P-lactam rings

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. 

Mechanistic studies (6,26,27,67) have shown that the acyl enzyme species is the ring opened compound (13), which can tautomerize to the transientiy inhibited amino acrylate (14), and both of these species can react further to give irreversibly inactivated enzyme. Three inactivated forms of the enzyme have been detected. Two, according to labeling studies, retain the complete clavulanate skeleton and the other retains only the carbon chain of the P-lactam ring. Stmcture (15) has been suggested as one possible inactivated form. 

P-Lactam antibiotics exert their antibacterial effects via acylation of a serine residue at the active site of the bacterial transpeptidases. Critical to this mechanism of action is a reactive P-lactam ring having a proximate anionic charge that is necessary for positioning the ring within the substrate binding cleft (24). 

Benzyl- and Phenoxymethylpenicillins, Ampidllin, Carbenicillin Cephalosporin C Cephaloglycine, Cephaloridine, Cephalothin Hydrolysis Corresponding p-lactam ring cleavage products Escherichia coli Streptomyces aibus Pseudomonas aeruginosa Enterobacter cloacae Streptomyces sp. 

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]. 

The anticancer activity of complex natural products having a cyclodecenediyne system [for a review see <96MI93>] has prompted the synthesis of 54 (X = CH2 and OCH2) <96CC749> and 55 (R = a-OH and p-OH) <95AG(E)2393> on the basis that such compounds are expected to develop anticancer activity as the P-lactam ring opens. This is because cycloaromatization can only occur in the monocyclic enediyne and the diradical intermediate in the cyclization is thought to be the cytotoxic species. 

As mentioned earlier in this chapter, penicillins are very unstable in aqueous solution by virtue of hydrolysis of the p-lactam ring. A successful method of stabilizing penicillins in liquid dosage forms is to prepare their insoluble salts and formulate them in suspensions. The reduced solubility of the drug in a suspension decreases the amount of drug available for hydrolysis. An example of improved stability of a... 

Scheme 2.76. MeLi-induced p-lactam ring-opening/intramolecular cyclization leading to anatoxin-a (2-326).

The answer is e. (Hardman, pp 1074—1076.) Penicillinase hydrolyzes the p-lactam ring of penicillin G to form inactive penicilloic acid. Consequently, the antibiotic is ineffective in the therapy of infections caused by penicillinase-producing microorganisms such as staphylococci, bacilli, E, call, P aeruginosa, and M tuberculosis,... 

Penicillins are the most widely used of the clinical antibiotics. They contain in their structures an unusual fused ring system in which a four-membered P-lactam ring is fused onto a five-membered thiazolidine. Both rings are heterocyclic, and one of the ring fusion atoms is nitrogen. These heteroatoms do not alter our understanding of molecular shape, since we can consider that they also have an essentially tetrahedral array of bonds or lone pair electrons (see Section 2.6.3). 

This selectivity is not achievable by simple chemical hydrolysis, since the strained P-lactam ring is much more susceptible to nucleophilic attack than the unstrained side-chain amide function. Normally, the electron-donating effect from the lone pair of the adjacent nitrogen stabilizes the carbonyl against nucleophilic attack (see Section 7.9.2) this is not possible with the P-lactam ring because of the geometric restrictions (see Box 3.20). 

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 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. 

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