Michael I. Page

The mechanisms of reactions of β-lactam antibiotics

Publisher Summary This chapter explains that penicillin was the first naturally occurring antibiotic to be characterized and used in clinical medicine. It can be seen as the progenitor of the β-lactam family of antibiotics, which are characterized by the possession of the four-membered β -lactam ring. Penicillins were originally detected in fungi but subsequently were found in streptomycetes. The cephalosporins, second member of the β -lactam antibiotic family, were also originally discovered in fungi but later detected in streptomycetes. At present, p-lactam antibiotics can be classified into several groups according to their structure: penicillins (penams), cephalosporins (cephems), cephamycins, oxacephems, penems, clavulanic acid, thienamycin (carbapenems), nocardicins, and monobactams. The chapter highlights that large number of nuclear analogs of the β -lactam antibiotics have been prepared by complete chemical synthesis or by partial synthesis starting from a naturally occurring β -lactam. Of these only the oxacephems and the penems have been investigated clinically.

The Mechanisms of Catalysis by Metallo β-Lactamases

Class B β-lactamases or metallo-β-lactamases (MBLs) require zinc ions to catalyse the hydrolysis of β-lactam antibiotics such as penicillins, cephalosporins, carbapenems, and cephamycins. There are no clinically useful inhibitors against MBLs which are responsible for the resistance of some bacteria to antibiotics. There are two metal-ion binding sites that have different zinc ligands but the exact roles of the metal-ion remain controversial, and distinguishing between their relative importance is complex. The metal-ion can act as a Lewis acid by co-ordination to the β-lactam carbonyl oxygen to facilitate nucleophilic attack and stabilise the negative charge developed on this oxygen in the tetrahedral intermediate anion. The metal-ion also lowers the pKa of the directly co-ordinated water molecule so that the metal-bound hydroxide ion is a better nucleophile than water and is used to attack the β-lactam carbonyl carbon. An intrinsic property of binuclear metallo hydrolytic enzymes that depend on a metal-bound water both as the attacking nucleophile and as a ligand for the second metal-ion is that this water molecule, which is consumed during hydrolysis of the substrate, has to be replaced to maintain the catalytic cycle. With MBL this is reflected in some unusual kinetic profiles.

A facile and highly stereoselective approach to a polycyclic isoindolinone ring system via an N-acyliminium ion cyclization reaction

A highly diastereoselective synthesis of chiral ring-fused isoindolinone products, the skeleton of which is common to many naturally occurring and biologically active compounds, is achieved in only two synthetic steps from readily available precursors via an N-acyliminium ion cyclization reaction of an isoindolinone substrate.

Cysteinyl peptide Inhibitors of Bacillus cereus Zinc β-Lactamase

Several cysteinyl peptides have been synthesised and shown to be reversible competitive inhibitors of the Bacillus cereus metallo-β-lactamase. The pH dependence of pKi indicates that the thiol anion displaces hydroxide ion from the active site zinc(II). , -Peptides bind to the enzyme better than other diastereoisomers, which is compatible with the predicted stereochemistry of the active site

The mechanism of catalysis and the inhibition of β-lactamases

Formation of a tetrahedral intermediate by nucleophilic attack on the β-lactam carbonyl carbon of penicillins generates a lone pair on the β-lactam nitrogen which is syn to the incoming nucleophile, in contrast to the normal anti arrangement found in peptides. Ring opening of the β-lactam requires protonation of the β-lactam nitrogen by a general acid catalyst. The general acid/base catalyst in β-lactamases is probably a glutamate and a tyrosine residue in class A and C enzymes, respectively. Phosphonamidates inactivate class C β-lactamases by phosphonylation of the active site serine, the rate of which is enhanced by a factor of at least 106. The enzyme’s catalytic machinery used for hydrolysis is also used for phosphonylation. The rate enhancement may be greater than 109 if the mechanism occurs by an inhibitor assisted reaction involving intramolecular general acid catalysis. Class B metallo-β-lactamases are inhibited by thiol derivatives with Ki as low as 10 µM. The mechanism of hydrolysis of the metallo-β-lactamase involves a dianionic tetrahedral intermediate stabilised by zinc(II).

