateral mobility and rotational motion. This explains the poor immobilization of enzyme on polymerized vesicles containing non-polymerizable metal chelating lipid. Detailed studies on the kinetics of enzyme binding with non-polymerizable metal-chelated lipid inserted in polymer constrained environment of polymerized vesicles may provide an insight about the role of polymerization on enzyme immobilization. 

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