Lactamase-like Activity

The data were fitted to a model where nitrocefin first binds to [Co2(C02EtH2Ll)(CH3COO)2] (ki) and then is subsequently hydrolyzed by this complex (k2). Analysis of the data (300-700 nm, [complex] 0.05 mM, [substrate] 0.05 mM, pH 8, 37 °C) resulted in ki = 2.5 x 10 1.9 x 10 M min and k2 = 1.6 X 10 8.1 X 10 min It should be mentioned that the second equivalent was more slowly hydrolyzed by [Co2(C02EtH2Ll)(CH3COO)2] than with the respective Cd(II) complex (Chap. 5). It is assumed that this is due to the larger size of Cd(II) and thus, to the greater ability to accommodate and hydrolyse a second nitrocefin molecule efficiently. 

For [Co2(C02EtH2Ll)(CH3COO)2] the pH dependence with one equivalent nitrocefin resulted in a bell shaped profile with the two pKaS (pKai = 6.88 0.74 and pKa2 = 8.45 0.68) (Fig. 6.38). The first pKa is attributed to the deprotonation of an alkoxide ligand arm coordinated to Co(II). The second pKa is proposed to arise from a water molecule which, upon deprotonation, competes with the substrate for coordination, thus lowering hydrolysis rates. 

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Metallo-P-lactamase-like Activity Assays were conducted in the same aqueous multi-component buffer as mentioned above with the ionic strength controlled by 250 mM LiC104. Assays were carried out at 37 °C in 50 50 acetonitrileibufifer, with nitrocefin as substrate initially dissolved in acetonitrile (10 mM) and the complex dissolved in acetonitrile water (1 mM). Assays conducted to investigate pH dependence were 5 pM in complex and 50 pM in nitrocefin. [Substrate]... 

Some boronic acid-based enzyme inhibitors undergo strong yet reversible covalent attachment to a nucleophile at the enzyme s active site, while others simply act as competitive inhibitors in their borate conjugate base form. Boronic acid-based inhibition of thrombin has been achieved <93MI109>, and that of P-lactamases has been particularly effective <95TL8399, 96M1688>. When compared to other covalent transition-state analog inhibitors of P-lactamases like phos-... 

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. 

The antibiotic activity of certain (3-lactams depends largely on their interaction with two different groups of bacterial enzymes. (3-Lactams, like the penicillins and cephalosporins, inhibit the DD-peptidases/transpeptidases that are responsible for the final step of bacterial cell wall biosynthesis.63 Unfortunately, they are themselves destroyed by the [3-lactamases,64 which thereby provide much of the resistance to these antibiotics. Class A, C, and D [3-lactamases and DD-peptidases all have a conserved serine residue in the active site whose hydroxyl group is the primary nucleophile that attacks the substrate carbonyl. Catalysis in both cases involves a double-displacement reaction with the transient formation of an acyl-enzyme intermediate. The major distinction between [3-lactamases and their evolutionary parents the DD-peptidase residues is the lifetime of the acyl-enzyme it is short in (3-lactamases and long in the DD-peptidases.65-67... 

Like carbapenems, several penems have also been found to be susceptible to renal dehydropeptidase degradation [165-167], 6-Substituted methyl-idene-penems 5.56 are very potent broad-spectrum inhibitors of bacterial /3-lactamase, with the inhibitory activity residing predominantly in the (5R)-enantiomers. As a rule, the (5S)-enantiomers are less stable than the (5R)-enantiomers toward DHP-I [168],... 

In the search for structural diversity, and novel therapeutic agents, unique ring structures like the 1,2,5-thiadiazole have always captured the imagination of chemists. Often, as in the case of timolol (4), the interest is rewarded. In the early 1990s, a simple thiadiazole was appended to a penem in the development of the structure-activity relationships for a series of ) -lactamase inhibitors. The result was enhanced penetration of the bacterial membrane and a broader spectrum of activity versus clavulanic acid <9lJAN33l>. 

Monobactams like aztreonam are monocyclic, as opposed to bicyclic, beta-lactam antibiotics. They are beta-lactamase-resistant. The monobactams are active against gram negative rods but lack activity against gram positive bacteria or against anaerobes. They are administered intravenously and they are rapidly excreted in the urine. 

It is formed by acylases that cleave off the side chain of the penicillins, and can also be obtained by the selective chemical cleavage of the amide, leaving the lactam intact. After this, 6-APA can be easily acylated by any carboxylic acid, and this has yielded literally thousands of semisynthetic penicillins in the past 30 years, many showing improved stability and activity. Some of them are lactamase resistant (methicillin (9.41), oxacillin (9.42) and its halogenated derivatives), whereas others are broad-spectrum antibiotics, like the orally active ampicillin (9.43), which also inhibits Gramnegative bacteria but is sensitive to lactamase. Carbenicillin (9.44) is particularly active against Pseudomonas and Proteus infections, which are unaffected by natural penicillins. Piperacillin (9.45), a broad-spectrum compound, is spectacularly active against Pseudomonas. 

These drugs retain the antibacterial spectrum of penicillin and have improved activity against gram-negative organisms. Like penicillin, however, they are relatively susceptible to hydrolysis by 13 lactamases. 

These drugs have greater activity than penicillin against gram-negative bacteria because of their enhanced ability to penetrate the gram-negative outer membrane. Like penicillin G, they are inactivated by many 3 lactamases. 

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