Histidine residues alkaline phosphatase

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

Photooxidation of alkaline phosphatase in the presence of methylene blue and Rose Bengal causes loss of activity for both native and apo-enzyme. In the case of the native enzyme, zinc protects 2 to 3 of the 16 histidine residues. The rate of oxidation of tryptophan is not affected by zinc, and there was no loss of tyrosine. Also, photooxidation of the apoenzyme diminishes zinc binding. It would appear that histidine residues play a role in binding the two zinc ions necessary for enzymic activity (91). 

Treatment of the enzyme with A-bromosuccinimide oxidized 2 of the 8 tryptophan residues and 8 of the 20 tyrosine residues, but none of the histidine residues. This treatment causes the phosphotransferase activity with tris as an acceptor to double and the hydrolase activity to increase slightly. In the case of cobalt alkaline phosphatase, the above treatment caused a threefold increase in hydrolase activity and the generation of an even greater phosphotransferase activity (91). 

It is noteworthy that three His, Glu, Asp or Cys residues provide zinc ligands for all known enzyme catalytic zinc sites [ 30], Water is the fourth ligand and histidine is by far the most frequent amino acid among the catalytic site residues. Three histidines are found in human carbonic anhydrases 1 and II, p-lactamase, the DD-carboxypeptidase of Streptomyces albus G, adenosine deaminase and astacin [30]. Two histidines are characteristic of bovine carboxypeptidases A and B, thermolysin and Escherichia coli alkaline phosphatase... 

Enzymes may use any of the above mentioned modes of catalysis in order to catalyze a particular chemical reaction. For example, the imidazole ring of a histidine residue of the enzyme a-chymotrypsin (Section 4.4) can function as a general-base catalyst, while in the enzyme alkaline phosphatase, the same residue can function as a nucleophilic catalyst. Indeed, enzymes are complex catalysts which employ more than one catalytic parameter during the course of their action. It is by this successful integration of a combination of individual catalytic processes that a rate enhancement as high as 10 " may be achieved. Furthermore, it is this combination of factors which results in a specific catalyst. 

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