Zinc enzymes hydrolases

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. 

All presently known nonredox enzymes with a cubane as catalytic center are hydrolases (see later discussion). In these enzymes Fea is formally equivalent to the tetrahedral Zn(II) site in hydrolytic zinc enzymes. Thus, when the enzyme is ready to act but not yet in turnover (the substrate-free enzyme), its cluster is probably best described as structure ii in Figure 1 with X a hydroxyl group. In shorthand notation this could be written as [FeaOH—3Feb—4S]1+ (Cys)3 or as [Fea3Feb-4S]2+OH(Cys)3 where it has been assumed that, on average, each iron is formally Fe2 5+. 

Some of these sites can effect function in zinc proteins as well as stabilizing structure. The hydrolase class of zinc enzymes are good examples of this action. In this case, one or more amino acid residues within the active site may be provided by the amino acid spacers between zinc ligands (Figure 12). The side chain of these amino acids may be involved in substrate binding, bond cleavage or modulating the chemical enviromnent of the active site. In addition, other active-site residues are often provided by... 

Aeromonas proteolytica aminopeptidase [a-Aminoacylpeptide hydrolase (Aeromonas proteolytica)] (3.4.11.10) is produced. [A zinc enzyme. Acts most rapidily on L-leucylpeptides, amide and B-naphthylamide. Does not cleave Glu-and Asp-bonds. Similar aminopeptidases were isolated from E, coli and Staphylococcus thermophilus. ] Thermophilic aminopeptidase [a-Aminoacylpeptide hydrolase] (3.4.11.12) is produced. [Metalloen-zymes of high temperature stability and of broad specificity, releasing all N-terminal amino acids, including arginine and lysine. Isolated from Bacillus St ear othermophilus, Talaromyces duponti, Mucor. ]... 

Hydrolases represent a significant class of therapeutic enzymes [Enzyme Commission (EC) 3.1—3.11] (14) (Table 1). Another group of enzymes with pharmacological uses has budt-ia cofactors, eg, in the form of pyridoxal phosphate, flavin nucleotides, or zinc (15). The synthases, and other multisubstrate enzymes that require high energy phosphates, are seldom available for use as dmgs because the required co-substrates are either absent from the extracellular space or are present ia prohibitively low coaceatratioas. 

The matrix metalloprotease (MMP) family of zinc hydrolases are thought to play important roles in extracellular tissue remodeling in angiogenesis and other normal physiological processes, in some inflammatory processes and in metastatic processes in cancer. Like the zinc carboxypeptidases, the MMPs also utilize a zinc-coordinated water molecule to initiate attack on the scissile amide bond of protein substrates. These enzymes are synthesized by the ribosome in a latent form composed of a catalytic domain and an N-terminal extension, referred to as the prodomain the latent, or inactive form of the enzyme is referred to as a zymogen or... 

We have already seen the diversity of function in the lyases, hydrolases and oxidoreductases. Several other types of zinc coordination are found in a number of other enzymes, illustrated in Figure 12.8. These include enzymes with the coordination motif [(His)2(Cys) Zn2+-OH2], illustrated by the lysozyme of bacteriophage T7 this group also includes a peptidyl deformylase. 

The cytosolic enzyme leukotriene A4 hydrolase (EC 3.3.2.6), which ster-eoselectively converts leukotriene A4 (LTA4) to leukotriene B4 [56], This enzyme catalyzes the hydrolytic cleavage of the 5,6-epoxide ring in LTA4, but, in contrast to what happens with other EHs, the product is not a vicinal diol but a 5,12-diol. As a zinc metalloenzyme, LTA4 hydrolase does not appear to be related to any other epoxide hydrolase. 

Figure 6. Superimposition of the crystai structures of the cataiytic domains of LTA4 hydrolase (biue), angiotensin-converting enzyme (red) and thermoiysin (yeiiow), each with bound zinc ion (coioured accordingiy).

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. 

Another approach for hydrolysis of acid derivatives utilizes metal-containing hydrolases (see I in Fig. 17.6). In these enzymes, a metal, for example a zinc atom, is included in the active site of the enzyme (Fersht, 1985). The process begins with the association of the carbonyl oxygen (or its equivalent in other compounds) with a ligand position on this metal (II). As we found for the acid-catalyzed hydrolysis mechanism discussed in Chapter 13.3, this association with an electropositive metal... 

Penicillin and related antibiotics are inactivated by P-lactamases (Box 20-G), some of which resemble serine proteases in forming acyl enzymes with active site serine side chains.656 657 Others are zinc metallogen-zymes.658/659 Amidohydrolases such as asparaginase and glutaminase,660/661 deacetylases,662 and many other hydrolases can also be described as acyltransferases. 

The four types of nickel-containing enzymes are quite distinct in the coordination sites and catalytic function of the nickel centers. In urease, the nickel appears to be bound to oxygen and nitrogen ligands and appears to remain as Ni(II), a state which favors octahedral or square-planar coordination. The function of nickel in this unique case may be analogous to that of zinc in other hydrolases such as carboxypeptidase. 

Hydrolases represent a significant classes of therapeutic enzymes (Table 11. Another group of enzymes with pharmacological uses has built-in cofactors, c.g.. in the form of pyridoxal phosphate. Ilavin nucleotides, or zinc. 

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. 

Metallohydrolases are the third major class of hydrolytic enzymes. Unlike serine or cysteine hydrolases, the metallohydrolases achieve substrate hydrolysis via a zinc-activated water molecule [78-80] (Scheme 3). 

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