3-lactamases 3-lactam ring

Resistance. Resistance to the cephalosporins may result from the alteration of target pencillin-binding sites (PBPs), decreased permeabdity of the bacterial ced wad and outer membrane, or by inactivation via enzyme mediated hydrolysis of the lactam ring (80,81,138—140). This resistance can be either natural or acquired. Although resistance is often attributed speciftcady to one of these factors, in reaUty it reflects the interplay of several factors. In most instances, however, resistance results from the production of a P-lactamase enzyme, which opens the P-lactam ring as depicted in Figure 2. 

The antibacterial effectiveness of penicillins cephalospotins and other P-lactam antibiotics depends upon selective acylation and consequentiy, iaactivation, of transpeptidases involved ia bacterial ceU wall synthesis. This acylating ability is a result of the reactivity of the P-lactam ring (1). Bacteria that are resistant to P-lactam antibiotics often produce enzymes called P-lactamases that inactivate the antibiotics by cataly2ing the hydrolytic opening of the P-lactam ring to give products (2) devoid of antibacterial activity. 

An example for proteases are the (3-lactamases that hydrolyse a peptide bond in the essential (3-lactam ring of penicillins, cephalosporins, carbapenems and monobac-tams and, thereby, iireversibly inactivate the diug. 13-lactamases share this mechanism with the penicillin binding proteins (PBPs), which are essential enzymes catalyzing the biosynthesis of the bacterial cell wall. In contrast to the PBPs which irreversibly bind (3-lactams to the active site serine, the analogous complex of the diug with (3-lactamases is rapidly hydrolyzed regenerating the enzyme for inactivation of additional (3-lactam molecules. 

P-Lactamases are enzymes that hydrolyze the P-lactam ring of P-lactamantibiotics (penicillins, cephalosporins, monobactams and carbapenems). They are the most common cause of P-lactam resistance. Most enzymes use a serine residue in the active site that attacks the P-lactam-amid carbonyl group. The covalently formed acylester is then hydrolyzed to reactivate the P-lacta-mase and liberates the inactivated antibiotic. Metallo P-lactamases use Zn(II) bound water for hydrolysis of the P-lactam bond. P-Lactamases constitute a heterogeneous group of enzymes with differences in molecular structures, in substrate preferences and in the genetic localizations of the encoding gene (Table 1). 

P-Lactamases (EC 3.5.2.6) produced by bacteria cleave the P-lactam ring and are responsible for their resistance to P-lactam antibiotics. Lactamases are useful catalysts for the enantioselective hydrolysis of P-lactams and other cyclic amides. P-lactams shown in Figure 6.40 were resolved by whole-cell systems containing an amidase [106]. 

In cephalosporins suseeptible to / -lactamases, opening of the y8-lactam ring occurs with concomitant loss of the substituent at (except in cephalexin, where represents H see Fig. 5.4). This is followed by fragmentation of the molecule. Provided that they are not inaetivated by "-laetamases, the cephalosporins generally have a broad spectrum of activity, although there may be a wide variation. Haemophilus influenzae, for example, is partieularly suseeptible to eefuroxime see also Table 5.2. 

N-(o, m, or p-carboxy)phenyl azetidin-2-ones 19 (Fig. 11.12), which have two characteristic features of a [3-lactamase substrate, a [3-lactam ring and a carboxy... 

Like with primary amides (see Sect. 4.2.1), bacterial amidases can be useful for the transformation of secondary amides in drug synthesis. Bacterial amidases have been extensively studied in the presence of penicillins and other [i-lactam antibiotics, for which two hydrolysis reactions are possible. One of these is carried out by enzymes known as penicillinases or /3-lactamases that open the /3-lactam ring this aspect will be discussed in Chapt. 5. The second type of hydrolysis involves cleavage of the side-chain amide bond (4.47 to 4.48) and is carried out by an enzyme called penicillinacylase (penicillin amidohydrolase, EC 3.5.1.11). Both types of hydrolysis inactivate the antibiotic [29-31],... 

Now we turn to a discussion of the influence of a-substitution at C(6) or C(7) on the chemical reactivity of the lactam ring (Table 5.4,B). This substitution has been introduced mainly to improve lactamase stability (see Sect. 5.2.2.2). The insertion of an additional a-substituent at C(6) or C(7) of penicillins or cephalosporins, respectively, has a relatively small effect on the rate of base hydrolysis [82] [83], 6a-Methoxypenicillin is hydrolyzed at a rate that is approximately half that observed for the unsubstituted parent penicillin. This decrease is due mainly to unfavorable steric interaction between the... 

The previous sections have covered both degradation by microbial enzymes (/3-lactamases) and chemical hydrolysis. We now turn to considerations of the mammalian metabolism of /3-lactams involving the opening of the /3-lactam ring. 

The first carbapenem released for clinical use was imipenem (5.46), a compound with relatively high resistance to microbial /3-lactamases. The addition of the (iminomethyl)amino side chain renders imipenem chemically more stable than thienamycin. But, like thienamycin, imipenem is also easily hydrolyzed by renal dehydropeptidase I, producing a mixture of /3-lactam ring-opened 1-pyrrolidine epimers at C(3) [161], The renal metabolism of imipenem can be minimized by co-administration of cilastatin (5.53), a competitive inhibitor of DHP-I [15 6] [162],... 

