Zinc enzymes transferases

This review will give a subjective account of mechanistic studies on some representative zinc enzymes comprising the enzyme classes I-IV (oxidoreductases, transferases, hydrolases and lyases). It does not claim to be comprehensive, as a comprehensive review would be far too extensive for this work. Lowther and Matthews have reviewed the met-alloaminopeptidases and Noodleman and coworkers have reviewed calculational studies on metalloenzymes. We apologize in advance for any omissions and point to previous reviews on this and related subjects to be found, e.g., in References 7-11. 

The sirtuins (silent information regulator 2-related proteins class III HDACs) form a specific class of histone deacetylases. First, they do not share any sequence or structural homology with the other HDACs. Second, they do not require zinc for activity, but rather use the oxidized form of nicotinamide adenine dinucleotide (NAD ) as cofactor. The reaction catalyzed by these enzymes is the conversion of histones acetylated at specific lysine residues into deacetylated histones, the other products of the reaction being nicotinamide and the metabolite 2 -0-acetyl-adenosine diphosphate ribose (OAADPR) [51, 52]. As HATs and other HDACs, sirtuins not only use acetylated histones as substrates but can also deacetylate other proteins. Intriguingly, some sirtuins do not display any deacetylase activity but act as ADP-ribosyl transferases. 

Zinc is an essential trace element. More than 300 enzymes that require zinc ions for activity are known. Most catalyze hydrolysis reactions, but zinc-containing representatives of aU enzyme classes are known, such as, for instance, alcohol dehydrogenase (an oxidoreductase), famesyl-Zgeranyl transferase (a transferase), -lactamase (a hydrolase), carbonic anhydrase (a lyase) and phosphomannose isomerase. 

More recently, isotopic labeling experiments have assumed a major role in establishing the detailed mechanism of enzymic action. It was shown that alkaline phosphatase possesses transferase activity whereby a phos-phoryl residue is transferred directly from a phosphate ester to an acceptor alcohol (18). Later it was found that the enzyme could be specifically labeled at a serine residue with 32P-Pi (19) and that 32P-phosphoserine could also be isolated after incubation with 32P-glucose 6-phosphate (20), providing strong evidence that a phosphoryl enzyme is an intermediate in the hydrolysis of phosphomonoesters. The metal-ion status of alkaline phosphatase is now reasonably well resolved (21-23). Like E. coli phosphatase it is a zinc metalloenzyme with 2-3 g-atom of Zn2+ per mole of enzyme. The metal is essential for catalytic activity and possibly also for maintenance of native enzyme structure. 

We have already seen a number of models for the zinc(II) containing enzymes such as carbonic anhydrase in Section 11.3.2. Zinc is an essential component in biochemistry, and forms part of the active site of more then 100 enzymes, of which hydrolases (such as alkaline phosphatase and carboxypeptidase A), transferases (e.g. DNA and RNA polymerase), oxidoreductases (e.g. alcohol dehydrogenase and superoxide dismutase) and lysases (carbonic anhydrase) are the most common. In addition, the non-enzyme zinc finger proteins have an important regulatory function. In many of these systems, the non-redox-active Zn2+ ion is present as a Fewis acidic centre at which substrates are coordinated, polarised and hence activated. Other roles of zinc include acting as a template and playing a structural or regulatory role. 

The normal zinc content of the body amounts to 20-30 mmol (1.3-2.0 g). The daily dietary requirement is 10-15 mg. In the blood, zinc is bound to tt2-macro-globulin, albumin or amino acids, and a small amount is also bound to transferrin. Zinc is crucial to a variety of enzyme reactions. This applies especially to the liver. More and more attention has therefore been paid to the role of zinc in liver disease in recent years. Six enzyme groups (hydrolases, isomerases, ligases, lyases, oxidore-ductases and transferases) with a total of 35 zinc metal-loenzymes are listed. (98) Almost 200 enzyme reactions in the body are zinc-dependent ... 

Several enzymes bind both magnesium and zinc ions, using them for different purposes. For example, nucleotidyl transferases can use one metal ion, to which is coordinated a phosphate group and a carboxylate group, the latter serving to polarize the water nucleophile. When there are two metal ions present, the carboxylate group also binds the second metal ion which in turn activates the nucleophile. Therefore it appears that the second metal ion aids in the catalysis, replacing the action of an active-site side chain in those enzymes of that activity that only bind one metal ion. T vo enzymes will be described here—the alkaline phosphatase and the 3 -5 exonuclease from E. coli. 

Both famesyltransferases " and geranylgeranyl-transferases " have been characterized, and the three- dimensional structure of the former has been established. The fwo-domain protein contains a seven-helix crescent-shaped hairpin domain and an a,a-barrel similar to that in Fig. 2-29. A boimd zinc ion in the active site may bind the -S group of the substrate protein after the famesyl diphosphate has been bound into the active site. These enzymes are thought to fimction by a carbocation mechanism as shown in Eq. 22-3 and with the indicated inversion of configurafion. ... 

More than 300 reactions are catalyzed by zinc-containing enzymes [82]. Zinc can be found with insulin, in the reproductive tract, in the DNA-binding proteins, and in oxidoreductases, transferases, lyases, isomerases, and ligases. 

One of the main biochemical roles of zinc is its influence on the activity of over 300 enzymes, which are distributed into the six classes oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. 

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