Metallo- 3-lactamase

The class B metallo- 3-lactamases have emerged more recently as a clinical problem but they are particularly dangerous since many of them hydrolyse all know (3-lactams, with the exception of monobac-tams. In particular, they hydrolyse the suicide substrates mentioned above, as well as carbapenems that usually escape the activity of all the SXXK enzymes, with the exception of the NMCA group. 

N. V. Kaminskaia, B. Spingler, S. J. Lippard, Hydrolysis of /3-Lactam Antibiotics Catalyzed by Dinuclear Zinc(II) Complexes Functional Mimics of Metallo-/3-Lactamases ,. /. Am. Chem. Soc. 2000, 122, 6411-6422. 

Figure 7.5 Deconvoluted, zero charge state mass spectrum demonstrating a hit from a DCL-targeting metallo-[3-lactamase (Bell). The dominant peak corresponds to anrora A kinase linked to extender 23, which is in turn linked to fragment 24 to give 25 (dynamic hit ). Reprinted from Reference 27, with permission from Elsevier, Copyright (2008).

Selevsek, N. Tholey, A. Heinzle, E. Lienard, B. M. R. Oldham, N. J. Schofield, C. J. Heinz, U. Adolph, H.-W. Frere, J.-M. Studies on ternary metallo-[3-lactamase inhibitor complexes using electrospray ionization mass spectrometry. J. Am. Soc. Mass Spectrom. 2006,17, 1000-10004. 

Three of the four classes of (3-lactamases, A, C, and D are serine nucleophile-based enzymes, while the fourth, class B, contains zinc metallo-(3-lactamases. Among the serine (3-lactamases, classes A and C are currently the most intensely studied. However, extended spectrum class D enzymes have, of late, been growing in clinical importance [309-314]. 

Similarly, metallo- 3-lactamases share with metallo-proteases activation of a hydrolytic water molecule by interaction with an active-site Zn + ion. These lactamases have gained clinical prominence in the past few years as a result of their association with carbapenem resistance. The carbapenems, such as meropenem and imipenem, are fS-lactam antibiotics that have been introduced to circumvent Ser-fS-lactamase activity. The trans stereochemistry across the 6-5 bond rather than the cis geometry found in most other fS-lactams (Fig. 7) contributes to... 

Lactamases may be chromosomal or plasmid-borne, inducible or constitutive, and for this reason their terminology can be confusing. A number of classification systems have been proposed, including classes A-D based on peptide sequence. Classes A, C and D have a serine at the active site, whereas class B enzymes have four zinc atoms at their active site and these are also called metallo-(3-lactamases. Class A enzymes are highly active against ben-zylpenicillin, class B (3-lactamases are effective against penicillins and cephalosporins. Class C en-... 

Metallo-(3-lactamases catalyze the hydrolytic ring opening of a (3-lactam ring in antibiotics containing this structural unit (e.g. penicillins, cephalosporins, carbapenems),... 

Fig. 20 Structural features of the active site zinc centers in metallo- 3-lactamases.

Model studies for metallo- 3-lactamases have been performed using mononuclear zinc hydroxide complexes.99,129,130 The breadth of (3-lactam hydrolysis reactivity of hydro-tris(pyrazolyl)borate-ligated mononuclear zinc hydroxide complexes has been explored.129 Treatment of the mononuclear zinc hydroxide complex [(Tpph Mc)Zn OH] with simple 3-lactams ( 3-propiolactam, 4-phenyl-(3-propiolactam, Scheme 21) does not result in ring opening, but instead results in the formation of 3-lactamide complexes and water. Treatment of [(Tpph,Me)Zn-OH] with /V-alkyl or -aryl 3-lactam derivatives instead results in no reaction (Scheme 21). Use of natural derivatives of penicillin and cephalosporin (Scheme 22) did not yield 3-lactam hydrolysis, but instead coordination of the carboxylate moiety of the antibiotic derivatives to the mononuclear Zn(II) center and release of water. 

Mechanistic studies of the reaction catalyzed by the metallo-(3-lactamase from Bacteroides fragilis have been performed using nitrocefin (Scheme 25) as a substrate.127,131,132 Several theoretical studies pertinent to possible steps in the mechanistic pathway have also been reported.133-139 Issues stemming from these studies that remain unresolved include whether the nucleophilic hydroxide is bridging or terminal, and the chemical nature of an intermediate that is detected during the hydrolysis of nitrocefin. 

An important conclusion from these comparative studies is that mononuclear Zn(II) complexes can be as efficient for nitrocefin hydrolysis as binuclear systems, thus providing evidence that the second zinc center (Zn2) in metallo- 3-lactamases is not required for nucleophile activation or catalytic activity. 

The hope that promiscuity is predictable is also supported by the identification of systematic patterns of promiscuity. For example, lactonases, and in particular lactonases that favor hydrophobic lactones, show a consistent tendency to promiscuously catalyze the hydrolysis of phosphotriesters. This pattern has now been seen in lactonases from three different superfamilies PLLs (TIM-barrels from the amidohydrolase superfamily Table 1, entry 11) PONs, or serum paraoxonases (calcium-dependent six-bladded /3-propellers Table 1, entry 13) and AHA (a lactonase from the metallo-/3-lactamase superfamily Table 1, entry 12). That very different scaffolds and active-sites configurations share the same promiscuity pattern suggests that these reactions share a key feature, probably in the geometry of their transition states. This feature must be distinct, also because many of these lactonases do not hydrolyze esters that are much closer to lactones than phosphotriesters, and should thus be amenable to structural analysis and prediction. 

Figure 11 Proposed catalytic mechanisms for (a) mononuclear and (b) binuclear metallo-/3-lactamases.

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