A. Matagne, J. Lamotte-Brasseur, G. Dive, J. M. Frere, Interaction between Active Site-Serine /3-Lactamases and Compounds Bearing a Methoxy Side Chain on the a-Face of the /3-Lactam Ring Kinetic and Molecular Modelling , Biochem. J. 1993, 293, 607-611. 

Resistance to antibiotics is usnally due to the acquisition of genes that express enzymes that can inactivate the antibiotics (e.g. P-lactamase degrades the lactam ring in penicillin (see below) or that can modify the structure of proteins that are necessary for the antibiotic to enter the... 

An additional disadvantage with many penicillin and cephalosporin antibiotics is that bacteria have developed resistance to the drugs by producing enzymes capable of hydrolysing the P-lactam ring these enzymes are called P-lactamases. This type of resistance still poses serious problems. Indeed, methicillin is no longer used, and antibiotic-resistant strains of the most common infective bacterium Staphylococcus aureus are commonly referred to as MRSA (methicillin-resistant Staphylococcus aureus). The action of P-lactamase enzymes resembles simple base hydrolysis of an amide. 

Note again that the strained P-lactam ring is more susceptible to nucleophilic attack than the unstrained side-chain amide function. However, by increasing the steric bulk of the side-chain, the approach of a P-lactamase enzyme to the P-lactam ring is hindered in the semi-synthetic antibiotic, giving it more resistance to enzymic hydrolysis. 

Note that penicillins and structurally related antibiotics are frequently deactivated by the action of bacterial -lactamase enzymes. These enzymes also contain a serine residue in the active site, and this is the nucleophile that attacks and cleaves the P-lactam ring (see Box 7.20). The P-lactam (amide) linkage is hydrolysed, and then the inactivated penicillin derivative is released from the enzyme by further hydrolysis of the ester linkage, restoring the functional enzyme. The mode of action of these enzymes thus closely resembles that of the serine proteases there is further discussion in Box 7.20. 

Finally, the most important mechanism of resistance to beta-lactam antibiotics is the production of beta-lactamase by the bacteria. Beta-lactamases break the C-N bond in the beta-lactam ring of antibiotics. Since its existence is absolutely necessary for reacting with PBP, a break in the beta-lactam ring leads to a loss of antibacterial activity. 

The cephalosporin nucleus is synthesized with a beta-lactam ring attached to a six-membered dihydrothiazine ring. Unlike the penicillin nucleus, the cephalosporin nucleus is much more resistant to beta-lactamase. Moreover, it has large areas for possible modifications. Modifications Rj in the acyl side chain alter the antibacterial activity, while modifications of R2 are associated with changes in the pharmacokinetics and metabolic parameters of the drug. 

Unlike penicillins and cephalosporins, which have a side aminoacyl group joined to the beta-lactam ring, imipenem has a a-hydroxyethyl side chain. Significant resistance to hydrolysis by beta-lactamases is observed in this compound, evidently thanks to the fran -configuration of the side chain, while the side chain of penicillins and cephalosporins have a cis-configuration. 

It is believed that the methyl group at position 4 increases the stability of the beta-lactam ring with respect to most beta-lactamases, and at the same time it does not induce formation of beta-lactamase as cephalosporins and imipenems do. 

The penicillins are a large group of bactericidal compounds. They can be subdivided and classified by their chemical structure and spectrum of activity. The structure common to all penicillins is a (3-lactam ring fused with a thiazolidine nucleus (Fig. 45.1).The antimicrobial activity of penicillin resides in the (3-lactam ring. Splitting of the (3-lactam ring by either acid hydrolysis or (3-lactamases results in the formation of penicilloic acid, a product without antibiotic activity. Addition of various side chains (R) to the basic penicillin molecule... 

Structure-activity correlations in the P-lactam antibiotic field have required drastic re-evaluation in view of the novel structures described above. Apparently, only the intact P-lactam ring is an absolute requirement for activity. The sulfur atom can be replaced (moxalactam) or omitted (thienamycin), and the entire ring itself is, in fact, unnecessary (nocardicin). The carboxyl group, previously deemed essential, can be replaced by a tetrazolyl ring (as a bioisostere), which results in increased activity and lactamase resistance. The amide side chain, so widely varied in the past, is also unnecessary, as shown in the example of thienamycin. There is a considerable literature analyzing the classical structure-activity relationships of the penicillin and cephalosporin groups. 

Many bacteria produce a p-lactamase or penicillinase which opens the (3-lactam ring rendering some p-lactams ineffective. The problem of p-lactamase-producing bacteria can, in part, be overcome by the addition of a p-lactamase inhibitor such as clavulanic acid (as in co-amoxiclav). 

The penicillins share features of chemistry, mechanism of action, pharmacology, and immunologic characteristics with cephalosporins, monobactams, carbapenems, and -lactamase inhibitors. All are 3-lactam compounds, so named because of their unique four-membered lactam ring. 

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