0-lactamases

The enantioselective esterification of 2-arylpropionic acids catalysed by a lipase was discussed earlier.26 Steady-state kinetics of the Pseudomonas cepacia lipase-catalysed hydrolysis of five analogous chiral and achiral esters (R)- and ( S)-(235 R1 = Me, R2 = H), (R) and (S)-(235 R1 = H, R2 = Me), and (235 R1 = R2 = H) were studied in emulsified reaction mixtures of water-insoluble substrates.212 The Km values were all the same and the apparent A cat values reflected the binding abilities of the alcoholate ions for the fast-reacting enantiomers. All the substrates are believed to be 

The kinetic results for the lipase-catalysed enantioselective hydrolysis of the esters (236)-(240) can be interpreted in terms of frontier orbital localization.213 The porcine pancreatic lipase (PPL)-mediated optical resolution of 18 racemic esters can be explained by a mechanistic model involving a W-shaped active conformation of the substrate lying in a diastereo-discriminating plane.214 

TS analogues and inhibitors of the class C /i-lactamasc of Enterobacter cloacae P99 have been mentioned earlier.17 The same enzyme from the same bacteria has been used to hydrolyse cephalexin (241) at pD 6.4 and 8.215 The hydrolysis product is the cephalosporoate intermediate (242) which undergoes tautomerization of the double bond in the dihydrothiazine ring from position 3/4 to 4/5 and there is the uptake of a proton at C(3). 

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Strynadka et al., 1996] Strynadka, N., Eisenstein, M., Katchalski-Katzir, E., Shoichet, B., Kuntz, I., Abagyan, R., Totrov, M., Janin, J., Cherflls, J., Zimmerman, F., Olson, A., Duncan, B., Rao, M., Jackson, R., Sternberg, M., and James, M. Molecular docking programs successfully predict the binding of a /3-lactamase inhibitory protein to TEM-1 /3-lactamase. Nature Struct. Biol. 3 (1996) 233-239... 

First (3-lactamase inhibitor combination. First monobactam and a synthetic product. First carbapenem. 

Compared to penicillins, cephalosporins are generally effective against a broader range of organisms and are more resistant to /3-lactamases. /3-Lactamases are bacterial enzymes that efficiently hydrolyze /3-lactam antibiotics to inactive species in which the /3-lactam bond has been cleaved. Cephalothin was the first cephalosporin to be marketed and continues... 

The most thoroughly studied mode of /3-lactam hydrolysis is that catalyzed by the enzyme /3-lactamase (EC 3.5.2.6). The elaboration of this enzyme is one of the three principal ways in which a bacterium can obtain resistance to a /3-lactam antibiotic (see Section 5.11.5.1), and much of the chemical work carried out on the penicillin molecule has been related to attempts to deal with this problem. A discussion of the /3-lactamases is beyond the scope of this work. The reader is referred to (B-79MI51101) for a recent review. 

Susceptible Gram-positive organisms such as Streptococcus pneumoniae and /3-lactamase-negative Staphylococcus aureus. 

Certain penicillins and penam derivatives have the ability to inhibit the /3-lactamase enzyme, and can provide a variable degree of protection to susceptible /3-lactam antibiotics... 

Design, synthesis and evaluation of azapeptides, oxapeptides and related heterocycles as inhibitors of D,D-peptidase and (3-lactamase 95F455. 

Molecular bases for interactions between (3-lactam antibiotics and (3-lactamases 97ACR162. 

Widespread clinical acceptance continues to be accorded to the cephalosporins, and the field is extremely active as firms search for the ultimate contender. Among the characteristics desired is retention of the useful features of the older members (relatively broad spectrum, less antigenicity than the penicillins, relative insensitivity toward 3-lactamases, and convenience of administration) while adding better oral activity and broader antimicrobial activity (particularly potency against Pseudomonas, anaerobes, meningococci, cephalosporinase-carrying organisms, and the like). To a considerable extent these objectives have been met, but the price to the patient has been dramatically increased. 

Penicillins and other /3-lactam antibiotics (see the Focus On in this chapter) typically develop a resistance to bacteria due to bacterial synthesis of /Mactamase enzymes. Tazobactam, however, is able to inhibit the activity of the /3-lactamase by trapping it, thereby preventing resistance from developing. 

Other examples of a-keto acid-dependent enzymes are mammalian proline hydroxylase and bacterial clavaminate synthase [113]. The latter enzyme is of particular interest as it is responsible for the catalysis of three individual steps in the biosynthesis of the (3-lactamase inhibitor clavulanic acid (Scheme 10.30). 

All feed streams are sterilised before being metered into the fermentation vessel. Contaminants resistant to the antibiotic rarely find their way into the fermenter. When they find a way to contaminate media, their effects are so catastrophic that prevention is of paramount importance. A resistant, (3-lactamase producing, fast-growing bacterial contaminant can destroy the penicillin.5 The contaminants not only consume nutrients intended for the fungus, but also cause loss of pH control and interference with the subsequent extraction process. 

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. 

According to their genetic relationship and their biochemical mechanism of action (3-lactamases are divided into enzymes of the serine-protease type containing an active-site serine (molecular class A, C, and D enzymes) and those of the metallo-protease type (molecular class B enzymes), which contain a complex bound zinc ion. 

Antibiotic Resistance. Figure 1 According to Bush, Jacoby and Medeiros [2] four molecular classes of (3-lactamases can be discriminated based upon biochemical and molecular features. Classes 1, 2, and 4 included serine-proteases, while metallo enzymes are included in class 3. The substrate spectrum varies between different subclasses and the corresponding genes can be part of an R-plasmid leading to a wider distribution or are encoded chromosomally in cells of specific species. 

The class A enzymes have Mx values around 30,000. Their substrate specificities are quite variable and a large number of enzymes have emerged in response to the selective pressure exerted by the sometimes abusive utilization of antibiotics. Some of these new enzymes are variants of previously known enzymes, with only a limited number of mutations (1 4) but a significantly broadened substrate spectrum while others exhibit significantly different sequences. The first category is exemplified by the numerous TEM variants whose activity can be extended to third and fourth generation cephalosporins and the second by the NMCA and SME enzymes which, in contrast to all other SXXK (3-lactamases, hydrolyse carbapenems with high efficiency. Despite these specificity differences, the tertiary structures of all class A (3-lactamases are nearly superimposable. 

Inactivators of class A (3-lactamases (clavulanate, sulbactam, tazobactam) are themselves (3-lactams and act as suicide substrates. They can be used in... 

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. 

Methicillin-resistent staphylococci are strains of staphylococci, which show resistance to a wide variety of antibiotics. They are named for their resistance to methicillin, a (3 -lactamase-resistant penicillin. Methicil-lin-resistante Staphylococcus aureus (MRSA) has become a serious problem particularly in hospitals. 

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