Copper(I)-Catalyzed Amination of Aryl Halides in Liquid Ammonia

The amination of aryl halides in liquid ammonia (LNH(3)) is catalyzed by a copper(I) salt/ascorbate system to yield primary aromatic amines in good to excellent yields. The low concentrations of catalyst required and the ease of product isolation suggest that this process has potential industrial applications. Commonly used ligands for analogous metal-catalyzed reactions are not effective. The rate of amination of iodobenzene in liquid ammonia is first order in copper(I) catalyst concentration. The small Hammett ρ = 0.49 for the amination of 4-substituted iodobenzenes in liquid ammonia at 25 °C indicates that the C-I bond is not significantly broken in the transition state structure and that there is a small generation of negative charge in the aryl ring, which is compatible with the oxidative addition of the copper ion being rate limiting.

Mutational analysis of the two zinc-binding sites of the Bacillus cereus 569/H/9 metallo-β-lactamase

The metallo-beta-lactamase BcII from Bacillus cereus 569/H/9 possesses a binuclear zinc centre. The mono-zinc form of the enzyme displays an appreciably high activity, although full efficiency is observed for the di-zinc enzyme. In an attempt to assign the involvement of the different zinc ligands in the catalytic properties of BcII, individual substitutions of selected amino acids were generated. With the exception of His(116)-->Ser (H116S), C221A and C221S, the mono- and di-zinc forms of all the other mutants were poorly active. The activity of H116S decreases by a factor of 10 when compared with the wild type. The catalytic efficiency of C221A and C221S was zinc-dependent. The mono-zinc forms of these mutants exhibited a low activity, whereas the catalytic efficiency of their respective di-zinc forms was comparable with that of the wild type. Surprisingly, the zinc contents of the mutants and the wild-type BcII were similar. These data suggest that the affinity of the beta-lactamase for the metal was not affected by the substitution of the ligand. The pH-dependence of the H196S catalytic efficiency indicates that the zinc ions participate in the hydrolysis of the beta-lactam ring by acting as a Lewis acid. The zinc ions activate the catalytic water molecule, but also polarize the carbonyl bond of the beta-lactam ring and stabilize the development of a negative charge on the carbonyl oxygen of the tetrahedral reaction intermediate. Our studies also demonstrate that Asn(233) is not directly involved in the interaction with the substrates.

The Chemical Reactivity of β-Lactams, β-Sultams and β-Phospholactams

β-Lactam antibiotics display a range of biological activities. The origin of this diverse biological activity is discussed with reference to the chemical reactivity of the small ring system. The reactions of β-sultams and β-phospholactams with simple nucleophiles are reported and their potential as mechanism based inhibitors of bacterial and mammalian serine proteases is described.

Metal-ion catalysed hydrolysis of some β-lactam antibiotics

The metal(II)-ion catalysed hydrolysis of some pencillin and cephalosporin derivatives in water at 30° shows saturation kinetics. A 1 : 1 complex is formed between the metal ion and penicillin which is attacked by hydroxide ion up to 108 fold faster than the unco-ordinated compound. The site of co-ordination of the penicillins and copper(II) ions is the β-lactam nitrogen and the carboxylate group. The association constants for the cephalosporins and metal ions are greater than those for the penicillins but the transition states for the cephalosporin reaction with hydroxide ion bind less tightly to metal ions. The order of rate enhancement brought about by the metal ion is CuII > ZnII > NiII∼ CoII. The metal ions are thought to stabilise the tetrahedral intermediate formed by hydroxide ion attack on the β-lactam. Some comments are made about metal ions as electrophilic catalysts in enzymes.

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFBs) readily undergo solvolysis in liquid ammonia and 2-nitrofluorobenzene is about 30 times more reactive than the 4-substituted isomer. Oxygen nucleophiles, such as alkoxide and phenoxide ions, readily displace fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. Using the pK(a) of the substituted phenols in liquid ammonia, the Brønsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative of the removal of most of the negative charge on the oxygen anion and complete bond formation in the transition state and therefore suggests that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second-order rate constants generate a Brønsted β(nuc) of 0.36 using either the pK(a) of aminium ion in acetonitrile or in water, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate. Nitrobenzene and diazene are formed as unusual products from the reaction between sodium azide and 4-NFB, which may be due to the initially formed 4-nitroazidobenzene decomposing to give a nitrene intermediate, which may then give diazene or be trapped by ammonia to give the unstable hydrazine which then yields nitrobenzene.