P-Lactamase enzymes

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. 

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. 

A. G. Brown (1985). Clavulanic acid and related compounds inhibitors of P-lactamase enzymes. 

It shares with penicillin extremely low toxicity but some danger of allergic reactions. Other semisynthetic penicillins are resistant to P-lactamases, enzymes produced by penicillin-resistant bacteria which cleave the four-membered (i-lactam ring of natural penicillins and inactivate them. 

All penicillins contain a common nucleus composed of a thiazolidine ring and a p-lactam ring connected to a side chain. An intact P-lactam ring is necessary for biologic activity, but the side chain primarily determines the antibacterial spectrum, susceptibility to destruction by gastric acid and P-lactamase enzymes, and pharmacokinetic properties. 

Fig. 19 (top) Types of P-lactam antibiotics and (bottom) hydrolytic ring opening catalyzed by P-lactamase enzymes. 

More recently the whole picture of antibiotic resistance has become better understood as a very complex phenomenon. Even with P-lactam antibiotics, resistance factors other than the mere production of hydrolytic P-lactamase enzymes are operative. In the late 1960s methicillin-resistant staphylococci began to appear as nosocomial infections. In fact these resistant strains show multiple resistance both to P-lactams as well as to antibiotics with different structures and different mechanisms of action.14... 

Attachment of 7-hydroxy-4-methylcoumarin as the releasable group of this side chain generated a penicillin structure that can function as a fluorescence-based reporter substance/diagnostic for the presence of low levels of p-lactamase enzyme in solution (Scheme 117), [320]. 

There are several naturally occurring variations on the lactam-thiazolidine or lactam-dihydrothiazine structures, leading to other useful antibiotics or to inhibitors of the p-lactamases, enzymes that hydrolyze the p-lactam unit. One group, termed carbapenems 5 has a five-membered ring in which the thiazolidine sulfur is replaced with CH2- Such compounds may still contain sulfur in a thioethylamine side chain (derived from L-cysteine) as in thienamycin 6, originally isolated from Streptomyces cattleya (Scheme 2). 

The alternative to combination treatments is the use of P-lactamase enz5 me inhibitors such as davulanic add, sulbactam and tazobactam (Fig. 22.14), which hearstmc-tural resemblance to the P-lactam antibiotics and subsequently compete for the active site of the P-lactamase enzyme. 

The initial interaction between the hydrolytic enzjone and the inhibitor involves the attack on the electrodeficient P-lactam carbon. With the P-ladam antibiotics, the attack on the carbon leads to the disruption of the C-N bond and subsequent loss of antibaderial activity. The P-lactamase enzyme is regenerated after disruption of the P-lactam ring. With the P-lactamase inhibitors, a... 

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